Fullscreen HTML5Med Res (??MB)HTML4

A CAPACITOR LEAKAGE ADAPTOR FOR DMMS By JIM ROWE Here’s a cut-down version of the Digital Capacitor Leakage Meter we described in December 2009. Instead of using a PIC microcontroller and an LCD panel to display the leakage current, this version connects to your DMM to provide the readout. It provides the same range of seven different standard test voltages (from 10V to 100V) and can measure leakage currents down to 100 nanoamps! 28  Silicon Chip siliconchip.com.au W hy would you need to measure capacitor leakage current? In case you missed the December 2009 article, here’s a summary of the introduction we provided there. In theory, capacitors are not supposed to conduct direct current apart from a small amount when a DC voltage is first applied to them and they have to ‘charge up’. With most practical capacitors using materials like ceramic, glass, polyester or polystyrene - even waxed paper - as their insulating dielectric, the only time they do conduct any DC is during charging. That’s assuming they haven’t been damaged, either physically or electrically. In that case they may well conduct DC as a steady ‘leakage current’, showing that they are faulty. But as many SILICON CHIP readers will be aware, things are not this clear cut with electrolytic capacitors, whether they be aluminium or tantalum. All brand new electrolytic capacitors conduct a small but measurable DC current, even after they have been connected to a DC source for sufficient time to allow their dielectric oxide layer to ‘form’. In other words all electrolytic capacitors have a significant leakage current, even when they are ‘good’. The range of acceptable leakage current tends to be proportional to both the capacitance and the capacitor’s rated voltage. Have a look at the figures given in the Leakage Current Guide opposite. The current levels listed there are the maximum allowable before the capacitor is regarded as faulty. So an instrument capable of measuring the leakage current of capacitors can be very handy in many areas of electronics. Commercially available capacitor leakage current meters are expensive (ie, over $1000) and even the Capacitor Leakage Meter we described in the December 2009 issue will probably cost you over $100 to build. That’s why we’ve developed a cut-down version described in this article, which lets you make all of the same measurements with your existing digital multimeter (DMM). The Adaptor is easy to build and will have a much lower cost than the December 2009 meter while still providing the same choice of seven different standard test voltages: 10V, 16V, 25V, 35V, 50V, 63V or 100V. It is also able to make current measurements from 10mA down to a fraction of a microamp. So it’s capable of making leakage current tests on the vast majority of capacitors in current use. It’s built into a compact UB1 size jiffy box and is battery powered (6 x AA alkaline cells). This makes it suitable for the workbench or the service technician’s tool kit. CAPACITOR LEAKAGE CURRENT GUIDE TYPE OF CAPACITOR Maximum leakage current in microamps A) at rated working voltage 10V 16V Ceramic, Polystyrene, Metallised Film (MKT, Greencap etc.), Paper, Mica 25V 35V 50V 63V 100V LEAKAGE SHOULD BE ZERO FOR ALL OF THESE TYPES Solid Tantalum* < 4.7 F 1.0 1.5 2.5 3.0 3.5 5.0 7.5 6.8 F 1.5 2.0 3.0 4.0 6.5 7.0 9.0 47 F 10 10 15 16 17 19 24 Standard Aluminium Electrolytic# <3.3 F 5.0 5.0 5.0 6.0 8.0 10 17 5.0 6.0 8.0 12 15 23 8.0 13 18 25 35 50 4.7 F 5.0 10 F 15 F 8.0 11 19 25 38 100 230 100 F 50 230 300 330 420 500 600 150 F 230 280 370 430 520 600 730 680 F 500 600 780 950 1100 1300 1560 1000 F 600 730 950 1130 1340 1500 1900 4700 F 1300 1590 2060 2450 2900 3300 4110 * Figures for Solid Tantalum capacitors are after a charging period of one minute. # Figures for Aluminium Electrolytics are after a charging/reforming period of three minutes. source (on the left) which generates one of seven different preset voltages when the TEST button is pressed and held down. The second section is a simple current to voltage converter (on the right) which is used to generate a voltage proportional to the direct current passed by the capacitor under test, so that it can be measured easily using your DMM. Any direct current passed by the capacitor being tested flows down to ground via resistor R2, which therefore acts as a current shunt. The voltage drop across R2 is then passed through an output buffer which feeds your DMM. The DMM is set to its 0-2.0V DC voltage range, which allows its readings to be easily converted into equivalent current levels. So that’s the basic arrangement. The reason for resistor R1, in series with the output of the test voltage source, is How it works to limit the maximum current that can be drawn from the source, in any circumstances. This prevents damage to The Adaptor’s operation is straightforward, as you can either the voltage source or the current-to-voltage converter see from the block diagram of Fig.1. There are two funcsections, in the event of the capacitor under test having tional circuit sections, one being a selectable DC voltage an internal short circuit. It also protects CAP UNDER TEST R2 and the output buffer from overload + when a capacitor (especially one of high R1 + +Vt + TEST SELECTABLE value) is initially charging up to one of + – OUT TO DMM OUTPUT DC VOLTAGE R2 the higher test voltages. (1V = 10mA BUFFER SOURCE OR 100 A) (S2) TEST – – R1 has a value of 10k, which was cho(7 VOLTAGES) TERMINALS sen to limit the maximum charging and/ (IC1) (IC2) or short circuit current to 9.9mA even Fig.1: block diagram of the adaptor shows it has two elements: a selectable on the highest test voltage range (100V). DC voltage source and a simple current-to-voltage converter. At this stage you may be wondering siliconchip.com.au April 2010  29 how the Adaptor can allow your DMM to read leakage currents down to less than a microamp, when it also has to cope with charging currents of up to 9.9mA. The answer is that the current-to-voltage converter section of the Adaptor actually has two current ranges, which are selected by switching the value of shunt resistor R2. The default value of R2 is 100, which provides a 0-10mA range for the capacitor’s charging phase (ie, when TEST button S2 is first pressed). But when (and if) the measured current level falls below 100A, pushbutton S4 can be pressed to switch the value of R2 to 10k, providing a 0-100A range for more accurate leakage current measurement. The 270k resistor forms the top arm of the feedback divider, while the 36k and 2.4k resistors from pin 5 to ground form the fixed component of the lower arm. These give the divider an initial division ratio of 308.4k/38.4kor 8.031:1, to produce a regulated output voltage of 10.04V. This is the converter’s output voltage when selector switch S1 is in the ‘10V’ position. When S1 is switched to any of the other positions, additional resistors are connected in parallel with the lower arm of the feedback divider, to increase its division ratio and hence increase the converter’s output voltage. For example, when S1 is in the ‘25V’ position, this connects the 270, 8.2k, 5.1k, 2.0k, 200, 2.4k, 150 and 3.6k resistors (all in series) in parallel with the divider’s lower arm, changing the division ratio to 283.954k/13.954k or 20.35:1. This produces a regulated output voltage of 25.44V. The same kind of change occurs in the other positions of S1, producing the various preset output voltages shown. (Although the test voltages shown are nominal, with the specified 1% tolerance resistors used for the divider resistors, they should all be well within ±4% of the nominal values because the 1.25V reference inside the MC34063 is accurate to within 2%.) Note that IC1 only generates the selected test voltage Circuit description Now have a look at the full circuit schematic of Fig.2. The selectable DC voltage source is based around IC1, an MC34063 DC/DC controller IC, used here in a ‘boost’ configuration in conjunction with autotransformer T1 and fast switching diode D2. We vary the circuit’s DC output voltage by varying the ratio of the voltage divider in the converter’s feedback loop, connecting from the cathode of D2 back to IC1’s pin 5 (where the voltage is compared with an internal 1.25V reference). D3 1N4004 POWER +8.4V K A S3 470 F 16V 9V BATTERY (6xAA ALKALINE) Q1 BC327 +8.4V T1 1 15T DrC GND SwE 4 Cin- A K 200 100V SET TEST VOLTS 63V 5.1k 25V 10V TP1 TPG 36k TEST TERMINALS 2.4k 100nF 100 – IC2: LM358 3 1k 100 F 16V LOW LEAKAGE + OUT TO DMM (0–1V) RLY1 K 6 K 10k 6 ZD1 10V 5 A D1 IC2b 470 7 – 4 A 7,8 8.2k 1k 270 2 D1: 1N4148 A K CAPACITOR LEAKAGE ADAPTOR FOR DMMS ZD1 A 1N4004, UF4003 K A K BC327 LEDS K A Fig.2: here’s the complete circuit diagram for the adaptor. At the beginning of each test it measures on its 10mA range but if the current drops below 100A it can switch to a 100A range. 30  Silicon Chip 470 1 IC2a 100 1,14 1M 2 8 + – 16V 33k 10mA RANGE  LED2 K 2.0k 35V A + 50V S1 SC 10k 2.4k  LED1 2010 2.2 F 250V MET. POLY +1.25V 150 TEST VOLTS K 270k 3.6k 2 68k 45T 8 1 IC1 SwC MC34063 5 Ct 820pF B D2 UF4003 A 7 Ips 6 Vcc 3 S4 PRESS FOR 100 A RANGE C TEST S2 2.2k E B E C siliconchip.com.au when test pushbutton S2 is pressed and held down. This is because IC1 only receives power from the battery when S2 is closed, allowing the converter circuit to operate and thereby charging the 2.2F/250V metallised polyester reservoir capacitor. The test voltage is then made available at the positive test terminal via the 10k current limiting resistor, R1. Now let us look at the current-tovoltage converter section, which is virtually all of the circuitry below and to the right of the negative test terminal. The 100, 1M and 10k resistors connected between the negative – 470 F 9V BATTERY (UNDER) POWER IC2 LM358 1 S3 TEST S2 0102 © 4004 10140240 D3 470 470 + – Test voltages: ........... 10V, 16V, 25V, 35V, 50V, 63V or 100V. Leakage current: ...... from 10mA down to less than 100nA (0.1A), via two ranges: 0-10mA (default) and 0-100A (manually selected). Both ranges convert these current values into an output voltage range of 0-1000mV DC, allowing all measurements to be made on the DMM’s 0-1V or 0-2V range. The Adaptor’s default 10mA range is current limited to provide protection from damage due to shorted capacitors or the charging current pulse of high-value capacitors. Power:......................... Internal 9V battery (6 x AA alkaline cells). Current drain:............ Varies between 1mA and 125mA, depending on the test voltage          and the current range in use. E GAKAEL R OTI CAPA C S M MD R OF R OTPADA + OUTPUT BANANA JACKS (TO DMM) Specifications 100nF 10V ZD1 TEST VOLTS S1 LED1 4 7 1.0 3 6 5 8.2k 270 1 IC1 34063 2 SET VOLTS 1 2.2k 33k 1k 820pF T S 10mA RANGE 5.1k 2.0k 100 A RANGE S4 68k Q1 BC327 LED2 36k 2.4k 1k 3.6k T1 150 2.4k 200 15T + 40T F D1 4148 RLY1 2.2 F 250V METAL POLYESTER 270k TPG 1M 10k 100 F LL T+ 10k TEST TERMINALS 100 T– D2 4003 100 TP1 Fig.3: with the exception of the test terminals, DMM output jacks and three of the switches, all components mount on one PC board. siliconchip.com.au Here’s a photograph which matches the diagram at left. In this case, the terminals and the two push-button switches are not shown on the board because they mount on the front panel and connect to the PC board via short lengths of tinned copper wire (one of the last steps in assembly). April 2010  31 NEGATIVE TEST TERMINAL (POSITIVE TERMINAL BEHIND IT) LED2 BEHIND S4 T1 LED1 BEHIND S1 PC BOARD MOUNTED BEHIND LID USING 4 x 25mm M3 TAPPED SPACERS S1 RLY1 S2 IC2 ZD1 100n TRANSFORMER T1 POTCORE HELD TO PC BOARD USING 25mm x M3 NYLON SCREW WITH NUT & FLAT WASHERS 470F S4 NEGATIVE OUTPUT JACK (POSITIVE JACK BEHIND IT) S3 BEHIND S2 6xAA CELL HOLDER (CUT DOWN FROM 10xAA HOLDER) MOUNTED IN BOTTOM OF BOX USING DOUBLESIDED TAPE Fig.4: a side-on view “through” the wall of the jiffy box, showing how everything goes together. The 6xAA cell holder must be mounted at one end, as shown here, to avoid fouling the screw holding the transformer to the PC board. test terminal and ground correspond to the current shunt labelled R2 in Fig.1, with the contacts of reed relay RLY1 used to change the effective shunt resistance for the Adaptor’s two ranges. For the default 0-10mA ‘charging phase’ range RLY1 is energised and connects a short circuit across the parallel 1M/10k combination, making the effective shunt resistance 100. For the more sensitive 100uA range RLY1 is turned off, opening its contacts and connecting the parallel 1M/10k resistors in series with the 100 resistor to produce an effective shunt resistance of 10k. Relay RLY1 is turned on or off by transistor Q1. When power is first switched on via switch S3, Q1 is switched on by forward bias current applied to its base via the 68k resistor to ground. It therefore conducts about 10mA of collector current, which energises RLY1 and also causes LED2 to light – indicating that the Adaptor is operating in the 10mA current range. But if the capacitor’s current reading (on the DMM) drops down to below 100A, pressing pushbutton switch S4 and holding it down causes Q1 to switch off. As a result LED2 and RLY1 both turn off as well, switching the Adaptor to its 0-100A range. The 100F low leakage capacitor in parallel with the shunt routes any AC signal from the capacitor being tested around the shunt. This prevents ripple from the switchmode supply from corrupting the reading. Regardless of which current range is in use, the voltage drop developed across the shunt resistance (as a result of any current passed by the capacitor under test) is passed to the non-inverting input of IC2a, one half of an LM358 dual op amp. IC2a is configured as a voltage follower with a voltage gain of unity, feeding the positive output terminal of the Adaptor via a 470 isolating resistor. So what is the purpose of IC2b? It is connected as a voltage follower in much the same way as IC2a, except that its non-inverting input is connected directly to ground and its output is used to drive the negative output terminal. Its purpose is to balance out most of the input offset of IC2a, so that the Adaptor’s effective output voltage, when there is no current flowing through the test terminals, is much less than 1mV. All of the Adaptor’s circuitry operates directly from the 9V battery, via polarity protection diode D4 and of course 32  Silicon Chip S3. The total current drain when in ‘standby’ (ie, with TEST button S2 not pressed) is about 11mA in the default 10mA current range or 1mA if S4 is pressed to switch it into the 100A range. The current level increases to between 25mA and 125mA when S2 is pressed and held down to generate the test voltage and perform the actual leakage current test. Construction Virtually all of the circuitry and components used in the Capacitor Leakage Adaptor are mounted on a single PC board measuring 145 x 84mm and coded 04104101. This is mounted under the lid (which becomes the Adaptor’s front panel) of a UB1 jiffy box (157 x 95 x 53mm) via four 25mm long M3 tapped spacers. Six AA alkaline cells provide power, mounted in a cut-down 10-cell holder secured to the bottom of the box. Both the voltage selector switch (S1) and the DC/DC converter’s step-up transformer (T1), wound on a 26mm ferrite pot core, mount on the board, the latter using a 25mm long M3 Nylon screw and nut. The only components not mounted directly on the main board are power switch S3, pushbutton switches S2 and S4, the two test terminals and the two output banana jacks. These are all mounted on the box front panel, with their rear connection lugs extended down via short lengths of tinned copper wire to make their connections to the board. All of these assembly details should be fairly clear from the diagrams and photos. To begin fitting the components on the PC board I suggest you fit the wire link, located just to the right of the position for rotary switch S1. Next fit the four 1mm terminal pins to the board – two for the test point at upper left and two at upper right for the battery clip lead connections. Follow these with the sockets for IC1 and IC2, which are both 8-pin devices. Now fit the fixed resistors. These are 1% tolerance metal film components, apart from the 1.0 resistor just to the right of T1 and to the left of IC1. This resistor should be a 0.5W carbon composition type. Check each resistor’s value with a DMM as you insert and solder them to ensure they all go in the right places. Next, you can fit the two lower-value capacitors and the siliconchip.com.au large 2.2F metallised polyester capacitor, followed by the (polarised) 470F electrolytic. Then fit the mini DIL relay, making sure its locating groove is at the top end. Then you can fit voltage selector switch S1, which mounts with its indexing spigot at 3-o’clock. Just before you fit it you should cut its spindle to a length of about 13mm and file off any burrs, so it’s ready to accept the knob during final assembly. After it has been fitted to the board, remove its main nut/ lock washer combination and turn the spindle by hand to make sure it’s at the fully anticlockwise limit. Then refit the lock washer, making sure that its stop pin goes down into the hole between the moulded ‘7’ and ‘8’ digits. Check that the switch is now ‘programmed’ for the correct seven positions, simply by clicking it around through them by hand. With S1 fitted, you can add the four diodes. Don’t mix them up! D1 is a low power 1N4148 ‘signal’ diode, D2 is a UF4003 ‘fast’ rectifier, D3 is a 1N4004 1A power diode and A zener. Use the overlay diagram as a guide ZD1 is a 10V/1W to their orientation when you’re fitting each one to the board. Next fit transistor Q1, followed by the two 5mm LEDs. The red one is used as LED1 and 60.5the green one as LED2. They are both mounted vertically with their leads left at almost full length, so that the lower surface of their bodies 38.5 5 is about 23mm above the surface of the board. E This allows them to just protrude through the matching B B holes in the lid/front panel when the board assembly is attached behind it. 9.5 At this stage your board assembly is very close to complete, with the main task remaining being to wind transE 9.5 former T1 and fit it to the board. You’ll find the full details on how to do this in the separate panel. C Once the transformer has beenF fitted to the board, you can attach the four 25mm M3 tapped spacers to it as well. 38.5 These each attach very close to each corner of the board, using 6mm long M3 screws passing up from the underside. 37 Now all that remains to complete the board assembly is to plug IC1 and IC2 into their sockets. Place it aside while you prepare the case to receive it. Preparing the case There are no holes to be drilled in the lower part of the case (the battery holder can be held securely in place using strips of ‘industrial’ double-sided adhesive foam tape) but the lid does need to have holes drilled for the various switches, LEDs and input/output connectors. The location and dimensions of all these holes are shown in the diagram of Fig.5, which is actual size so it (or a photocopy) can also be used as a drilling template. The larger holes are easily made by drilling them all first with a 7mm twist drill and then carefully enlarging them to size using a tapered reamer. We have prepared an artwork for the front panel if you would like to make it look neat and professional. This can be either photocopied (Fig.6) or downloaded as a PDF file A from our website and then printed out. Either way the resulting copy can either be covered with self-adhesive clear film or, better still, laminated, for HOLE protection60.5 against finger grease, etc before it isDIAMETERS: glued to the lid/panel. A: 3.0mm B: 5.0mm Mount switches S2, S3 and S4 on the panel, followed 7.0mm by the binding posts used as the meter’s testC: terminals D: 8.0mm D and the banana sockets used for Dthe output connections E: 9.0mm to your DMM. F: 12.0mm Tighten the binding post and banana socket mounting 9.5 nuts firmly, to make sure that they cannot come loose with C use. Then use the second nut of each post andLsocket to 9.5 D attach 16.5 a 4mm solder lug plus a 4mm lockwasher to make sure they don’t work loose either. Now you can turnF the lid assembly over and solder ‘extension wires’ to the connection lugs of the three switches, and also the solder lugs fitted to the rear of the binding posts and sockets. These wires should all be about 30mm long 37 and cut from tinned copper wire (about 0.7mm diameter). A A A ALL DIMENSIONS IN MILLIMETRES A CL 60.5 38.5 5 E D B B D 9.5 9.5 9.5 HOLE DIAMETERS: A: 3.0mm B: 5.0mm C: 7.0mm D: 8.0mm E: 9.0mm F: 12.0mm 60.5 CL E D 16.5 F 9.5 F C 38.5 37 37 A A siliconchip.com.au CL Fig.5: a 1:1 drilling template for the front panel ALLof the DIMENSIONS specified jiffy IN box. MILLIMETRES April 2010  33 Parts List – Capacitor Leakage DMM Adaptor 1 PC board, code 04204101, 145 x 84mm 1 UB1 jiffy box, 158 x 95 x 53mm 1 Single pole rotary switch, PC mounting (S1) 2 SPST mini pushbutton switch (S2, S4) 1 SPDT mini toggle switch, panel mounting (S3) 1 Mini DIL reed relay, SPST with 5V coil (RLY1) 2 Premium binding posts, 1 x red and 1 x black 2 4mm banana jack sockets, 1 x red and 1 x black 1 16mm diameter fluted instrument knob 1 Ferrite pot core pair, 26mm OD 1 Bobbin to suit pot core 1 3m length of 0.5mm diameter enamelled copper wire 1 25mm M3 Nylon screw and nut and two flat washers 2 8-pin DIL IC sockets 4 1mm dia. PC board terminal pins 4 25mm long M3 tapped spacers 8 6mm long M3 machine screws, pan head 1 10x AA battery holder (flat, side by side) Semiconductors 1 MC34063 DC/DC converter controller (IC1) 1   LM358 dual op amp (IC2) 1   BC327 PNP switching transistor (Q1) 1   10V 1W zener diode (ZD1) 1   5mm red LED (LED1) 1   5mm green LED (LED2) 1   1N4148 100mA diode (D1) 1   UF4003 fast 1A diode (D2) 1   1N4004 1A diode (D3) Capacitors 1   470F 16V PC electrolytic 1 100F 16V low leakage electro 1  2.2F 250V (or 100V) metallised polyester 1  100nF multilayer monolithic ceramic 1  820pF disc ceramic Resistors (0.25W 1% unless specified) 1 1M 1 270k 1 68k 1 36k 1 33k 1 22k 2 10k 1 8.2k 1 5.1k 1 3.6k 2 2.4k 1 2.2k 1 2.0k 2 1k 2 470 1 270 1 200 1 150 2 100 1 1.0 0.5W carbon (5%) 34  Silicon Chip “Opened out” view showing the PC board “hanging” from the front panel. The next step is to prepare the battery holder. Because you can’t buy a six-way flat AA holder (at least we couldn’t find one!) we cut down a tenway AA holder. The last three cell positions are removed altogether (at the ‘negative lead’ end) and then the eyelets are drilled out and used to attach the contact spring for the sixth cell position and also the contact spring and negative lead connection lug at the end of the removed section. This will allow you to re-attach the negative lead’s connection lug to the contact spring for the sixth cell using a 6mm long M2 machine screw and nut. The seventh cell position is still retained to support the sixth cell connection spring and the negative lead connection lug. The converted battery holder can now be fitted inside the main section of the box at lower right, with the connection lead side uppermost. Mount it using double-sided adhesive foam as mentioned earlier, or simply a strip of ‘gaffer’ tape. You should now be ready for the only slightly fiddly part of the assembly operation: attaching the PC board as- sembly to the rear of the lid/front panel. This is only fiddly because you have to line up all of the extension wires from switches S2, S3 and S4, the two test terminals and the output banana sockets with their matching holes in the PC board, as you bring the lid and board together. At the same time you have to line up the spindle of switch S1 and the two LEDs with their matching holes in the front panel. This is actually easier to do than it sounds, so just take your time and the lid will soon be resting on the tops of the board mounting spacers. Then you can secure the two together using four 6mm long machine screws. Now it’s simply a matter of turning the complete assembly over and soldering each of the switch and terminal extension wires to their board pads. Once they are all soldered you can clip off the excess wires with sidecutters. If you find this description a bit confusing, refer to the assembly diagram in Fig.4. This will hopefully make everything clear. Next solder the bared end of the red (positive) battery holder lead to the positive battery terminal pin at the upper right on the PC board, and the siliconchip.com.au Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o No. 1 1 1 1 1 1 2 1 1 1 2 1 1 2 2 1 1 1 2 1 Value 1M 270k 68k 36k 33k 22k 10k 8.2k 5.1k 3.6k 2.4k 2.2k 2.0k 1k 470 270 200 150 100 1.0 (0.5W) black (negative) battery holder lead to the negative pin alongside. You can now fit six AA-size alkaline cells into the battery holder (make sure you fit them with the correct polarity) and your new Capacitor Leakage Adaptor should be ready for its initial checkout. Initial checkout You’ll need to use a twin test lead to connect the Adaptor’s output to the input jacks of your DMM. The DMM should also be set to measure DC voltage, and to its 0-1V or 0-2V range if it’s not auto ranging. Switch on the Adaptor’s power using S3 and green LED2 should light – showing that the Adaptor is operating, in standby mode and in the default 10mA current range. Then if you press S4, the range change button, LED2 should go dark. This shows that the range switching circuitry is operating. But your DMM should still be giving a zero reading. At this point you can stop pressing S4. Next try pressing test button S2. This should cause red LED1 to glow, indicating that power is now being applied to the test voltage generation circuitry. If there is no capacitor or other component connected across the test terminals, your DMM should still be giving a reading of zero. Assuming all has gone well at this point, your Adaptor is probably worksiliconchip.com.au 4-Band Code (1%) brown black green brown red violet yellow brown blue grey orange brown orange blue orange brown orange orange orange brown red red orange brown brown black orange brown grey red red brown green brown red brown orange blue red brown red yellow red brown red red red brown red black red brown brown black red brown yellow violet brown brown red violet brown brown red black brown brown brown green brown brown brown black brown brown brown black gold gold (5%) ing correctly. However if you want to make sure, try shorting the two test terminals. Then set S1 to the ‘100V’ position, and press Test button S2. The DMM reading should change to a value corresponding to 9.9mA (i.e., 990mV), representing the current drawn from the nominal 100V source by the 10k current limiting resistor and the 100 current shunt resistor inside the Adaptor. Don’t worry if the current reading is a bit above or below the 9.9mA figure, by the way. As long as it’s between about 9.2mA (920mV) and 10.6mA (1.06V), things are OK. With the terminals still shorted together, you can try repeating the same test for each of the other six test voltage positions of switch S1. You should get a reading on the DMM corresponding to approximately 6.25mA (625mV) on the 63V range, 4.95mA (495mV) on the 50V range, 3.46mA (346mV) on the 35V range, 2.48mA (248mV) on the 25V range, 1.58mA (158mV) on the 16V range and 990uA (99mV) on the 10V range. If the readings you get are close to these, your Capacitor Leakage Adaptor is working correctly. This being the case, switch off the power again via S3 and then complete the final assembly by lowering the lid/ PC board assembly into the case and securing the two together using the four small self-tapping screws supplied. 5-Band Code (1%) brown black black yellow brown red violet black orange brown blue grey black red brown orange blue black red brown orange orange black red brown red red black red brown brown black black red brown grey red black brown brown green brown black brown brown orange blue black brown brown red yellow black brown brown red red black brown brown red black black brown brown brown black black brown brown yellow violet black black brown red violet black black brown red black black black brown brown green black black brown brown black black black brown Make sure you also remove the shorting wire between the test terminals. Using it The Capacitor Leakage Adaptor is very easy to use, because all you have to do is connect the capacitor you want to test across the test terminals (with the correct polarity in the case of solid tantalums and electrolytics), after connecting the Adaptor’s output sockets to the input jacks of your DMM. Then turn on the DMM and set it to measure DC volts. Now set the Adaptor’s selector switch S1 for the correct test voltage and turn on the power (S3), whereupon LED2 should light. Then to begin the actual test, press and hold down Test button S2. What you may see first on the DMM is a reading of the capacitor’s charging current, which can be as much as 9.9mA (with high value caps) but will then drop back as charging continues. How quickly it drops back will depend on the capacitor’s value. With capacitors below about 4.7F, the charging may be so fast that the first reading will often be less than 100A (10mV). If the capacitor you’re testing is of the type having a ‘no leakage’ dielectric (such as metallised polyester, glass, ceramic or polystyrene), the current should quickly drop down to less than 10A (1mV). April 2010  35 Winding autotransformer T1 The step-up autotransformer T1 has around the hole in a circle, with a diam60 turns of wire in all, wound in four eter of 10mm. Your ‘gap’ washer will 15-turn layers. And as you can see then be ready to place inside the lower UPPER SECTION from the assembly diagram at right, all half of the pot core, over the centre hole. OF FERRITE POT CORE four layers are wound on a small Nylon Once the gap washer is in position, bobbin using easily handled 0.5mm you can lower the wound bobbin into the diameter enamelled copper wire. Use pot core around it, and then fit the top BOBBIN WITH this diagram to help you wind the half of the pot core. The transformer WINDING (4 x 15T OF 0.5mm DIA is now be ready for mounting on the transformer correctly. ENAMELLED COPPER Here’s the procedure: first wind on main PC board. WIRE, WITH TAP AT END 15 turns, which you’ll find will neatly First place a Nylon flat washer on the OF FIRST LAYER & INSULATING TAPE take up the width of the bobbin provid25mm-long M3 Nylon screw that will be BETWEEN LAYERS) ing you wind them closely and evenly. used to hold it down on the board. Then Then to hold them down, cover this first pass the screw down through the centre FINISH layer with a 9mm-wide strip of plastic hole in the pot core halves, holding them TAP insulating tape or ‘gaffer’ tape. (and the bobbin with gap washer inside) START Next take the wire at the end of this together with your fingers. first layer outside of the bobbin (via one Then lower the complete assembly 'GAP' WASHER OF 0.06mm of the ‘slots’) and bend it around by 180° down in the upper left of the board with PLASTIC FILM at a point about 50mm from the end of the ‘leads’ towards the right, using the the last turn. This doubled-up lead will bottom end of the centre Nylon screw to be the transformer’s ‘tap’ connection. locate it in the correct position. When you LOWER SECTION The remaining wire can then be used are aware that the end of the screw has OF FERRITE POT CORE to wind the three further 15-turn layers, passed through the hole in the PC board, making sure that you wind them in keep holding it all together but up-end the same direction as you wound the everything so you can apply the second first layer. M3 Nylon flat washer and M3 nut to the (ASSEMBLY HELD TOGETHER & SECURED TO Each of these three further layers end of the screw, tightening the nut so PC BOARD USING 25mm x M3 NYLON SCREW & NUT) should be covered with another 9mmthat the pot core is not only held together wide strip of plastic insulating tape just as but also secured to the PC board. This is to provide a thin magnetic ‘gap’ in the you did with the first layer, so that when all Once this has been done, all that four layers have been wound and covered pot core when it’s assembled, to prevent the remains as far as the transformer is pot core from saturating when it’s operating. concerned is to cut the start, tap and fineverything will be nicely held in place. The washer is very easy to cut from a ish leads to a suitable length, scrape the The ‘finish’ end of the wire can then be brought out of the bobbin via one of the piece of the thin clear plastic that’s used enamel off their ends so they can be tinned slots (on the same side as the start and for packaging electronic components, like and then pass the ends down through tap leads) and your wound transformer resistors and capacitors. their matching holes in the board so they This plastic is very close to 0.06mm thick, can be soldered to the appropriate pads. bobbin should be ready to fit inside the which is just what we need here. So the idea two halves of the ferrite pot core. Don’t forget to scrape, tin and solder Just before you fit the bobbin inside is to punch a 3-4mm diameter hole in a piece BOTH wires which form the ‘tap’ lead – the bottom half of the pot core, though, of this plastic using a leather punch or similar, if they are not connected together, the there’s a small plastic washer to prepare. and then use a small pair of scissors to cut transformer won’t produce any output. And if you press button S4 on the Adaptor to switch down to the 100A range, you should be able to see the DMM reading fall down to zero. That’s if the capacitor is not faulty, of course. On the other hand if the capacitor is one with a tantalum or aluminium oxide dielectric with inevitable leakage, the current reading will drop more slowly as you keep holding down the Test button. In fact it will probably take up to a minute to stabilise at a reasonably steady value in the case of a solid tantalum capacitor and as long as three 36  Silicon Chip minutes in the case of an aluminium electrolytic. (That’s because these capacitors generally take a few minutes to ‘reform’ and reach their rated capacitance level.) As you can see from the guide table earlier the leakage currents for tantalum and aluminium electrolytics also never drop down to zero but instead to a level of somewhere between about 4.1mA and 1A depending on both their capacitance value and their rated working voltage. So with these capacitors, you should hold down the Adaptor’s test button to see if the leakage current reading drops down to the ‘acceptable’ level as shown in the guide table and preferably even lower. If this happens the capacitor can be judged ‘OK’ but if the current never drops to anywhere near this level it should definitely be replaced. What about low leakage (LL) electrolytics? Well, the current levels shown in the guide table are basically those for standard electrolytics rather than for those rated as low leakage. So when you’re testing one which is rated as low leakage, you’ll need to make sure that its leakage current drops well below the maximum values shown siliconchip.com.au CAPACITOR LEAKAGE MEASUREMENT ADAPTOR POWER TEST VOLTS 10mA RANGE + – + PRESS FOR 100A RANGE 25V 35V 16V 63V 10V Fig.6: same-size front panel artwork. in the guide table. Ideally it should drop down to no more than about 25% of these current values. A final tip: when you’re testing nonpolarised (NP) or ‘bipolar’ electrolytics, PRESS TO APPLY VOLTS 50V OUT TO DMM – 100V SELECT TEST VOLTAGE these should be tested twice – once connected to the terminals one way around and then again connected with the opposite polarity. That’s because these capacitors are essentially two polarised types, internally connected in series, back-to-back. If one of the dielectric layers is leaky but the other is OK, this will only show SC up in one of the two tests. Why risk your intellectual property with any other prototype maker? SOS Components has the widest range of product development technology in Australia, all contained in one place. We keep your IP safe by keeping it in-house, under lock and key. Whether your idea requires an inexpensive rapid prototype or full production - rest assured it will look good, function perfectly and remain 100% your property. To find out how cost-effectively SOS Components can bring your ideas to life: phone 07 3267 8104, email sales<at>3dprinting.com.au or go to www.3dprinting.com.au. siliconchip.com.au April 2010  37