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Got a bunch of unknown diodes and zener diodes? Check ’em all with this . . . Zener Diode Tester This zener diode tester plugs into your digital multimeter and you can directly check any zener diode rated from 2.2V up to 100V. You can also check the forward voltage of diodes and test low-voltage Schottky diodes. By JOHN CLARKE W HILE MOST DIGITAL multimeters (DMMs) do include a diode test function, they do not test zener diodes. So how many zener diodes do you have stashed away which are not used because their value is unknown? In many cases, the type number will be missing or partially rubbed off or it is difficult to read because the print is so small. And even if it can be read, the type number will not directly give you the voltage rating. So unless you can look up the data for that type number, you are still “in the dark”. This Zener Tester is the answer. It plugs directly into your DMM, so that you can easily read the breakdown voltage of the zener being tested. The unit can measure all the common types, from very low values of around 2.2V right up to 100V. It’s best for 400mW and 1W power devices, although it will also provide reasonably accurate measurements for 3W zener diodes. The Zener Tester can also measure the breakdown voltage of other diode types such as tran82  Silicon Chip siliconchip.com.au sient voltage suppression (TVS) diodes, as well as standard and Schottky diodes with PIV (peak inverse voltage) ratings below 100V. That makes it suitable for testing many Schottky diodes that break down at 20, 30 or 40V depending on the type (eg, 1N5819 or 1N5822). As with a standard diode tester, you can also measure the forward voltage, which is typically in the range of 0.2-0.8V. Fig.1: the typical zener characteristic. In the reverse direction, there is very little current flow until the “knee” is reached, at which point the zener breaks down and the voltage remains reasonably constant over a wide current range. siliconchip.com.au (FORWARD CONDUCTION) KNEE Vz VOLTAGE –Vr 0.7V VOLTAGE +Vf Idmax 10 10% OF MAXIMUM POWER How zener diodes work Zener diodes are manufactured to provide a specified breakdown voltage where current will flow in the reverse direction. This is known as the “zener” voltage, after Clarence Zener who discovered the effect. The zener diode effect is the predominant operating mechanism for zener diodes with breakdown voltages up to 5.6V. Above this voltage, the “avalanche” effect is more predominant. However, avalanche effect diodes continue to be called zener diodes regardless. Zener diodes (breakdown below 5.6V) have a negative temperature coefficient and avalanche diodes (breakdown above 5.6V) have a positive ­­ temperature coefficient for their break­ down voltage. Zener diodes with a breakdown of around 5.6V have a zero temperature coefficient and so the breakdown voltage does not vary with temperature. Fig.1 shows the typical zener characteristic. In the forward direction, the zener behaves as a diode and begins to conduct at about 0.7V. Conversely, in the reverse direction, there is very little current flow until the “knee” is reached. At this point, the zener breaks down and the voltage remains relatively constant over a wide current range. However, the voltage does increase with increasing current and the slope of voltage against current is the zener impedance (or resistance). This impedance can range from 10Ω for lowvalue zener diodes to above 350Ω for 100V zener diodes. Fig.1 highlights three operating con­ ditions for a zener diode and the two of particular interest are maximum power and 10% of maximum power. These define the normal operating range of the zener. Note how the current/voltage slope is almost a straight line between these points. At less than 10% of rated power, CURRENT Id Idmax 4 25% OF MAXIMUM POWER I ZENER IMPEDANCE = SLOPE (V/I) V MAXIMUM POWER (100%) Idmax the zener voltage is much less than its rated value. On the other hand, operation at or above the maximum power rating will destroy the device (unless it is subjected to brief pulses of current). In any case, zener diodes are not normally operated at maximum power since they must be de-rated for ambient temperatures above 25°C. Note: some zener diode types have a very sharp “knee” which enables the diode to operate at very low currents, well below 10% of maximum power, while maintaining their rated breakdown voltage. Testing zener diodes Testing zeners might seem simple; just apply current so that it operates between 10% of maximum power and maximum power. That’s done by supplying a voltage that’s greater than the zener diode breakdown voltage and by limiting the current. However, in practice, it’s not that simple. Some zener testers apply a constant 5mA to the zener and then read off the value of breakdown voltage. That fixed current is suitable for the BZX79 series of zener diodes (or similar) that are specified for zener voltage at 5mA. That current applies for zener diodes ranging from 2.2V to 25V. A 2mA specification applies to zener diodes from 25V to 60V. Other zener diodes are not characterised for 5mA and the current needed to test a low-voltage zener is vastly different to that required for a higher voltage type. In other words, a fixed 5mA is unsatisfactory, as we need to ensure that the test current runs the zener somewhere between the 10% and 100% power conditions. The 1N5728 (4.7V) to 1N5757 (75V) series of 400mW zener diodes and the 1N4728 (2.2V) to 1N4764 (100V) series of 1W zener diodes are designed to operate at their specified zener voltage at a current that is 25% of maximum power. For a 3.3V 400mW diode, this equates to 30.3mA while for a 75V 400mW zener, the 25% condition is achieved at 5.3mA. It will not matter too much if the current doesn’t precisely give the 25% full-power rating since the breakdown voltage will only change slightly due to the zener impedance. But it is imNovember 2011  83 S1 POWER + 9V BATTERY REFERENCE (LED1, IC1a) ERROR AMPLIFIER IC1b + PULSE CONTROLLER (IC2) K CONVERTER (Q1, T1, D3) METER A – ZENER UNDER TEST – CURRENT FEEDBACK Fig.2: block diagram of the Zener Tester. It uses a DC/DC converter to step up the voltage from a 9V battery so that high-voltage zener diodes can be tested. The error amplifier and pulse controller ensure that a constant power is delivered to the zener diode under test, for a wide range of zener voltages. portant that we do not drop below the zener knee. Fig.8 (later in the article) shows the curves for both 1W and 400mW zener diodes for voltages from 2.5V to 100V. The lower two plots show the 40mW (10% of 400mW) and the 100mW (10% of 1W) power curves. The upper two traces show the maximum power curves for 400mW and 1W. To properly test both 400mW and 1W diodes, we must have the zener diode operating between the 100mW and 400mW curves. In this way, we will be above the 10% power point and below their maximum limits for both wattage types. For our Zener Tester, the current typically follows the 200mW curve. The constant 5mA current zener test is also shown on the graph. This reveals that in this condition, 400mW zener diodes below 8V operate at less than 10% of maximum power (ie, 40mW) while the maximum power rating is exceeded above 80V. For 1W zener diodes, the test power is below the minimum 100mW for any voltage rating below 20V. So the constant current method does not work well in practice. means that at high zener voltages, the output current is low and at low voltages, it is higher. A standard digital or analog multimeter can be used to read the zener voltage. Block diagram The full circuit for the Zener Tester is shown in Fig.3. IC2 is a 7555 timer configured as an astable oscillator to drive Mosfet Q1 with a square wave. This in turn drives step-up transformer T1. The output of the transformer is rectified by fast-recovery diode D3 and the resulting DC voltage is applied to the zener diode under test. Error amplifier IC1b monitors the current through the 1Ω source resistor for Mosfet Q1. IC1b has a gain of 470 and it amplifies the difference between the feedback voltage at its pin 6 and the reference voltage at pin 5 to generate an error voltage. IC1b then drives pin 5 of the 7555 (its control voltage terminal) to modulate the output pulse width. The operating frequency of IC2 hovers around 67kHz. If the current through Q1 is too high, IC1b pulls pin 5 of IC2 slightly lower, so that the width of the gate pulse fed to Q1 is reduced. This pulls the current back to the required level. Conversely, if the current is too low, IC1b pulls pin 5 of IC2 higher. This increases the duty cycle of the drive to Q1’s gate and thus increases the current. The reference voltage at the noninverting input of IC1b (pin 5) is derived from a red LED via IC1a. Note that LK1 is installed if the power pushbutton switch used has no LED, in which case the reference voltage is provided by LED1 instead. IC1a monitors the battery voltage via a voltage divider comprising 100kΩ and 1.2kΩ resistors, connected to its pin 2. The 100kΩ feedback resistors The Zener Tester is based on a 9V to 125V DC-DC step-up converter. The block diagram is shown in Fig.2. It has four sections: a voltage reference, error amplifier, pulse controller and the converter itself. Error amplifier IC1b monitors the current supplied to the converter and adjusts pulse controller IC2 to maintain a constant current to the converter from the 9V battery. The reference circuit also compensates for falling battery voltage as it discharges, so the power delivered to the converter and thus to the zener diode under test is also constant. With the power being constant, this Features & Specifications Main Features • • • • • Tests 400mW and 1W zener diodes Test range from 0.6V to 100V Constant power testing (about 200mW) Reading displayed using a digital multimeter Battery powered (9V) Specifications Diode test power: typically 200mW from 3.3V up to 30V, tapering to 150mW at 75V and 2.2V; 70mW at 100V. Test power variation with supply voltage (6-9V): 0% (8.2V zener); 21% (3.3V zener); 12% (75V zener) Battery current drain: from 51mA (9V) up to 84mA (6V) Open circuit test voltage: ~125V Short circuit output current: 100mA 84  Silicon Chip How it works siliconchip.com.au 1k 2 K A K A 7 1 IC2 7555 D1–D3 1.5nF 6.8k 6 3 5 10nF 470k IC1: LM358 4 IC1a 2011 SC  K * ALTERNATIVE TO SWITCH LED K A 9V BATTERY  LED1* 1.2k 4.7k LK1 100k  A SWITCH LED ZENER DIODE TESTER 3 2 100k 1k S1 K A D1 1N5819 ZD1–ZD3 A K ZD1 10V 10 D2 UF4003 4 8 7 IC1b 6 8 5 1k 1 100k 100nF 100k Voltage limiting Fig.3: the complete circuit diagram of the Zener Tester. IC1b is the error amplifier and this controls the duty cycle of oscillator IC2. IC2 in turn drives Q1, and this switches the primary winding of step-up transformer T1. The secondary output of T1 is then rectified by D3 and applied to the zener diode. S D K A LED1 S 1 Q1 STP16NE06 D G 10 K A A ZD2 27V K 100 F IC2 is configured in a slightly unusual way; the 1.5nF timing capacitor is charged and discharged directly from the output. Normally this results in a fixed 50% duty cycle. However, because IC1b overrides the control voltage, the oscillator ramp voltage is not necessarily symmetrical any more. For example, if IC1b pulls the control voltage below the normal 2/3VCC, the 1.5nF capacitor charges faster than it discharges, because the voltage across the 6.8kΩ resistor is higher than usual when the output is high and lower than usual when it is low. As a result, the output duty cycle is lower. The reverse is also true and hence IC1b controls the duty cycle at Q1’s gate. siliconchip.com.au D G 10 K A 17T ZD3 27V 100nF 100 F Timer configuration Zener diodes ZD2 & ZD3 limit the voltage spike which occurs at the drain of Mosfet Q1 each time it switches off. What happens is that as the drain voltage rises above about 54V, zener diodes ZD2 and ZD3 begin to conduct and pull the gate of Q1 above 0V. This switches on Q1 to suppress any excess voltage and so the drain voltage is limited to a value which is the sum of the voltages across ZD2, ZD3, diode D2 and the gate on-threshold voltage. STP16NE06 – – + METER ZENER UNDER TEST + 10M 10nF 250V 40T K D3 UF4003 A T1 connected to pins 1 & 2 give IC1a a gain of -1 for this signal path. Similarly, the 1.8V across LED1 is divided using 100kΩ and 4.7kΩ resistors to give about 80mV at pin 3 of IC1a. IC1a then amplifies the difference by a factor of 2 (1 + 100kΩ / 100kΩ) to give 160mV. To understand how this all works in practice, let’s assume that the battery supply is 9V. In this case, the voltage across the 1.2kΩ resistor will be 106.7mV and so the output (pin 1) of IC1a will be at 160mV - 106.7mV = 53mV. However, if the power supply falls to 7.5V (for example), then the voltage across the 1.2kΩ resistor will be 89mV. The pin 1 output of IC1a will now be at 160mV - 89mV = 71mV. Thus, as the supply voltage goes down, the reference voltage applied to pin 5 of IC1b goes up. This ensures that greater current is supplied with lower voltages, to maintain constant power. As the accompanying specifications panel shows, the scheme works well, with the power remaining constant for a supply of between 9V and 6V for an 8.2V zener diode. November 2011  85 SWITCH LED RETSET ED OID RE NE Z LED1 100nF PRIMARY 17 TURNS 4003 CABLE TIE T1 IC2 7555 27V 27V ZD3 10 4003 D2 10 ZD2 ZD1 10V 6.8k 10 1.5nF 4.7k 100k 1.2k 100k 100nF 100 F ZENER DIODE TESTER Fig.4: install the parts on the PCB as shown here, taking care to ensure that all polarised parts (including toroidal transformer T1) are correctly orientated. The two ICs can be directly soldered to the PCB. (LID OF CASE) REAR OF TEST TERMINALS S1 + – T1 SECONDARY 40 TURNS D3 CABLE TIE Q1 1k 1k 100k 10nF 250V 1 K 10nF IC1 LM358 100k 11101140 5819 470k 9V BATTERY 10M A + D1 LK1 A 100 F + K 1k - - Fig.6: T1 is wound using 0.25mm enamelled copper wire with 17 turns for the primary and 40 turns for the secondary. The winding direction is important, so follow the way the windings are shown for both the primary and the secondary. indicated by the brightness of the LED. If LED1 is dim, then it’s time to change the battery. The fact that the circuit works below 6V means that battery life is good. An alternative battery check is to measure the output voltage when the Zener Tester is plugged into the multimeter, without anything connected across the terminals. If the output is above 100V then the battery condition is satisfactory. Construction CABLE TIES END OF CASE - + K A 5819 4003 RETSET ED OID RE NE Z + BANANA PLUGS 27V 27V 10V 11101140 4003 9V BATTERY Fig.5: the switch, binding posts and banana plugs are connected to PC stakes on the PCB via medium-duty hook-up wire. Use heatshrink tubing over the PC stake connections and at the ends of the binding posts to stop the connections from breaking due to vibration. Typically, this will be just over 60V and this in turn limits the maximum voltage that can be delivered by the transformer’s secondary winding (with no zener diode connected across the test terminals) to something less than 145V. In practice though, the limit appears 86  Silicon Chip to be about 115V (depending on the battery condition). Power supply Power for the circuit is derived from a 9V battery via reverse polarity protection diode D1 and pushbutton switch S1. The battery condition is Construction of the Zener Tester is straightforward, with most of the parts mounted on a PCB coded 04111111 and measuring 61 x 107mm. This is housed in a plastic utility box measuring 130 x 68 x 44mm. The PCB clips into slots moulded into the sides of the case. Corner mounting holes are provided on the PCB for other applications. Check the board for faults and repair it if necessary. Also check that the PCB mounting holes and the holes for the battery leads are the right size (3mm). Fig.4 shows the assembly details. Begin by fitting all resistors. The resistor colour code table can be used to read their values however it’s best to check them with a digital multimeter in Ohms mode. The diodes, including zener diodes ZD1-ZD3, can then be installed and must be mounted with the orientations shown. Note also that D1 is a 1N5819, while D2 and D3 are UF4003 or 1N4936 types. There are two different zener diode types (10V & 27V) so don’t get them mixed up. IC1 & IC2 go in next. 8-pin DIL sockets may be used but are not necessary; the ICs can be soldered the PCB. siliconchip.com.au In either case, take care to orientate them correctly, with the notch/dot positioned as shown. Solder the PC stakes next, then the capacitors. The electrolytic types must be orientated with the correct polarity, ie, longest lead through the hole marked “+”. LED1 is mounted flush against the PCB (it’s there to provide a reference voltage only). Make sure the anode (longer lead) is placed in the hole marked “A”. That done, the 2-way pin header can be installed, followed by Mosfet Q1 which is installed vertically. Be sure to orientate Q1 as shown. Winding the transformer T1 is wound as shown in Fig.6. It uses 0.25mm enamelled copper wire with 17 turns for the primary and 40 turns for the secondary. The winding direction is important so follow the way the windings are shown on Fig.6 for both the primary and the secondary. When winding is completed, the transformer can be installed on the PCB. Use a sharp knife or emery paper to strip the enamel insulation at each end of both wires, then tin them and solder them to the appropriate PCB pads. The transformer is held in place with cable ties that pass through holes in the PCB. Preparing the case Use the front panel artwork (Fig.7) as a guide to drill the holes in the lid for the switch and binding posts. Start with a small pilot drill, then enlarge the holes and ream them out to the correct size. The binding post holes must be 8mm in diameter but the switch hole will depend on the switch used. You also need to make holes in the This is the view inside the case after the wiring has been completed. The metal battery holder is secured to the side of the case using an M3 x 9mm tapped spacer and machine screws. end of the box for the banana plugs, with the standard 19mm spacing. These go 12mm down from the top edge of the box. Drill the holes out smaller than the screw thread of the banana plugs so that these can be screwed into the plastic box, forming a thread in the process. Finally, a 3mm hole is also required for the battery holder screw support in the opposite end of the box. This hole is positioned 13mm down from the top Resistor Colour Codes o o o o o o o o o o siliconchip.com.au No.   1   1   4   1   1   1   3   3   1 Value 10MΩ 470kΩ 100kΩ 6.8kΩ 4.7kΩ 1.2kΩ 1kΩ 10Ω 1Ω 4-Band Code (1%) brown black blue brown yellow violet yellow brown brown black yellow brown blue grey red brown yellow violet red brown brown red red brown brown black red brown brown black black brown brown black gold brown edge of the box, then countersunk on the outside. The next step is to prepare the front panel label. This can be downloaded from the SILICON CHIP website (in the November 2011 downloads section) or Table 2: Capacitor Codes Value 100nF 10nF 1.5nF µF Value IEC Code 0.1µF 100n 0.01µF    10n .0015µF    1n5 EIA Code   104   103   152 5-Band Code (1%) brown black black green brown yellow violet black orange brown brown black black orange brown blue grey black brown brown yellow violet black brown brown brown red black brown brown brown black black brown brown brown black black gold brown brown black black silver brown November 2011  87 onto the panel. Once the label is in place, use a hobby knife to cut out the holes. If it isn’t self-adhesive, affix it to the panel using an even smear of neutral cure silicone sealant or spray contact adhesive. For plastic film, if you are affixing to a black coloured panel, use coloured silicone such as grey or white so that the label can be seen against the black. Wiring These waveforms illustrate the operation of the step-up converter. The yellow trace is the waveform fed to the gate of Mosfet Q1. Each time the gate signal goes positive, the Mosfet turns on and its drain is pulled low, as shown by the green trace. As the gate pulse goes low again, the Mosfet turns off and the drain voltage swings high and rings at a high frequency, producing a peak voltage of around 60V. This is stepped up in the transformer and rectified by diode D3 to charge the 10nF capacitor. When the diode stops conducting, the ringing at the drain continues at a lower frequency until the Mosfet is switched back on by the next positive gate pulse. photocopied from this article. You can either print it onto paper and laminate it, or print it onto sticky-backed photo paper or plastic film. When using clear plastic film (ie, overhead projector film) you can print the label as a mirror image so that the ink is behind the film when placed Begin the wiring by removing the banana plugs, then solder short lengths of hook-up wire to the rear of each one (if you solder it with them in the box, the box will melt). That done, screw them back in, allowing the wires to rotate freely as you do so, so they don’t get twisted. Fig.5 shows how the wiring is done. The 9V battery leads are looped through the holes in the PCB and then soldered to the PC stakes with heatshrink tubing over the soldered joint. It’s important to loop the wire through the holes provided in the PCB, to improve retention and to prevent the wires from breaking off the PC stakes when the battery is changed. The wiring shown assumes switch S1 has an integral LED. If not, simply omit the two additional wires. Use regular hook-up wire for the connections and as with the battery, heatshrink the joints to the PC stakes as well as where the wires join to the binding posts. Parts List: Zener Diode Tester 1 PCB, code 04111111, 61 x 107mm 1 plastic utility box, 130 x 68 x 44mm 1 9V alkaline battery 2 banana line plugs 1 red binding post 1 black binding post 1 9V battery clip connector 1 9V battery holder (Jaycar PH-9237, Altronics S5050) 1 momentary push-on switch with red LED indicator (Jaycar SP-0706, Altronics S1086) (S1) OR 1 momentary pushbutton switch 1 ferrite toroid, 18 x 10 x 6mm (Jaycar LO-1230 or equivalent) 1 1.3m length of 0.25mm enamelled copper wire 88  Silicon Chip 1 M3 x 6mm panhead screw 1 M3 x 6mm countersunk screw 1 9mm M3 tapped spacer 1 2-pin header (2.54mm pitch) 1 shorting plug for header (LK1) 10 PC stakes 4 100mm cable ties 1 100mm length of 3mm-diameter heatshrink tubing 1 30mm length of 5mm-diameter heatshrink tubing 200mm of red hook-up wire 200mm of black hook-up wire 120mm of white hookup wire Semiconductors 1 LM358 dual op amp (IC1) 1 7555 CMOS timer (IC2) 1 STP16NE06 60V Mosfet (Q1) 1 1N5819 Schottky diode (D1) 2 1N4936 or UF4003 fast recovery diodes (D2, D3) 1 10V zener diode (ZD1) 2 27V 1W zener diodes (1N4750; ZD2, ZD3) 1 3mm red LED (LED1) Capacitors 2 100µF 16V PC electrolytic 2 100nF MKT 1 10nF 275VAC X2 class MKP 1 10nF MKT 1 1.5nF MKT Resistors (0.25W, 1%) 1 10MΩ 1 1.2kΩ 1 470kΩ 3 1kΩ 4 100kΩ 3 10Ω 1 6.8kΩ 1 1Ω 5% 1 4.7kΩ siliconchip.com.au Zener Diode Power Curves Zener Diode Tester 100 95 90 ++ 85 80 Press To Test A + K 75 70 + 65 SILICON CHIP Fig.7: this artwork can be copied and used as a drilling template for the front panel. It’s also available in PDF format from our website, to make a front panel label. Once the wiring is complete, secure it using cable ties as shown. With the board in place and wired up, install the battery holder. Use a machine screw to connect the 9V battery clip to the M3 tapped spacer, then attach the other end of the spacer to the box using an M3 countersunk screw. Testing If you are not using a power switch with integral LED, install a shorting block on LK1. Otherwise, leave it out. Press S1 and check that the LED lights. If not, check the LED and switch wiring. The LED may be wired or installed backwards. Now plug the unit into a multimeter and set it to read DC volts. Press power button S1 and check that the output produces 115-125VDC. If not, check that T1 is wound correctly, as shown in Fig.6. You can swap the two primary connections if necessary; there is no need to rewind it if it is wrong. If it still doesn’t work, check other voltages on the circuit. The supply for IC1 (between pins 8 & 4) and IC2 (between pins 8 & 1) should be about 0.3V less than the battery voltage. Check for around 80mV at pin 3 of IC1a. siliconchip.com.au Zener Current (mA) 60 55 50 45 40 35 1W 30 25 20 400mW 15 5mA Constant 10 Current Test 5 200mW 100mW 40mW 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Zener Voltage (V) Fig.8: the voltage versus current curves for both 1W and 400mW zener diodes for voltages from 2.5 to 100V. The lower two traces show the 40mW (10% of 400mW) and the 100mW (10% of 1W) power curves. Our Zener Tester typically follows the 200mW power curve. To check operation of the Zener Diode Tester under load, connect a 1kΩ resistor across the test terminals. The multimeter should indicate a reading of about 14V. This means that close to 200mW (14V2 ÷ 1kΩ) is being delivered to the resistor. Further testing can be done using zener diodes with known breakdown voltages. Note that zener diodes can have a tolerance of 10%, 5%, 2% or 1% and that the measured voltage can also SC depend on the zener current. November 2011  89