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Design by JEFF MONEGAL Get some real grunt with this . . . 10A DCC booster for model railways Most DCC base stations have puny current capabilities which are exposed if you want to run more than a few locos and peripherals on your model railway layout. Problem is, DCC boosters are expensive, with a well-known 5A booster costing over $200. Problem no longer; build this 10A beauty at a fraction of the cost. I F YOU ARE a model railway enthusiast you probably already know about the current trends in model railways, with Digital Command Control or DCC being the standard control system of today. A beginner’s guide to the DCC standard was published in the February 2012 issue of SILICON CHIP. 64  Silicon Chip The advantage of DCC is that many model trains can be run on the same layout at the same time and all under individual control. In fact, many of the DCC systems available today can control or address up to 9999 trains and peripherals at the same time. Apart from being able to address so many locos and peripherals, DCC greatly simplifies the wiring to model railways. There is no need to have umpteen hundreds of wires going to points, lights, track blocks etc. Since the whole system can be regarded as a serial bus (much like Ethernet or USB), you need only connect a pair of wires siliconchip.com.au to every device and the individuallyaddressable DCC decoders take care of everything. Given that a DCC system can handle such a huge number of model locomotives and other equipment, you might wonder how much current a typical system needs to deliver. The current requirement for DCC locos varies wildly. If the loco is large, with a sound decoder and a smoke generator (in the case of a steam loco), then the current required may be 1A or more. On the other hand, a small shunting loco may require less than 200mA. All of which makes it difficult to calculate the current requirements of any layout. To give an extreme example, on a recent trip to a large model train layout the author noticed that the layout used a huge power supply. I asked the club techo and he said it was an 18V DC power supply capable of supplying 60A; that’s 1080 watts! The supply was fitted with voltage and current meters. At the time, the current meter was showing the total layout load to be 32A. I was a bit shocked at this but was informed that the DCC system was siliconchip.com.au running more than 25 locomotives, all fitted with sound decoders and some with smoke, right at that moment. At the same time, it was powering a lot of lighting with in excess of 80 lamps and signal LEDs. As well, all the point decoders were powered from the DCC system. Incidentally, he told me that the power supply often runs for hours at this level yet uses only two small computer fans for cooling. That’s what I call design efficiency! But even if you’re not running a large DCC layout you will quickly find that you run up against the limits of typical DCC command (base) stations. Some low-cost systems can only supply 1A while the higher priced systems can typically supply 3-4A. The only way to get more current capacity is to add a DCC booster. The problem with most boosters is the cost. A well-known brand of DCC booster supplying 5A costs around $200. Other boosters rated at only 3-4A cost well over the $100 mark. But let’s be serious, if you want a booster, you don’t want a flyweight; you want a BOOSTER! The booster presented here can supply up to 10A and you can build it for a fraction of the cost of commercial boosters. It has been tested on several brands of DCC system and it operated without any problems. It is fully compatible with NMRA (National Model Railway Association) standards for DCC systems and so should operate with all systems that conform to the NMRA standards. Incidentally, you can view these standards and many more on the NMRA web site: www. nmra.org As presented, our DCC booster is a PCB module measuring 127 x 77mm. It will need to be housed in a suitable case but it does not require any heatsinks or fan cooling. It needs to be teamed with a DC power supply capable of delivering 16-18V and 10A. The booster module has six LEDs to indicate its status and a piezo beeper which can sound a number of alarms if fault conditions occur on the layout. Circuit details The full circuit is shown in Fig.1 and it does feature a PIC microcontroller but in this case the micro is performing something of a cameo role which we will detail later. The heart of the circuit actually consists of four IRF2804 Mosfets (Q3-Q6) which operate in bridge configuration to feed the track on your DCC layout. Made by International Rectifier, these are specifically intended for automotive applications and are rated for supply rails up to 40V DC and 75A. They are particularly suitable for our booster design because they have a very low on-resistance; RDS(on) is only two milliohms (2mΩ)! That means that their power loss when conducting at 10A is only 200mW each. Other key devices in the circuit are the 6N138 optocoupler (IC2) and the two IR2110 high and low side Mosfet drivers (IC5 & IC6). The DCC signal from the base or command station can be either the full track voltage or the 5V signal typically available from an RJ12 6-pin or other modular connector. This connector will have pins for +5V, 0V and the DCC signal. Either source can be used but they must be completely isolated from the circuitry in the booster. This is where the 6N138 optocoupler (IC2) comes into the picture. As shown, DCC track signals (if used) are terminated to two pins on CON2, each labelled “Track DCC”. One “Track DCC” line is passed via a 1kΩ resistor to pin 2 of the 6N138. This is the anode of the internal LED. The cathode of the LED at pin 3 connects via the 3-way header socket to either the other “Track DCC” line or to the output (pin 1) of IC1a, one half of an LM358 dual op amp. Alternatively, if the 5V DCC signal is used, this is buffered by IC1a which is configured as a comparator. Note that it uses the 5V supply from the base station connector. LED6 is there to indicate if the DCC 5V supply is present. The output of the 6N138 optocoupler drives a 74HC14 hex Schmitt trigger inverter. All six inverters in the package are used, firstly to buffer the signal from the 6N138 (ie, by IC3a & IC3f) and then to generate complementary (out-of-phase) signals to drive the IR2110 high and low side drivers. Dead-time is essential Dead-time is essential to ensure that each pair of Mosfets (ie, Q3 & Q4 or Q5 & Q6) are not both turned on at any time. If that did happen, it would effectively short the 16V supply rail to ground and the result would range July 2012  65 0.1  5W +16-18V +16 -18V REG1 7805 0.1  5W POWER IN IN 560 10 F 470nF +5V OUT 10 F 10 F GND LOW ESR LOW ESR LOW ESR 1k GND 560 CON1 E 1 4.7k 2 560 820 47k A  FROM CONTROLLER 560 8 A A    K RA2 RB6 RB0 RB1 RA4 9 K K LED1 LED2 LED3 LED4 LED5 POWER DCC OK FAULT V+ OK OVER LOAD 12 4.7k B Q2 BC548 E 3 17 OSC1 RB4 RB3 RA1 OSC2 RB5 5 K 10 CURRENT CONTROL 18 11 10 F LOW ESR Vss  K 13 RA0 IC4 PIC16F628 16 RB2 15 A RB7 C 10k 1k TRACK DCC 270 +5V 47k 330 3 IC1a 4 A K +5V OK 47k 10 F LOW ESR A K D7 1N4148 IC1: LM358 8 2 1k  LED6 0V 7 RA5/MCLR RA3 6 560 560 470nF A DCC SIGNAL (+5V) Vdd 4 B C 47k 14 560 Q1 C8550 PIEZO BEEPER K 1 D8 1N4148 6 5 IC1b A DCC 7 A SOURCE SELECT +5V SIGNAL TRACK LK1 B TRACK DCC CON2 SC 2012 10 AMP DCC BOOSTER Fig.1: the DCC Booster circuit can be regarded as a high power buffer. It takes the 5V DCC or track DCC signals from a command station and feeds exactly the same pulse signal to the layout tracks with a much higher current capacity of up to 10A. And while it has a DC input of 16V (typical), it delivers a track signal of ±16V by virtue of its Mosfet bridge output stage. from increased dissipation through to power supply malfunction and possibly even destruction of the Mosfets themselves. Dead-time is achieved as follows. First, one signal path goes via diode D1 in parallel with a 560Ω resistor and bypassed by a 2.2nF capacitor before driving IC3e. The diode means that the positive edge goes through without 66  Silicon Chip delay but the negative edge is delayed by the RC filter. That means that the inverted pulse produced by IC3e has its positive edge delayed but its negative edge is not, resulting in a pulse which is shorter than the output from IC3a/f. IC3b and IC3c and a similar diode/ RC filter network are used to generate a complementary (ie, out-of-phase) pulse but in this case the resultant pulse is slightly longer. The net result is that these two pulses have “deadtime” whereby they are both at 0V each time their polarity is swapped. So far then, we have generated suitable complementary gate signals and now we need to look at how these turn on their respective Mosfets. Note that the supply rail to the Mosfet bridge circuit is between 16V & 18V but the siliconchip.com.au +16 -18V +5V 10 F 10 F LOW ESR 10 F A D5 BA159 10 9 3 Vdd Vcc Hin 10 F LOW ESR LOW ESR 6 Q3 Q5 D IRF2804 IRF2804 D 470nF 22 5 Vs 9 Vdd Hin Vb 10 Hout 100k (TO TRACK) A 7 3 Vcc S CON3 100k IC5 IR2110 470nF 22 G G S SD K 6 7 Hout 11 D6 BA159 K Vb LOW ESR A 5 B IC6 IR2110 Vs SD 11 CON4 D 12 Lout Lin Vss 22 1 S COM 13 G D Q4 IRF2804 8  7 3 6 S IC3d 8 470nF 13 5 1 14 IC3f IC3a 1nF IC3b 100k 470nF IC3c 5 6 2.2nF K A 560 K A K A 560 4 D1 1N4148 LEDS 13 2 A K A 7 CER 12 Vss COM D3 1N4148 D2 1N4148 3 Lin K 12 2 Lout D4 1N4148 1k IC3: 74HC14 2.2k 1 100k 9 IC2 6N138 22 G 100k 2 +5V 2 Q6 IRF2804 11 IC3e 10 2.2nF C8550 1N4148, BA159 A K B B C E typical DCC signal fed to the tracks on model railway layout has an amplitude of around 30V to as much 44V peak-topeak. To obtain such a large signal we need to drive the four Mosfet in bridge mode whereby the 16V is alternately connected in one direction and then the other. In practice, this done by turning on Q3 & Q6 and then turning on Q5 & Q4. siliconchip.com.au BC548 E G C In the first instance, Q3 connects one side of the track (A) to +16V and Q6 connects the other side (B) to 0V. Then Q5 & Q4 do the opposite, connecting “A” to 0V and “B” to 16V. This happens at the DCC frequency of about 4.5kHz and the resultant track voltage becomes 32V peak-to-peak. Note that there is negligible voltage loss across the Mosfets when they are 7805 IRF2804 D D S GND IN GND OUT switched on, since their RDS(on) is so low at 2mΩ. The high and low-side drivers, IC5 & IC6, handle the gate signals to the Mosfets. These ICs perform a number of functions. First, they take the 5V signals generated by IC3 and boost them to 16V, equal to the Vcc rail at pin 3 of each device. Turning on the lower Mosfets, Q4 & Q6, is pretty straightforJuly 2012  67 ward really; just feed in the requisite positive 15V pulse signals which are referred to the 0V line. But driving Q3 & Q5 is a problem because the gate pulse voltage must be 15V above the respective source electrodes, otherwise they would not turn on. The IR2110s manage this by using the switching action of the external Mosfets. For example, considering IC6, Q5 & Q6, when Q6 is turned on, the Vs line at pin 5 is pulled down to 0V and this causes the 470nF capacitor between pins 5 & 6 to be charged to Vcc via diode D6. Then, when Q6 is turned off and Q5 is turned on, pin 5 is jacked up to Vcc and it thereby pushes pin 6, the top of the 470nF capacitor, above Vcc by an amount equal to Vcc minus the voltage drop across D6. In other words, pin 6 of IC6 is now pulled to almost 2Vcc or about 32V, assuming at Vcc is 16V. So Vb is the internal gate supply for the high-side driver and IC6 connects Vb to pin 7 and thence the gate of Q5, each time Q5 is turned on. This a classic case of “boot-strap” operation. The final wrinkle in driving the Mosfets involves feeding the gate signals from IC3’s inverter stages to IC5 & IC6. For example, IC3e drives pin 10 of IC6 (and thereby Mosfet Q5) as well as pin 12 of IC5 (and thereby Mosfet Q4). Similarly, IC3c drives pin 10 of IC5 (and Mosfet Q3) as well as pin 12 of IC6 (and Mosfet Q6). This gives the alternate switching of the Mosfets referred to above. 68  Silicon Chip LED4 V+ OK OUTPUT 1 D6 BA159 D5 BA159 IRF2804 100k 100k IRF2804 Q4 LED5 O/LOAD Now we come to the microcontroller, IC4. It has a number of monitoring and control functions. The first of these involves IC3d and the diode pump involving D3 & D4. This generates a DC voltage while ever the DCC signal is present. The “DCC present” signal is fed to pin 11 of the micro. If it is not present, IC4 pulls the SD (shut-down) line to pin 11 on IC5 & IC6 high, thereby removing any DCC voltage from the tracks. Secondly, the micro monitors the incoming supply voltage from CON1 via a resistive divider. This divider is connected across the main 16V rail and its output fed to pin 18. The resistor values have been selected so that if the DC supply drops below 10.8V, the micro again shuts down IC5 & IC6. Thirdly, the micro monitors the current drain, using PNP transistor Q1 to sense the voltage across two parallel 0.1Ω 5W resistors. If the current drain rises above 10A, the collector of Q1 goes high, pulling pin 2 of IC4 high. Again, the micro responds by shutting down IC5 & IC6. However, the story is a little more involved at this point. Momentary shorts across the track do not cause the microcontroller to shut off the gate switching signals because the 470nF capacitors at the emitter of Q1 and pin 2 of IC4 provide a short delay. This means that momentary shorts which can occur in a DCC layout when a locomotive crosses the points in a reverse loop are ignored – a very good feature. OUTPUT 2 + IC5 IR2110 MWJ Fig.2: follow this parts layout diagram to build the DCC Booster. The LEDs can either be mounted on the PCB or on the front panel of the case that’s used to house the unit. 10 F Q3 22 560 LED3 FAULT + 92 K 8K298 + 22 IRF2804 22 560 560 Q6 1102 YAM LED2 DCC OK 100k 470nF 2.2nF 560 47k LED1 ON 10 F 10 F IC4 PIC16F628A 1nF DCC 4kmBOOSTER RETSOOBmk4 CCD BEEPER 4.7k 10 F 10 F 470nF + 100k 470nF Q2 4148 22 IC6 IR2110 4148 D4 10k 47k D1 4148 JWM 74HC14 560 + 1k 560 2.2k DCC SOURCE D2 1k BC548 + SIG IC2 6N138 4148 10 F 1k 820 4.7k D9 LED6 TRK 47k 4148 0V TRK DCC D8 DCC SIG IC1 LM358 +5V 10 F 470nF IC3 1k 47k 270 330 TRK DCC 560 KAL 2 x 0.1 /5W FFEJ IN PARALLEL 0V IRF2804 Q5 + 560 +16+18V 470nF + C8550 470nF 10 F 100k 4148 D3 Q1 REG1 7805 10 F 2.2nF C 560 YELTAO SCINORTCELE This view shows the completed prototype. Note how the two 0.1Ω 5W resistors are installed by mounting one on top of the other. Slightly longer duration shorts cause the micro to pull its pin 10 high and this shuts down IC5 & IC6. At the same time it flashes the Fault and Overload LEDs and causes the piezo beeper to sound three times. The micro then waits 4s and then pulls its pin 10 low, restoring DCC signals to the track. However, it does this in a clever way since DCC locomotives, especially those with in-built sound decoders present a difficult load at switch-on. This is because all decoders, and particularly sound decoders, have large electrolytic capacitors following the bridge rectifier which is connected across the DCC track supply. Typically, this capacitor is 1000µF or so but it can be 3300µF or more. So you can imagine that a large layout which might have 10 or more locomotives, with sound decoders, could easily have a total capacitance in excess of 35,000µF. When the DCC track signal of 30V peak-to-peak is applied to the track, the initial switch-on surge current can be very large, well in excess of the 10A rating of this booster circuit. So at switch-on and when restoring power after a short-circuit, the micro does not simply switch its pin 10 from high to low. Instead, it ramps it down with a varying PWM signal over a 1.5-second period, so that all those decoder power supply capacitors are charged at a manageable rate. Furthermore, if a short-circuit con­ dition is maintained, the microcontroll­ er will cycle continuously between siliconchip.com.au Table 2: Capacitor Codes Value 470nF 2.2nF 1nF shut-down and then “having a look” to see if the condition has been correct­ ed. The result is that, in the face of a permanent short-circuit, the Fault and Overload LEDs will flash, the beeper will sound three times and then it will repeat after four seconds. These loud beeps and the flashing LEDs will leave you in no doubt that a fault is present. Q2 drives the PCB-mounted beeper. As well as giving an audible warning when overloads occur, it gives a couple of quick beeps at switch on, as well – just because we can. As well as static tests to verify its current rating and ability to handle short-circuits, the booster has been tested with DCC systems from various manufacturers. These included Bachmann, Fleischmann, NCE, Lenz µF Value IEC Code EIA Code 0.47µF 470n 474 .0022µF 2n2 222 0.001µF   1n 102 to its maximum you will need a DC power supply capable of delivering 15-18V (preferably close to 16V) at 10A or more. The cheapest and most compact approach will be to use a switchmode open frame supply which can be mounted in the same case as the DCC Booster itself. If you don’t need to run the booster at maximum output and can manage with, say, 7A or 8A, a laptop PC supply delivering close to 16V will be ideal for the job. Note that if you do use a laptop power supply which inevitably will not be able to supply the full 10A (or more), you will need to change the point at which the DCC Booster’s overload circuit cuts in, otherwise any overload on the model layout will overload the power supply rather than the DCC Booster. So if your laptop supply is capable of supplying 7A, we suggest reducing the DCC Booster’s short-circuit current to about 5.4A by increasing the two parallel 0.1Ω 5W resistors to 0.22Ω 5W. Alternatively, you could build a large conventional power supply with a 160VA (minimum) 12VAC trans­ former, a 35A bridge rectifier and a minimum 20,000µF capacitor bank rated at 25V. That will work but will probably cost more and not be as efficient as a switchmode DC supply and you would need to be sure that its and MRC Advance. All these systems follow NMRA standards. When 10A is being supplied to the track (using a resistive load), the four Mosfets run very slightly warm; no heatsinks are required. However, the two paralleled 0.1Ω 5W wirewound resistors do become hot under these circumstances and if you envisage running the DCC Booster at close to is maximum rating for protracted periods, you might want to mount these two resistors off the PCB, as will be discussed in a moment. Of course, if you do envisage needing such high currents for your DCC layout, that is an argument for building two of these boosters. Power supply requirements If you want to run this DCC Booster Table 1: Resistor Colour Codes o o o o o o o o o o o o o siliconchip.com.au    No.     5     4     1     2     1     4     1     9     1     1     4     2 Value 100kΩ 47kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 820Ω 560Ω 330Ω 270Ω 22Ω 0.1Ω 5W 4-Band Code (5%) brown black yellow gold yellow violet orange gold brown black orange gold yellow violet red gold red red red gold brown black red gold grey red brown gold green blue brown gold orange orange brown gold red violet brown gold red red black gold not applicable 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown grey red black black brown green blue black black brown orange orange black black brown red violet black black brown red red black gold brown not applicable July 2012  69 Parts List 1 PCB, code K298, 128 x 80mm 1 3-pin PCB-mount terminal block 1 2-pin PCB-mount terminal block 3 2-pin high-current PCB-mount terminals (CON1,CON3, CON4) 2 8-pin IC sockets 3 14-pin IC sockets 1 18-pin IC socket 1 PCB-mount DC piezo beeper 1 3-pin header strip (LK1) 1 shorting link Semiconductors 1 LM358 dual op amp (IC1) 1 6N138 optocoupler (IC2) 1 74HC14 hex inverter (IC3; do not use 74C14) 1 PIC16F628A microcontroller programmed with program boost_mk4.asm (IC4) 2 IR2110 half-bridge Mosfet drivers (IC5, IC6) 1 C8550 PNP transistor (Q1) 1 BC548 NPN transistor (Q2) 4 IRF2804 Mosfets (Q3-Q6) 6 1N914, 1N4148 signal diodes (D1-D4, D7-D8) 2 BA159 Schottky diodes (D5, D6) 1 7805 regulator (REG1) 1 5mm yellow LED (LED1) 2 5mm green LEDs (LED2, LED4) 3 5mm red LEDs (LED3, LED5, LED6) Capacitors 9 10µF 50V low-ESR electrolytics 6 470nF MMC ceramic 2 2.2nF greencap or ceramic 1 1nF ceramic Resistors (0.25W, 5%) 5 100kΩ 1 820Ω 4 47kΩ 9 560Ω 1 10kΩ 1 330Ω 2 4.7kΩ 1 270Ω 1 2.2kΩ 4 22Ω 4 1kΩ 2 0.1Ω 5W wirewound DC output did not exceed 18V with light loads. Assembly All the parts go on a double-sided PCB (128 x 80mm) with platedthrough holes. The heavy currentcarrying tracks on the top and bottom of the PCB are paralleled to increase their current-carrying capacity. 70  Silicon Chip Fig.3: this scope grab shows the output waveform from the DCC Booster which had a DC input of 16V. Note that the long-term average value of DCC waveforms is 0V. This waveform can only be measured if you have a floating power supply (ie, not earthed) or an oscilloscope with differential inputs. Synchronising the scope display with a DCC waveform is very difficult; this waveform is taken with sweep stopped. Fig.2 shows the parts layout on the PCB. Assembly is a straightforward process and you can start with the small components such as the resistors and diodes. Make sure you check each resistor using a digital multimeter as you install it. The diodes must be installed with the correct polarity. It’s important that you install all components correctly the first time because removing and re-installing them on a PCB with plated-through holes is not easy. Having installed the resistors and diodes, you can continue with the other small components such as the capacitors and the two transistors. Again, make sure that you correctly install the electrolytic capacitors and transistors and make sure you don’t inadvertently swap transistors Q1 & Q2. The DC piezo beeper must also be installed with correct polarity. Mounting the 5W resistors You need to decide whether you want to mount the two paralleled 0.1Ω 5W resistors on the PCB or not, in view of the fact that they will get quite hot if you run the DCC Booster up to its maximum 10A rating. If you decide to mount them on the PCB, first piggy- back and solder them together before soldering the combination into the PCB. The piggy-backed resistor must be spaced off the PCB by about 4-5mm, to improve ventilation and prevent eventual discolouration (of the PCB). Alternatively, if you are going to run the DCC Booster at close to its maximum ratings, use an aluminiumclad 0.05Ω 10W chassis-mount resistor such as this one from Element14: http://au.element14.com/te-connectivity-cgs/ths10r05j/resistor-al-clad-10wr05-5/dp/1259281?Ntt=125-9281 Such resistors are not expensive and by mounting them on the metal chassis of the finished DCC Booster, you can be sure that they will always run reasonably cool. With the sensor resistor wired in, you can fit the PCB-mount screw terminal connectors. Two types have been specified: low current for CON2 and high current for CON1, CON3 & CON4. The low-current connectors are not critical but the high-current types should be rated at 16A. As you can see in the photos, they are substantially taller than those used for CON2. You can either mount the LEDs on the PCB or, as we think most constructors will, mount them on the front siliconchip.com.au Where To Buy A Kit A complete kit of parts is available from Oatley Electronics who own the copyright for this kit. Cost of the kit is $70 plus $10 for postage & packing. Fully constructed and tested units will be available on request. These units will come with a 6-month warranty. Cost will be $100. Contact the project designer via email for details. Oatley Electronics can be contacted by email at sales<at>oatleyelectronics.com Kits can also be ordered by phone on (02) 9584 3563 or by logging onto their web site: www.oatleyelectronics.com All technical enquires can be forwarded to the project designer at jeffmon<at> optusnet.com.au All enquires will be answered but please allow up to 48 hours for a response. panel of the DCC Booster’s chassis or case so that their indications can be clearly seen. The last components to be installed are the four IRF2804 Mosfets. By the way, don’t use substitutes for these devices unless you know that their RDS(on) values are at least as good as those specified here. Initial tests At this stage leave the microcontroller (IC4) out of its socket. First, connect a 16-18V DC supply to CON1. You don’t need a heavy current supply at this stage. Switch on the power and check that 5V DC is between pins 8 & 5 of IC2, pins 14 & 7 of IC3 and pins 14 & 5 of the socket for IC4 (the microcontroller). This checks the function of the 7805 5V regulator, REG1. If all is OK, switch off and insert the microcontroller into its socket. Make sure the jumper link at LK1 is set to position B, ie, to select track signals. Note that no DCC signal should be connected at this stage. Switch on power and check that all the LEDs come on for about 1s and that the piezo beeps twice. The Fault and DCC OK LEDs should then flash and the piezo should also sound twice every few seconds. If that happens, then so far so good. You can now connect a DCC signal source (or a square-wave oscillator set to 4kHz with an amplitude of about 12V) to the “Track DCC” pins on CON2. Now switch the power on again. All LEDs should flash and after a few seconds the “DCC OK” LED should be steady and the Fault LED should be off. Now slowly wind the supply voltage down to less than 11V. The Fault LED and the “V+ OK” LEDs should then flash alternately and the beeper should siliconchip.com.au give one beep every four seconds or so. At the same time, the micro will have shut down the Mosfet drivers, IC5 & IC6. You can check this by measuring the voltage at pin 10 of IC4; it should be close to +5V. If the DCC Booster has performed as stated so far then it is a safe bet that the it is working correctly. Switch off and set the jumper link to position B, ie, connecting a 5V DCC signal. This can be supplied from the 5V connector on your DCC command station or it can be a 5V 4kHz squarewave (DC-coupled) from a function generator. Connect it to the appropriate terminals on CON2 and you will also need to connect a separate 5V supply to power IC1. Now the DCC Booster should perform as before. Of course, if you are not going to use this facility, there is no need to test it. In fact, you could omit all the components associated with IC1, including diode D1 and LED6. Overload protection check The overload protection can be simulated using a small screwdriver to short the collector and emitter leads of Q1. The Fault LED and the Overload LED should start flashing together within half a second and the beeper should give a series of beeps every few seconds. Again, pin 10 of the micro should go to +5V. Finally, you can connect a high current supply set to around 16V DC and run a fair-dinkum short circuit test by using a clip lead to short the output pins on CON3 or CON4. This time, you will draw sparks, the Fault and Overload LEDs should start flashing and the beeper will sound as before. Then, when you remove the short-circuit, normal operation will be restored. That’s it – enjoy. SC Helping to put you in Control Control Equipment Ambient Light Sensor A 4-20mA loop powered ambient light sensor with human eye response, outputting 4 –20mA over 0 to1000 lux. The sensor is enclosed in a IP65 wall mount box with cable gland. KTA-274 $99.00+GST Pulse Stepper Designed to provide step and direction signals to two stepper motor drivers. Speed is controlled by a single potentiometer in the range 70 Hz to 4.8 kHz. KTA-276 $39.95+GST AC Current Transducer These current transformers have a 4-20mA output. Available in ranges of 0-30A or 0-50A they are ideal for measuring motor currents WES-005 $59.95+GST Temperature Indicator. The indicator comes with a waterproof NTC sensor on 3m cable. The cable can easily be extended to 50m. Indicators available for thermocouples and RTD. NOI-001 $69.00+GST Enclosure with Prototype Board An aluminium enclosure with prototype board and 18 screw terminals ENC-032 $29.00+GST Car Diagnostics Kit Interface with your car's OBD-II bus. Provides a serial interface using the ELM327 command set and supports major OBDII standards such as CAN and JBUS. SFK-003 $59.00+GST Pressure Transmitter A pressure transmitter with a –1 to 2Bar range and 4 to 20 mA output (2 wire). Features an accuracy of 0.5% Full Scale. The sensor mounts with a ¼" NPT thread.. AXS-149 $149.00+GST Contact Ocean Controls Ph: 03 9782 5882 www.oceancontrols.com.au July 2012  71