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By PETER SMITH This simple project is ideal for testing DC power sup­plies, shunt regulators & constant current sources. It’s also a great way to check battery capacity and can even be used as a current limiter for an existing DC supply. I F YOU’RE INVOLVED with servicing or building power sup­plies, you’ll wonder how you ever managed without this ultra-useful test­bench tool! This electronic load enables you to ob­serve DC power circuits under a variety of load conditions, all of which can be quickly “dialled-in” using a single potentiome­ter. 58  Silicon Chip An electronic load is good for testing batteries too. But why use an electronic load instead of a resistive load? Let’s find out. Resistance is futile Electronic loads are often called “dummy” loads. This name refers to the fact that they replace or simulate a real load. For example, a dummy load might be used at the output terminals of a DC power supply to allow measurement of ripple voltage at differ­ent current levels. The dummy load enables us to conveniently program any load resistance (and thus current flow) that we desire. Of course, a dummy load need not be electronic – it could consist of a rheostat or even a bunch of high-power resistors in series and/or parallel. However, these methods tend to be rather inflexible and lack adjustment range and resolution. Rather than providing a variable load resistance, the elec­tronic version presented here provides variable curwww.siliconchip.com.au CON4 + CURRENT MUST NOT EXCEED 50W CURRENT *10A *PRODUCT OF THE VOLTAGE AND 50W *50V VOLTAGE MAXIMUM INPUT RATINGS 1k R12 R14 .01 3W 1k 10k C5 1.5nF R10 6 IC1b 1k 7 2 3 S D SC 2002 G E B C _ CON2 + SIMPLE 50W DC ELECTRONIC LOAD CURRENT SET S2 RANGE SELECT 10A STW34NB20 BC327 _ + CON1 1A 2 CON3 VR1 2k 10A ADJ. _ R3 10k 1 R2 47k +V R1 180 1W REF1 ICL8069 +1.2V + ZD1 10V 1W S1 POWER Y VR3 50k 10T X VR2 100k 1A ADJ. C1 47F 16V R4 510k C2 100nF 3 Z R5 1k 4 C3 1nF Q2 BC327 C R9 IC1: LMC6062 4 IC1a 8 1 C4 1nF 10k R13 5 D3 R8 22 G R11 S bSTW34NB20 C7 100nF 100V C6 47F 100V NP bQ1 D www.siliconchip.com.au 9 - 12V DC INPUTS Fig.2 (right): the final circuit for the Electronic Load. IC1b amplifies the voltage across feedback resistor R14 by a factor of 10 and this allows R14 to be substantially reduced in value (which, in turn, reduces its power dissipation). Q2 and diodes D1-D3 clamp the output of IC1a when the load voltage is very low, to protect Q1. +V B E +V How it works The Simple 50W Electronic Load is based around an adjustable precision current sink. Fig.1 shows the elements of a basic current sink. It consists of op amp IC1, power MOSFET Q1 and resistor R1 and operates as follows: Initially, both the inverting and non-inverting inputs of IC1 are at 0V, so the output is also at 0V. When a voltage (VIN) is applied to the non-in- R7 1k R6 100k Current limiting Earlier on, we stated that the Electronic Load could be used to provide current limiting for an existing power supply. How do we do that? Simple – just connect the load terminals in series with the negative supply lead. It’s then just a matter of winding up the pot to set the required current limit. 2x 1N4148 D2 D4 1N4148 D1 1N4148 rent sinking. This means that regardless of the applied voltage, the current that it “swallows” remains exactly as set. The required load current is simply “dialled in” via a multi-turn potentiometer, up to a maximum of 10A. Note that, to handle both low and high-power circuits, we’ve included 1A and 10A switch-selectable current ranges. POWER _ FAST BLOW F1 12A LOAD TERMINALS Fig.1: the basic scheme for a current sink. The current through R1 depends on the voltage applied to IC1’s non-inverting input and is independent of the supply voltage. September 2002  59 Table 2: Capacitor Codes Value Alt. Value IEC Code EIA Code 100nF  0.1uF 100n 104 1.5nF .0015uF  1n5 152  1nF .001uF   1n 102 reduce its power dissipation to sensible levels. The basic current sink described above has one major draw­back when used in high-current applications, however. Consider the case where a certain current is “dialled-in” but little or no voltage is present across the load terminals. In this case, insufficient current flows in the circuit to generate enough voltage across R14 to satisfy the feedback loop. This means that IC1a’s output will be at the supply rail voltage, turning Q2 fully on. If a low-impedance source is now connected to the load terminals, a massive instantaneous current will flow, limited only by the drain to source “on” resistance of Q4 and the .01Ω feedback resistor. Result – exit one MOSFET! To prevent this, we’ve included a clamping arrangement for the op amp, formed by diodes D1-D4, transistor Q1 and resistor R6. This circuit works as follows: when the load voltage is below a certain threshold, Q2 turns on (via D4) and so pin 1 of IC1a is effectively clamped to four diode drops above pin 2 – ie, approximately 2.9V. As a result, IC1a’s output is effectively below the MOSFET’s gate threshold voltage and so the current flow through this device is kept to a very low level. Conversely, when the load voltage rises above the threshold, Q2 turns off and plays no further role in the circuit – ie, feedback control is now via IC1b. Fig.3: install the parts on the PC board as shown in this wiring diagram. The .01Ω resistor looks like a thin metal U-shaped band and is mounted by pushing it down until its shoulders contact the board (see photo). Fig.4: if you can’t get a multi-turn pot (or just want to save money), here’s how to wire up two low-cost pots as coarse and fine controls instead. verting input, the op amp’s output begins to rise rapidly towards the positive supply rail. When this voltage exceeds the MOSFETs gate threshold voltage, it begins to conduct, causing current (IO) to flow. Obviously, the current flow through resistor R1 causes a voltage drop across it: V = IO x R1 This, in turn, is fed back to the inverting input of IC1. The op amp’s output voltage will continue to rise until this feedback voltage equals the voltage on the non-inverting input (VIN). Therefore, we can say that: IO = VIN/R1 As you can see, the current flow (IO) in the circuit is independent of the applied voltage (V+). Instead, it de­pends on the voltage applied to the op amp’s non-inverting input (VIN). Our final design (see Fig.2) expands on the above by adding an additional op amp stage (IC1b) in the feedback loop. This stage amplifies the voltage across the feedback resistor (R14) by a factor of 10, as set by resistors R10 and R12. And that, in turn, allows us to reduce the value of R14 and thus Table 1: Resistor Colour Codes  No.   1   1   1   1   3   3   1 60  Silicon Chip Value 510kΩ 180Ω 5% 100kΩ 47kΩ 10kΩ 1kΩ 22Ω 4-Band Code (1%) green brown yellow brown brown grey brown gold brown black yellow brown yellow violet orange brown brown black orange brown brown black red brown red red black brown 5-Band Code (1%) green brown black orange brown not applicable brown black black orange brown yellow violet black red brown brown black black red brown brown black black brown brown red red black gold brown www.siliconchip.com.au ELAN Audio The Leading Australian Manufacturer of Professional Broadcast Audio Equipment Featured Product of the Month PC-BAL PCI Format Balancing Board Interface PC Sound Cards to Professional Systems Not only do we make the best range of Specialised Broadcast "On-Air" Mixers in Australia. . . We also make a range of General Audio Products for use by Radio Broadcasters, Recording Studios, Institutions etc. This is the completed PC board assembly, ready for attachment to the heatsink. Note that the standoffs fitted to the rear of the board should be removed once the heatsink is attached. This arrangement provides a much smoother current ramp, with less overshoot when cycling the input. The load current is controlled by external potentiometer VR3, which varies the voltage applied to the non-inverting input of IC1a. With range switch S2 in the 1A position, the maximum output from VR3 is 100mV. Alternatively, when S2 selects the 10A position, the maximum output is about 1V. To ensure that the set current remains stable with tempera­ ture and input voltage variations, a precision voltage reference IC (REF1) is used to provide a steady 1.2V to the divider net­works. Trimpots VR1 and VR2 allow for full-scale adjustment of each range, if required. Unlike many electronic load circuits, this unit sources its supply voltage independently of the load terminals. This ensures that the circuit continues to operate, even when the voltage at the load terminals drops to just a few volts. As the circuit draws only about 390µA, it can be powered from a 9V PP3 battery. Alternatively, a 2.5mm DC socket is provided for 9-12V DC plugpack operation. Construction All parts except the potentiometer (VR3) and range switch (S2) mount on a 58 x 93.5mm single-sided PC www.siliconchip.com.au And we sell AKG and Denon Professional Audio Products For Technical Details and Professional Pricing Contact board. Fig.3 shows how the parts are installed. Begin by installing the two wire links and follow with all the 0.25W resistors. Diodes D1-D4 and zener diode ZD1 can go in next but watch their orientation – the cathode (banded) ends must be aligned as shown. Once the diodes are in, install all remaining components in order of their height. Transistor Q2 and power resistor R14 should be left until last. Before soldering R14, make sure that its shoulders are seated firmly against the PC board surface. The mounting position for Q2 will depend on the chosen heatsink. On our prototype, we inserted it into the PC board just far enough for proper soldering. With 10mm spacers fitted to the board, this placed Q2 near the centre line of the heatsink for best heat dissipation. Elan Audio 2 Steel Crt South Guildford WA 6055 Phone 08 9277 3500 08 9478 2266 Fax email sales<at>elan.com.au WWW elan.com.au Subscribe & Subscribe & Get this FREE!* Get this FREE!* *Australia only. Offer valid only while last. *Australia only. Offer validstocks only while stocks last. External hardware The current set potentiometer (VR3) and range switch (S2) are connected to the PC board via terminal block CON3. You can use light-duty hook-up wire for this job. The circuit diagram (Fig.2) shows the pinouts for CON3. If you don’t need the fine resolution of the 1A range, then you can save wiring (and money) and connect VR3 directly to the 10A circuit, eliminating the need for the range switch. Note Buy a 1- or 2-year subscription to S ILICON we’llsubscription mail you a free Buy a 1-CHIP or and 2-year to copy of “Computer Omnibus”. you SILICON CHIP and we’ll mail youOr a free can “Electronics Testbench”. copychoose of “Computer Omnibus”. Or you can choose “Electronics Testbench”. Subscribe now by using the handy order form in this or the callorder (02)form 9979 Subscribe nowissue by using in this issue8.30-5.30 or call (02) 9979 5644,with 8.30-5.30 5644, Mon-Fri your Mon-Fri with details. your credit card details. credit card September 2002  61 Parts List 1 PC board, code 04109021, 58 x 93.5mm 1 SPDT PC-mount sub-miniature slide switch (S1) (Altronics S-2060, Jaycar SS-0823) 1 SPDT miniature panel-mount toggle switch (S2) 1 heatsink to suit (0.6°C/W thermal resistance or lower) 4 2 way 5mm pitch terminal blocks (CON2 - CON4) 3 M3 x 6mm cheese head screws 2 M3 x 10mm tapped spacers 1 M3 flat washer 2 PC-mount 3AG fuse clips 1 12A 3AG fast-blow fuse 1 BC327 PNP transistor (Q2) 1 ICL8069 1.23V voltage reference (REF1) (Farnell 410-895) 4 1N4148 diodes (D1-D4) 1 1N4740A 10V, 1W zener diode (ZD1) Semiconductors 1 LMC6062IN dual CMOS op amp (IC1) (Farnell 270-854) 1 STW34NB20 N-channel MOS­FET (Q1) (Farnell 498-180) Resistors (0.25W, 1%) 1 510kΩ 3 10kΩ 1 180Ω 1W 5% 3 1kΩ 1 100kΩ 1 22Ω 1 47kΩ that the wire length should be kept as short as possible to reduce potential noise pick-up. It may help to tightly twist the wires to VR3 or, even better, use a length of shielded cable. The cable shield should be connected to ground (CON3, pin 3) at one end and to terminal “Y” and the metal shell of the potentio­ meter at the other end. For most applications, a 10-turn wire-wound potentiometer is preferred for VR3. However, these can be expensive and difficult to obtain. An alternative arrangement using standard carbon track potentiometers is shown in Fig.4. Here we’ve shown Capacitors 1 47µF or 56µF 100V non-polarised axial-lead electrolytic (Altronics R-6415, Jaycar RY-6916) 1 47µF 16V PC electrolytic 2 100nF 100V MKT polyester 1 1.5nF 63V MKT polyester 2 1nF 63V MKT polyester how a 50kΩ dual-gang pot (VR3a & VR3b) and a 500Ω pot (VR4) can be wired together to give both coarse and fine adjustments. Keeping your cool Apart from aesthetic reasons, there is no real need to house your completed work. For long service life, it can simply be mounted on a thick aluminium baseplate. However, if you prefer to build it into a case, then allow for plenty of ventilation. If the heatsink fins are vertically arranged, then you should install small spacers under the heat­ sink to allow airflow up through the Fig.5: this is the full-size etching pattern for the PC board. 62  Silicon Chip 1 0.01Ω 3W 1% power resistor (Welwyn ‘OAR’ series) (Farnell 327-4718) Potentiometers 1 2kΩ miniature horizontal trimpot (VR1) 1 100kΩ miniature horizontal trimpot (VR2) 1 50kΩ multi-turn linear potentiometer (VR3) (Farnell 351-817) -or1 50kΩ dual-gang linear potentiometer (VR3) (coarse adjustment) -and1 500Ω linear potentiometer (VR4) (fine adjustment) Miscellaneous Heatsink compound, 50mm-length (approx.) tinned copper wire for links, light duty hook-up wire fins. Ventilation holes positioned directly above and below the fins will make the most of the “chimney” effect. Alternatively, if the fins are horizontally arranged, then you’ll almost certainly require forced air cooling of some kind. A single 3mm hole is required for attaching the power MOS­FET. Try to position this as close to the centre of the heatsink as possible and be sure to remove any sharp edges that result from drilling. You can deburr the hole using an oversize drill. Mounting the MOSFET 50W continuous power dissipation is quite a bit to ask from a single plas­tic power MOSFET, even in the larger TO-247 package. Therefore, we have to make sure that as much of the heat as possible flows out of the package and into the heatsink. In other words, proper mounting of the power MOSFET (Q1) is vitally im­portant! Unlike many other projects described in SILICON CHIP, the MOSFET should not be electrically isolated from the heatsink. To mount it, first apply a thin, even smear of heatsink compound to the entire rear face of Q1 as well as the area that it will contact on the heatsink. That done, attach Q1 to the heatsink using an M3 screw with a flat washer and tighten it up firmly. www.siliconchip.com.au Note: direct connection between the transistor and heatsink means lower thermal resistance but it does have a downside. Along with the centre pin, the metal contact area of the transistor is connected to the drain, so the heatsink is always at positive load terminal potential. That means that you have to make sure that the heatsink doesn’t short against anything when using the Electronic Load. Prototype performance We checked the full-power performance of our prototype with an infrared thermometer and an ambient temperature of 22°C. The heatsink temperature rose to 70°C, with Q1 running about 10°C hotter. Although the transistor temperature was within specifica­tion, we hadn’t expected the thermal resistance between it and the heatsink to be so high. A little investigation revealed that the heatsink surface was not completely flat, resulting in only partial contact with the transistor! Watch this point when buying a heatsink – make sure that the contact area is completely flat. Circuit protection The 12A fast-blow fuse included in the circuit provides only basic over-current protection. No over-voltage or overload protection has been included, which is why we’ve dubbed it the “Simple” Electronic Load. Having said that, the MOSFET we’ve selected for this circuit is a very robust device, so you’d have to exceed the ratings listed in Fig.2 by a fair margin in order to destroy it. If you’re interested in increasing the robustness even further, then one option might be to use a special “pro- NEW! HC-5 hi-res Vid eo Distribution Amplifier DVS5 Video & Audio Distribution Amplifier Five identical Video and Stereo outputs plus h/phone & monitor out. S-Video & Composite versions available. Professional quality. For broadcast, audiovisual and film industries. Wide bandwidth, high output and unconditional stability with hum-cancelling circuitry, front-panel video gain and cable eq adjustments. 240V AC, 120V AC or 24V DC. VGS2 Graphics Splitter High resolution 1in/2out VGA splitter. Comes with 1.5m HQ cable and 12V supply. Custom-length HQ VGA cables also available. Check our NEW website for latest prices and MONTHLY SPECIALS www.questronix.com.au Email: questav<at>questronix.com.au Video Processors, Colour Correctors, Stabilisers, TBC’s, Converters, etc. QUESTRONIX tected” MOSFET in place of the standard part specified for Q1. Manufac­ turer STMicrolelectronics produce a range of such devices, called OMNIFETs. Additional circuits built into these devices add ther­mal, short-circuit and over-voltage protection to normal MOSFET function. A suitable device from the range is the VNW100N04 (rated at 42V). This is available locally from Farnell Electronic Compon­ents. Note that the OMNIFET’s over-voltage protection is intended for transient protection only. This means that you should not apply a higher than specified maximum drain to source voltage across the load terminals. Check out the STMicrolelectronics web site at http://us.st.com for more details on these devices. Calibration Trimpots VR1 and VR2 provide full-scale trim for their respective ranges. To adjust them, insert a 10A or higher rated ammeter in series with the positive load terminal and connect a suitable power source. Set S2 to the 1A range and apply power. Wind VR3 All mail: PO Box 348, Woy Woy NSW 2256 Ph (02) 4343 1970 Fax (02) 4341 2795 Visitors by appointment only fully clockwise and adjust VR2 for a reading of 1.00A on your meter. Now toggle S2 to the 10A position and repeat the procedure, this time adjusting VR1. Be sure not to exceed the maximum power rating, which means that the input voltage must not be above 5V when the load is swallowing 10A. The position of VR4 should now correlate roughly with the desired percentage of full-scale current. For example, on the 10A range with VR4 at centre position, current draw should be about 5A. Of course, for real accuracy you’ll need to leave your ammet­er connected. Load modulation As presented, this project is intended as a DC current sink. However, the frequency response of the circuit is such that it should be possible to modulate the control voltage to IC1a by various external means should you have such a requirement. No promises though – we haven’t tried it! If you want to give it a go, we suggest a maximum modulation SC frequency of about 1kHz. K&W HEATSINK EXTRUSION. SEE OUR WEBSITE FOR THE COMPLETE OFF THE SHELF RANGE. www.siliconchip.com.au September 2002  63