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Versatile power-supply load uses light bulbs

-February 07, 2002

Improvising loads for bench-testing and designing power supplies is often a frustrating and sometimes hazardous experience. When you push large power resistors to their limit, they tend to burn benches and melt solder connections. Many electronic loads are on the market but are usually expensive and of laboratory-type precision and often represent overkill for the average designer. Incandescent light bulbs make excellent loads, able to handle large amounts of power. Moreover, they come in small packages and require no heat sinks. Furthermore, because they light up, you obtain instant feedback, rather like an analog versus a digital meter. The drawback is that the resistance of an incandescent lamp changes dramatically with the power input. The power into the load must therefore be controllable over a broad range, if the bulb is to be a useful current sink. A simple approach to this control problem is to pulse-width-modulate a power MOSFET in series with the load. This design uses a four-rail, 100W supply with outputs of 5, 12, and ±15V. For these voltage and power levels, 50W, 12V bulbs provide a suitable load. This application required three bulbs connected in parallel (Figure 1). Many automotive-supply dealers offer 50W, 12V bulbs.

Because these bulbs screw into an ordinary 115V light socket, you can create a virtually unlimited combination of loads. The use of old-fashioned porcelain-type sockets wired in parallel allows you connect any number of bulbs in the load circuit. The circuit in Figure 1 addresses supplies of 1 to 24V output levels, of positive or negative polarity, with power levels as high as 150W. You can use the same basic approach to load higher voltage supplies by using 115V light bulbs and appropriately sizing the power MOSFET and other components. The circuit uses a standard PWM 3843 IC, IC1, in open-loop mode. Potentiometer VR1 controls the duty cycle over its full range. The frequency is not critical and is approximately 37 kHz with the values shown in Figure 1. A small, modular plug-in transformer provides power, but you can use any source of approximately 18V dc at 50 mA.

T1 provides isolated drive to the power MOSFET, Q1. The transformer allows you to load negative as well as positive sources. The various components in the gate circuit provide efficient drive to Q1 over a broad range of duty cycles. The L1 choke isolates the input from the switching pulses in Q1. You could use either an analog or a digital current readout. This design uses an LED readout salvaged from an old power supply. You must size the current resistor, R1, for the power dissipation and the requirements of the current meter. In this application, three metal-oxide, 0.1Ω, 2W resistors connected in series met the requirements (maximum current of 4A). You must fasten Q1 to a heat sink adequate for the application. The circuit in Figure 1 uses an Aavid (www.aavid.com) 530101B00100. This heat sink is a U-shaped radiator measuring approximately 1.75×175 in. on each side. Applications requiring higher currents could use two MOSFETs in parallel. The gate-drive scheme shown has enough power to drive two MOSFETs.

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