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Dummy Load for power supplies


Homebrewed power supplies need some testing. Of course, you can use power resistors for that purpose. But if you want to know exactly at which load your supply is not working any more, you'll need something more flexible.

To be prepared for larger power supplies you'll need a 10 or 20 W (or even larger) design. That power should be dissipated over a longer period of time, too, e.g. to test the heat properties of the power supply. For that we need some larger power equipment.

Schematics of a universal version

Schematic of the Dummy Load The minus plug of the dummy load (GND) shall be tight to the heat sink, so we can install the power transistor without heat transfer isolation to the heat sink. For that we need a PNP transistor, where the case is the collector.

The dummy load should work from 3 V on, so we can test smaller power supplies as well. But it should work at 35 V as well, so we do not need a specially designed dummy load for each of our power supply devices. Therefore we need a constant voltage that already works at 3V but also on 35V. This is performed by the FET BF256B and the three diodes: the BF controls the current flow through the diodes and limits it to 8.5 mA. The diodes deliver voltages by 0.65, 1.30 and 1.95V below the positive voltage.

With the 1k potentiometer the voltage can be adjusted to between 0.65 and 1.95V below plus. This is fed into the base of the PNP power transistor. In its emitter line to plus a resistor is producing a current dependant voltage. The base-emitter voltage of the transistor is around two silicon diode voltages, because the transistor is a darlington. Its hFE is around 10,000, so to regulate a current of 1A we need less than 1 mA on the base. This one mA does not much influence the current though the diodes, as held constant with the FET.

Theoretically the base-emitter voltage of 2 * 0.65 V = 1.30 V gets lost. But, in practice that is not the case: with small base currents VBE is much smaller than 1.30 V. With an RE of 1 Ω and at VB of 1.30V already a current of 0.35 A resultet. Therefore the potentiometer is attached already to the first of the three diodes.

The Darlington has to dissipate the whole power that the power supply delivers. If the splly has an elevated power, the transistor gets very, very hot. It therefore needs a heat sink. In practice the transistor's temperature has a large influence on the VBE. Therefore the current increases with time until the transistor has reached its steady-state-temperature. With large heat sinks this needs longer times. So prepare to hand-regulate the current very often.

The dimension of the heat sink has to consider Without a heat sink the transistor has a heat resistance of 1.17°C/W between the silicon core and the metal case. A finger heat sink of 46-by-46-by-12,7 mm has 7 K/W,, a larger aluminium profile has 3.5 °C/W, very large sinks have 2.5 °/W. With 7 °/W and a 12V/1A power supply we come to a temperature of T = 25 + 12 * 1 * 7 = 109 °C.

Dummy Load with a small heat sink This here is the mounting on a small heat sink, for small power supplies. The sink has about 25 °C/W and can dissipate roughly 4 W.

The two schematics in Libre-Office-Draw-Format are here for Download.

Schematic for a more long-term stable version

Schematic of the long-term stable Dummy Load Dummy Load with 741 If your power supply has at least 5V and you do not want to re-adjust every 5 seconds, you can build this version. Here, an opamp takes over the task to drive the transistor's base pin.

Because of the OpAmp this works only with 5V and higher. With currents from 100 mA on the device works long-term stable and only small increases of the current need to be corrected, if the device gets hotter.

The many 47k resistors translate the voltages down to areas that the OpAmp can work with (near to the positive supply voltage the 741 is dump).

In practice the regulator starts already with 73 mA and not with zero Volts. Again, this is due to the OpAmp: it's output pin does not work at high voltages. So either you can live with that or you place a power diode between the emitter and the resistor. But in that case keep the 47k on the resistor.

When experimenting with that dummy load care for the temperature of the heat sink. It might not be a good idea to place the sink onto a piece of plastic, if the heat sink exceeds 80°C: you'll ruine that piece.

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