Variable power supply from fixed

[dankoch]

I've got a couple of Standard Power Inc. power supplies that I'd like to setup as a variable bench power unit. One is a dual 12-15 VDC, 1.6-1.8A, as well as a 5VDC/5A out, and the other is a single 24-28VDC .5A unit. Without reengineering these, what can I do to make-up a variable power supply? I don't have any real specs in mind except to be able to dial it down to zero. I might consider adding a DC-DC converter to get a higher range as well, but I don't want to reinvent the wheel, or spend $+ when I could buy a new unit for $.

[Chris Smith]

In some cases you can add a variac to a power supply, to lower the out put. <p>Not all PS will vary with this method, and then with the fixed LM 78xx series VRs, you can vary the ground pin with a pot to vary the out put Voltage.

[rshayes]

These are probably "open frame" power supplies that have regulated outputs at their specified voltages. Changing the input voltage with a variac will not change the output voltage until the input voltage becomes low enough that the regulators will not work properly. The outputs will not only be unregulated, but they will also have 120 hertz ripple under these conditions.<p>The simplest approach would be to set the power supply up for plus and minus 15 volt output and then add variable regulators to the two outputs. The LM317 (positive) and LM337 (negative) are fairly cheap and widely available. The maximum output of these devices is about three volts less than the input voltage. This would be 12 volts with a 15 volt source. The minimum voltage is 1.25 volts. This isn't quite zero, but it is close enough for most purposes. The fixed voltages can also be brought to binding posts, since some circuits may require the full 15 volts. Very few circuits will need a supply that can be varied through the 12 to 15 volt range.<p>The 28 volt supply would give you a range from 1.25 to 25 volts.<p>The additional regulators will need substantial heat sinks. Their maximum current is over 1 amp, so the possible dissipation is about 14 watts for the 15 volt source. The 28 volt supply is limited to .5 amp, so it would also dissipate about 14 watts under the worst condition.<p>If you need the full 1.8 amps, an additional transistor can be used to carry additional current. This should share the same heat sink as the regulator so that the thermal cutout in the regulator will still be at least partially effective. The heat sink would need to handle about 25 watts in this case.<p>Data sheets and ap notes for the LM317 and LM337 are available on the National Semiconductor web site (www.national.com). These parts are also available from several second sources and they may also have data sheets available. The TO-220 package will probably be the easiest to get.