November 4, 1998
Updated March 3, 2005
Fair Radio Sales used to carry a surplus 24VDC 10.2A power supply manufactured by Elpac Power Systems. Although this power supply has not been listed in their catalog since 1996, there are probably quite a few of these supplies out there. I ordered one of these and modified it to produce 28VDC, and I use it on my testbench for powering military radio gear which needs 28V. This supply is a simple, brute-force linear supply, which makes it work well for this application. This modification will probably seem pretty obvious to anybody who has worked with linear power supplies before, but I hope that this article may be helpful to somebody who's just getting started in electronics. Furthermore, it took quite a bit of work to reverse-engineer the schematic before I modified my supply, so I felt that I might as well share my results.
Warning! You might burn your power supply into a useless pile of
ashes by trying this. You've been warned... Have fun!
Here is a schematic that I reverse-engineered from my power supply, in both Adobe PDF and Postscript formats:
| elpac24.pdf | (61k) |
| elpac24.ps | (46k) |
The core of the power supply is an off-the-shelf voltage regulator IC made by Fairchild. It drives transistor Q1, which drives prediver transistor Q4, which, in turn, drives power transistor Q3 (a parallel combination of seven power transistors). Capicitor C7 filters the regulated DC power.
Unregulated DC voltage for the power circuit is generated by rectifying and filtering the lower secondary winding of T1 with diodes CR3 and CR4, and capacitors C2 and C6. The upper secondary winding generates a DC voltage higher than the unregulated DC power which feeds the power section, and this higher DC bus provides power for the predriver transistors.
The power supply also includes a crowbar overvoltage protection circuit (components R10-R13, VR2, Q2, SCR1 and C9). This circuit is designed to protect the load if the regulator fails, by shorting the power supply output and blowing the input fuses. Unfortunately, this often blows out many of the power supply components, too.
Examination of the schematic reveals that the power supply has three adjustments: a voltage adjustment, a current limiter adjustment, and an overvoltage protection adjustment. Converting the power supply to produce 28VDC is a bit more complicated than just turning up the voltage adjustment for a couple of reasons. First, the overvoltage protector may trip, so it must first be set to trip at an appropriate voltage (such as 32VDC). Second, output filter capacitor C7 is only rated for 25V, and it may blow at 28VDC. It must be replaced with a capacitor with an equal or greater capacitance and a high enough voltage rating. It may be difficult to find a capacitor which meets these requirements and fits in the same location as the original C7, because C7 is unusually small for a capacitor with its ratings. If you cannot find a small enough capacitor, you may need to rearrange the power supply's components.
After replacing C7, you will need to readjust the power supply. First, you need to adjust the overvoltage protector. I recommend the following procedure: First, disconnect the Elpac supply from the AC line, disconnect C7, and turn R10 fully counterclockwise. Next, drive a voltage into the Elpac supply's output with another current-limited power supply which is set to at least 32VDC and a maximum current of 100mA or so. Next, turn R10 counterclockwise until SCR1 just barely triggers, thus causing the other power supply to begin current-limiting. Temporarily disconnect the other power supply to allow SCR1 to reset, turn down the other power supply's output voltage, reconnect it, and verify that the overvoltage protector trips right when it gets to 32VDC (or whatever trip voltage you chose). Readjust R10 if necessary. Finally, disconnect the other power supply and reconnect C7 (C7 was disconnected during this procedure to avoid dumping a lot of current through SCR1 when it trips).
Now that you have adjusted the overvoltage protector, you can set the output to 28VDC with R8. Make sure you don't turn it high enough to trip the overvoltage protector or burn up C7! I recommend derating the output current to 8.75A, and adjusting R3 accordingly.
Good luck!