Repairing a diy power supply - Page 1
Forum section: electronics / beginners

author: MartinM87
author_rank: Newbie
timestamp: March 13, 2018, 07:22:07 pm
content: Hey if anyone can help with this one I would be very grateful.I made this 30v power supply from a schematic I found online,and it was working great untill recently,now it will only give max.voltage out no mather where the potentiometer is set.The pot. and the resistors are ok.but i don't know how to test the other components.Any help is welcome thanks.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 13, 2018, 07:26:49 pm
content: One or 2 of the transistors died, probably the 2N3055, start by replacing that one.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2018, 08:17:49 pm
content: The design is marginal. There simple isn't enough voltage headroom for 30V out, with a 24V transformer. You might get away with it at low currents, say under 100mA, because the transformer will output a higher voltage, than 24V and the voltage loss of the Darlington pair less, but forget 3A.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2018, 08:38:34 pm
content: Start by disconnecting the 2N3055 collector. The PSU should work normally like that with a small load - 100mA should be OK for a short time or if the BD135 is properly heatsinked. That will tell you whether or not the 2N3055 is blown. If not, disconnect the BD135 and connect LM723 pin 10 to pin 2, you can test the PSU with a very small load - say 10mA - to check the LM723 and the rest of the circuit. That will tell you if the BD135 is blown. If its still not working, check the LM723 by substitution.

author: MartinM87
author_rank: Newbie
timestamp: March 13, 2018, 09:18:53 pm
content: It does work when i disconect 2n3055 collector!I must have shorted the outputs accidently.Thanks so much for the help guys.

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2018, 09:43:14 pm
content: Quote from: MartinM87 on March 13, 2018, 09:18:53 pmShorting the output shouldn't cause Tr2 to fail. The current limiting circuit should kick-in and protect it. My guess is the heat-sinking on Tr2 is inadequate. Another possibility, is when short circuited, the safe operating area of the 2N3055 is being exceededand it's going into secondary breakdown. See the graph on the bottom of page 2 of the data sheet. The solution is to use two 2N3055s in parallel, with current sharing resistors, in series with the emitters.http://www.onsemi.com/PowerSolutions/document/2N3055-D.PDFEDIT:Actually it's not going into secondary breakdown, as the first decline in current rating vs voltage, is purely thermal, not secondary breakdown, but it's still being overstreached and needs another transistor to share the load.

author: Ian.M
author_rank: Super Contributor
timestamp: March 13, 2018, 10:06:59 pm
content: One of the current sharing emitter resistors can double up as the LM723 current sense resistor so multiple paralleled pass transistor does not force an increased dropout voltage.However you'll still run into problems with insufficient headroom (input voltage margin above dropout voltage) for your desired 30V full output, and jacking up the input isn't a simple option because of the LM723's 40V abs max V+ to V- (supply) voltage rating so its essential that the peak input voltage with no load and the mains supply at 10% above nominal doesn't exceed that.You can save 0.7V headrooom by using a PNP-equivalentSziklai pairin place of the NPN Darlington. Use the LM723 datasheet Figure 20. Positive Voltage Regulator (External PNP Pass Transistor) pass transistor and current sense circuit, keeping the 2N3055, but using a PNP (non-Darlington) driver transistor capable of 1A Ic at the max voltage drop. Hopefully you'll be able to get 25V at full load.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 13, 2018, 10:38:33 pm
content: If the BD135 is bad, a better replacement is the BD139 (same series, just a higher Vce of 80v not 45v).

author: MartinM87
author_rank: Newbie
timestamp: March 14, 2018, 08:52:15 am
content: OK I will try to implement these upgrades in the future.Though I just use it to test small bulbs and LEDs up to 12v