Elenco XP-581A - BEWARE!!!

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The Good: The 5V, 12V and -12V fixed supplies work well and I trust them. Large heat sink across the back but only convection cooling so it doesn't make a sound. I like the cabinet. You can stack other equipment on top. Meters are easy to read but wish they were back-lighted. The Bad: Arrived with rattles in the cabinet - found all 3 plastic standoffs on PCB had broken - replaced before power up. Found the Voltmeter was reading about 3V too low and the error could not be adjusted out - useless junk - plan to replace with a digital meter. The 3 fixed voltage supplies are well behaved but the variable supply was a circuit killer! Tested on a resistive 32 ohm load and just turning on the switch without turning up the voltage, the variable supply put out large (over 10V) oscillations dampening down to 0V after about 1/2 second. With output voltage set at minimum, about 3 seconds (varies with load on the variable and -12V supply) after turning off the supply, the variable supply output would drift up to about 6V (with a 32 ohm load!, it will go higher with less load) and then decay. Work-arounds: Disconnect the PNP pass transistor - this stops the huge oscillations on power up but limits output current to just what the LM317 can pass which is about 300mA just to be safe but can be more depending on Vin - Vout. Probable culprit is the 1000 uF cap on the base of the PNP - probably there to stop oscillations onder regulating conditions. Removing it or using a smaller cap might cause other problems to show but worth a try. The voltage rise on turn off is caused by the 0V output feature (normally the LM317's minimum output voltage is about 1.25V). The LM317's ADJ terminal is held two diode drops below ground by using the rectified -12V. Problem is the diode drop goes away and actually starts to float positive before the variable supply's capacitors discharge. So the output starts to drift up a few seconds after shutdown. This isn't some high impedance voltage, it's circuit-killer low impedance. Work around: disconnect the wire that connects the voltage adjust pot to the dropping diodes and ground the wire (you must disconnect the wire before grounding it) - now no more voltage after turn-off, i.e., ground won't go away like the -12V does. These two work-arounds give you a 1.25V - 20V (actually goes over 20) supply with about 300mA capacity but it is well behaved and won't kill your integrated circuits! It's now in the classic LM317 configuration, i.e., no current boosting PNP and no negative bias on ADJ to pull it down to 0V output. But if you're just going to use the variable supply to run light bulbs and motors, you won't care. It's too bad whom ever designed the variable supply didn't do their homework and didn't own an oscilloscope! Too bad it'll ruin about every integrated circuit that's connected to it! Too bad that the voltmeter is horribly inaccurate! It's too bad a gorilla on the assembly line broke all the PCB stand-offs and it's too bad no one tested this supply before they shipped it to a customer!!! Bottom Line: I've been fixing things my entire life and the Elenco XP-581A is just another thing that needs fixing. I'll probably replace the voltmeter with a digital and live with the 2 work-arounds but if I need the 0V feature and the extra current from the PNP, I'll probably use a delayed-turn-on immediate-turn-off relay on the output or replace the LM317 with something like an LT3080. Best complement I can offer: It's a nice TRIPLE output supply.

Jimmy Ford on Dec 15, 2017 I found this helpful (7)