DC Volts testing psu circuit. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 03, 2016, 07:53:32 pm
content: Just a quick one really. Could testing a psu circuit with power transistor emitter follower make it oscillate from two smps. As opposed to using a transformer, bridge rectifier.The oscillations conditons where two smps in series to obtain 24 Volts for testing the psu circuit . Or is this not likley to be the reason for oscillations in a psu circuit ? Thanks for reading, any help appreciated.

author: Simon
author_rank: Unknown
timestamp: December 03, 2016, 08:07:42 pm
content: you best post your schematic, why two SMPS ? if they are in series it would only be the one directly in front of your circuit that is likely to be a problem

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 03, 2016, 10:30:58 pm
content: Is just the well known LM7812 and a pnp power transistor emitter follower. I saw a thread on here where someone mentioned instability if using DC for testing power circuits. That meaning regulated smps DC, i wandered if it would lead to the regulator and transistor acting oddly. The smps where a couple of xbox psu units run in series, so doubled the voltage, and current staying the same as a single unit.It was a way back now, but i wanted to try and find out if the oscillating was an unusual input voltage source.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 03, 2016, 10:34:09 pm
content: Its just this simple regulator boost circuit, using same resistors and a 7812 Voltage regulator with an MJ11015 power transistor.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 03, 2016, 10:58:41 pm
content: I have made circuits with is simple design and there stable. But i was wandering if its the high gain darlington the MJ11015 or the smps input i tried on this circuit. At the time i gave no thought to a mock up 24 Volt smps input. I've just repaired an old Alti 3 - 5 Amp linear supply using this circuit and an LM317K and a single MJ4502. That works fine with normal linear input.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 03, 2016, 11:06:30 pm
content: Quote from: Simon on December 03, 2016, 08:07:42 pmI think the OP needed 24v &reallyput two 12v supplies in series to get it.One PSU,followed by another would be in"cascade".I know a lot of people use the wrong term,but that doesn't make it correct.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 04, 2016, 12:55:26 am
content: Yes i needed a voltage above the 7812, although 24 volts is a bit on the high side, its all i had at the time that could handle up to 10 Amps. Athough i was testing single pass trasistors up to 5 Amps. But also paralleled 4 x transistors. The TIP2955 transistors i used back then where fine with an smps input. But trying first a single MJ11015 darlington underload produced some unstable results. I wandered if it was the smps input.Two smps where in series to give me more than just 12 volts. I'm not convinced it caused the insability issues, but don't know. I might have to try again with a single MJ11015 and a linear input. On the plus side the high gain of the MJ11015, there was no voltage sag or drop up to 5 Amps. The oscillating was intermittent, voltage would drop a volt then climb a volt, then go stable if load was reduced. The smps where just a means to an end.If it was the smps input causing instability that would be great, because its unlikely to do it with a linear input. The desirability of the MJ11015 transistor is its ability not to drop much voltage under load, if i could get it stable. I've got another redundant linear psu that needs repair, i would use an LM317K and the MJ11015, in hope its stable..