DC Volts testing psu circuit.

[davelectronic]

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.

[Simon]

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

[davelectronic]

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.

[davelectronic]

Its just this simple regulator boost circuit, using same resistors and a 7812 Voltage regulator with an MJ11015 power transistor.

[davelectronic]

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.

[vk6zgo]

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

I think the OP needed 24v &  really put 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.

[davelectronic]

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..