LTSpiceIV - modelling a computer PSU

Thread Starter


Joined Dec 10, 2012
Hi Folks,
As will no doubt become abundantly clear, I am very new to this, but here goes . . . .

I am trying to model the power circuit for an old 1980s computer (Memotech MTX), just to try and understand it more than anything else.

The external "PSU" is nothing more than a step down transformer with multiple taps at nominal voltages of 22.3, 18 and 9. The regulation etc. is done on the mother board inside the main computer chassis.

I am trying to use LTspiceIV to model the circuit. I have tried to cobble together a transformer using some inductors to give me the approximate AC voltages. I have downloaed Spice models for the Diodes, regulators and TIP2955 that are not part of the LTspice standard distribution.

I have added resistors to the 12VDC, 5VDC and -5VDC to simulate the loads that I expect the computer to run. The model appears to work fine until I add the TIP2955 and then things go very wrong - giving wierd current and voltages. I am sure that the circuit topography is correct, but my modelling skills are suspect.

I'd appreciate any help/advice that anyone can give please?
I have attached my model and include files



Ron H

Joined Apr 14, 2005
I don't have any answers for you, but I have a few comments.
Your regulator input voltages are way too high. M78XX regulators need a minimum of about 3V across them for regulation. Anything higher wastes power and causes the regulators to get hotter than necessary. I realize you measured AC voltages, but I'm having trouble believing the designers would have built in that much headroom. I see you are in Scotland. Are you applying 220VAC to a supply that was designed for 110VAC?

The normal way of boosting the output of a linear regulator is to use a power PNP as shown, but with the collector connected to the output of the regulator. I tried that, and your simulation oscillated. I moved C39 to the other side of R3, and it still oscillated.
I don't understand the "value" of R2. What does that mean?

Thread Starter


Joined Dec 10, 2012
Hi there,

Thanks a lot for the reply.

The PSU is rated for 240VAC - it was 240 here in those days before we got "Eurofied".

The transformer tap voltages are printed on the PSU case, 22.5, 18, 9, it is not clear whether they are p-p or RMS though, but I would think they should be RMS?

The "value" for R2 was just a way to make the simulated 5v load vary a bit, I just look at one of the AC voltages and manipulate it a bit, then make sure the value of the resistor is within certain limits to keep the current value reasonable,