Help needed with understanding this basic PSU design

[stevenA]

Good evening

I am trying to relearn some of the basic electronics that I have long forgotten, and would appreciate some help understanding how the attached reverse engineered circuit from a broken no-name PS is supposed to work!

This should supply 12v DC at 1.2 amp from 230VAC (the circuit may not be correct but I have checked it several times)

The supply does not work and I suspect the LM358 is at fault (the 370 ohm resistor to the inverted input is clearly overheating). The cap is bad.

I think (!) that this is a dual rail supply, and am clear on the functioning of the +ve rail.

I am unclear as to the design of the -ve rail, as this appears at best to provide half wave rectification through the SCR. I am also unclear as to how it is referenced to the +ve rail, unless through Vs+ and Vs- (?) of the LM358

I do not remember what this was used to power, and it may be that the device smoothed the DC at input, but as it stands, I see only that the op amp is used as a comparator to check for the presence of a load, and to then activate the SCR to complete the circuit.

I also do not understand why the two windings are used on the transformer (rather than a single winding).

Any help in explaining how this circuit is intended to work would be greatly appreciated

Thanks

[tom66]

Re: Help needed with understanding this basic PSU design

It is a single rail supply, to be dual rail it must have a separate ground, which it does not, the output negative is your ground reference.

The 370 ohm resistor isn't a standard value, and it shouldn't be overheating - the inverting input of the op-amp is essentially infinite impedance so no current flows, this means that the op-amp is probably broken.

It looks like it might use the LM358 for shut down in case of failure, it's not clear?

[budm]

Re: Help needed with understanding this basic PSU design

I am not sure where you get this circuit from, but the LM117 is setup as constant current source and its output is fed through 470 Ohm resistor, so you want 1.2A flowing through 470 Ohm = 564V which is impossible, not sure what you draw up the this circcuit. The Thyristor will never conduct since the Cathode side is backward bias if you ever have the current flow throw the the load from 12v to the Negative point.
Can we see this power supply that you are trying to reverse engineering it?
You just want a 12VDC power supply that can supply current up to 1.2A that will run with 230VAC input?

[stevenA]

Re: Help needed with understanding this basic PSU design

Thanks for your replies. This is only an exercise to help me improve my understanding of power circuits - this one is unusual (?) and I cannot see how it is working, particularly how the +ve output is linked to the -ve output, as the negative output is only connected via the supply to the LM358 to the rest of the circuit!


I have attached some photos as requested - and have overlaid components on the back of the PCB.

There were some errors in my previous circuit , notably:

• Resistor Values
• The outputs were inverted (the thyristor as drawn should now be OK)
• Missing resistor on the non-inverting input
• The transformer windings were incorrect - the revised circuit is based on voltages measured
• The 370 ohm resistor which is overheating is 470 ohm and is on the output of the LM358, and not the input as originally drawn!

I have made some measurements, without any load, and have added these values to the circuit.

There are several things that appear unusual - there is only 1v across the 180 ohm resistor at the ouput of the LM317 (instead of 1.25) - could this be because there was no load - and I do not understand how there is only 25.5vdc across the cap, when based on the 29v rms input, there should be 39.8v according to my calcs

The no -load output of the supply is 12.8V.

Thanks again for your help.

[budm]

Re: Help needed with understanding this basic PSU design

Do you still have the case of the power supply? I would like to see what it is said on it.
I will draw out the sch later.

[stevenA]

Re: Help needed with understanding this basic PSU design

Thanks Budm

I think that this is not in fact a PSU, but a battery charger! Following your above question, I have found similar circuits for a NiMh charger using similar components. That being said I would still like to understand how it works!

I do have the case but there are very few markings unfortunately - I cannot even remember what this supply was originally for but have an inkling that it could have been for a very early gen transportable phone

The markings are:

Approval no: S1357/4/G/500454 (probably BABT)

Made in England

and on the base:

25D83599R
XDC 86 (49)

I have googled but found nothing with this.

As I said, my interest is to understand the concept.

After a days research, my only thoughts are that the LM317 is giving a ref voltage to the LM358 which is switching the TIC106D - logically this would be as overvoltage protection, but in reality, it seems to be to provide the positive rail! If this is a charger, then this would be to stop overcharging.

The only uncertainty in the circuit is the transfo - and there are no markings on it at all.

Hope this helps, and thanks for your interest

[budm]

Re: Help needed with understanding this basic PSU design

OK, from what I can see, this power supply will be putting out pulsing Half wave DC. Do you have the scope? It is more likely to be used for batteries charger using pulsed DC.
Is you meter TRUE R.M.S., if you trying to read DC output, it will only show the average DC output of the AC output (half wave).
You second drawing is correct, the resistor between the ADJ and the output pin of the 317 is set to put out constant current of about 7mA (i out = 1.25Vref/180 Ohm)

[stevenA]

Re: Help needed with understanding this basic PSU design

Thanks Budm. I doubt that the meter I am using has true RMS, and do not (yet) have a scope.

If I understand correctly, you are saying that the output is via the TIC106D, and that the full bridge is only used to provide a voltage reference for the LM358 via the LM317, and a return path.

[retiredcaps]

Re: Help needed with understanding this basic PSU design

I doubt that the meter I am using has true RMS, and do not (yet) have a scope.

Most multimeters that support True RMS will say True RMS right on the front of the multimeter.

[budm]

Re: Help needed with understanding this basic PSU design

The current return path is through the Bridge rectifier setup.