HandyMan

Hello everyone

I want to design a circuit. I'm a repairer of electronic goods but unfortunately I am not a circuit designer, not to the level whereby I can create a fairly complex circuit that works smoothly - my design tends to be a bit rough and ready.

Recently I've been repairing a good number of TFT monitors which operate on 12V DC supplies. Sometimes the monitors are supplied without the 12V chopper power supply adapters which means I have difficulty testing them as my benchtop power supply uses a standard mains transformer and a regulator connected to a pot which allows voltage control from 0 - 30V and the output current just isn't high enough despite the huge size of the transformer.

As I want the higher current but with the continued ability to alter the voltage level, I thought that a chopper power supply hooked up to a regulator would be better. I can use LM338T for voltage regulation.

I want to build the chopper power supply myself. I know a lot of people gawp at the thought because they are fairly complex circuits, but what about if a lot of the 'extras' in the circuit were removed, like the optocoupling and other perhaps unnecessary complexities.

Most TFT monitors (and laptops that come in for repair) need from 2.5A to 4A and a variable supply using a chopper circuit would be ideal, while giving me high current and variable voltage to use on other devices when needed.

I'd take mains voltage, rectifying and smoothing it I can design myself, but I'm not sure how to go about creating the high frequency which chopper circuits use. I have plently of chopper transformers about (from TV chopper supplies) and planned to use one winding with a center tap and then use a push-pull design at high-frequency. Nothing fancy, if that's workable.

I'd like to then use any of the secondary windings that produce around 35V, rectify, smooth, and supply to a lead.

Is it possible to design a very simple circuit to do this and does anyone feel up to the task of helping me please? I can make the PCB using photo-etch board and I have a good stock of components and can get hold of others easily.

I'd really appreciate any technical help and advice on this.

Thanks.

Russlk

If your time is worth anything, it is more efficient to buy than build. You can buy a 12 volt, 5 amp supply from Mouser (www.mouser.com) for $32. You can't buy the parts and build it for less. Mouser part number: 418-CFM60S120.

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HandyMan

Thanks for finding that for me, I appreciate it. I think I may go with your suggestion.

Incidentally, I don't suppose you or anyone else knows where a good place is on the net where I can learn more about the workings and theories to do with switchmode (chopper) circuits?

Thanks a lot,

James

Nigel Goodwin

I would suggest looking at IC manufacturers datasheets, however it's a very complex subject, and the really critical component is probably the transformer.

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HandyMan

Do you feel like explaining it and answering a few of my hundred and one questions?

I wouldn't blame you for wanting to switch your computer off right now!

James

Nigel Goodwin

I'll offer what advice I can, but my experience is in repairing them, not designing them.

As I see it, the BIG problem is when things go wrong, generally it's catastrophic - it's like a chain of dominoes, ones goes down - THEY ALL GO DOWN.

Have you considered using an old PC PSU?, there's been a number of threads on this in the last few months.

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HandyMan

Yes, I have! I've got 3 in the workshop from old computers, but they are the type which are turned on by a momentary switch on the computer case and apparently the circuits are so contrived that they aren't really suitable.

I did think of using one though, they have a great deal of current available and a 12V line...

Nigel Goodwin

If you search for the previous threads, they discuss how to make them suitable - basically just adding a minimum amount of load to make the supply run correctly.

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HandyMan

I'll have a search and try to get one up and running, sounds ideal although I'd like more than 12V available, it's good enough for now.

Thanks for your help so far. The main thing I don't understand with chopper circuits is how such small transformers can deliver so much current when an equivalent mains transformer has to be the size of a brick to do the same thing. I know the frequency difference is involved (~60KHz compared to mains transformer operating at 50/60Hz) but not sure how this fits into it.

Nigel Goodwin

As you say, it's down to the higher frequencies involved - basically the higher the frequency the smaller the transformer can be.

I presume it's down to core saturation, a lower frequency transformer needs to be larger to prevent the core saturating?. But I'm sure there are plenty of people here who will know!.

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k7elp60

I have built a designed a built a number of switching power supplies. All have been the buck(reduce) or boost(increase) the DC voltage. Even in these one has to be aware of the minimum load current to keep them stable. One thing I noticed was that the higher the current the lower the
value of the inductance.
Some were built from published data sheets and manufacture furnished software. These generally used a specific IC that had a lot of the circuit in the package.
Others where built around a specific IC like the LM3524 switching regulator. In this case the math for the design was sometimes extensive depending upon the application.

I have also built numerous linear power supplies, they are simple and seem to work for ever. Like Nigel says you can usually buy them cheaper than you can build them.

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HandyMan

I have found a good tutorial for learning about switch mode power supply circuits:

http://www.smpstech.com/tutorial/t00con.htm

Hope this helps others who wish to learn more about them.