Power supply : SMPS - how do i regulate it

[vwchico]

I bought a electronic transformer initially meant for halogen bulb lighting.
Its 60VA at 12V, figured it will be cheaper then buying a transformer and more efficient. Was cheap to about $8US. It seems its an ordinary SMPS

I also bought some digital multimeters which I want to dissassemble and use as voltage/ampre meters for $2US each, they were dirt cheap.

I want to use the above in a LM317T regulated power supply. But I tried it out today and it don't work.

When I put a bridge rectifier on the SMPS. I measure 6vdc. Stick a 1000uf cap on it and it measures 12VDC, exactly what I need. Then I connect the LM317T on it, then things go all weird. Voltage jumps between 2vdc and 12vdc every second on the output of the LM317T and the input of the LM317T. I know the LM317T circuit is working perfect becauseb I'm using it on a transformer power supply no problems.

Now why is the voltage jumping up and down. Is this what u guys refer to ripple on SMPS ? How do I get rid of it so I can actually use this thing with a voltage regulator. I tried sticking on a smaller cap then the voltage doesnt even get close to 12vdc.

What is my problem

 

[Nigel Goodwin]

It's not intended for that purpose, it's basically a very crude SMPS intended to feed specific bulbs - it's not suitable for a general purpose PSU.

 

[vwchico]

but there surely must be a way i can use it to make a SMPS and make it variable. How do they do it in PC's then.

 

[eblc1388]

but there surely must be a way i can use it to make a SMPS and make it variable. How do they do it in PC's then.

One thing you should know is that SMPS is usually meant to be used for a fairly constant load(both voltage and current) and most SMPS would go into keeping alive mode or even shutdown if insufficient loading is connected to its output.

The SMPS in the PC is not variable. It just has several fixed voltage outputs. This is possible using several windings and several set of rectifiers.

So, use a proper transformer for your variable supply.

 

[Nigel Goodwin]

but there surely must be a way i can use it to make a SMPS and make it variable. How do they do it in PC's then.

You could completely redesign it?, but then you're just making your own from scratch anyway!.

As 'eblc1388' mentioned, a PC supply isn't variable, and in some ways is actually quite crude!.

 

[eblc1388]

Hi vwchico,

After re-reading the postings, I now know what you really meant by "variable".

You have noticed that in BIOS, you can fine tune the CPU core voltages in steps of 0.05 or 0.1V. This is possible because there is a separate switching power regulator circuit on the motherboard, and it is designed to vary its output voltages while other part of the motherboard take up fixed voltages.

Notices the range of voltage change is small so this is a completely different situation as yours.

 

[vwchico]

I think i might have not made the question clear. What I want to do is make a variable power supply on the output of the SMPS not make the actual SMPS variable.

So in effect replace a trasformer with a SMPS, but it doesn't work with the 317 regulator

I want SMPS because they are more efficient then transformers and generate less heat.


Think i found my solution.

If I use a LM2576, you think I will have any problems ?

 

[eblc1388]

I think i might have not made the question clear. What I want to do is make a variable power supply on the output of the SMPS not make the actual SMPS variable.

We know but I have already told you that idea won't work unless your "load" is continuously connected and take a certain amount of current at all times.

Else the SMPS shutdown or going into "keep alive" mode and you have no/widely changing output voltage.

I want SMPS because they are more efficient then transformers and generate less heat.

A good transformer can achieve over 90% to 95% efficiency while a switching regulator at best can achieve over 80% efficiency. Thus a SMPS in general generates more heat than a transformer.

The advantages of using a SMPS instead of a transformer is its small overall size, weight and the ability to accept a very wide range of input voltages.

 

[Nigel Goodwin]

I think i might have not made the question clear. What I want to do is make a variable power supply on the output of the SMPS not make the actual SMPS variable.

So in effect replace a trasformer with a SMPS, but it doesn't work with the 317 regulator

An LM317 will work fine with an SMPS, it's your SMPS which is the problem - it's NOT a 'proper' SMPS, it's designed ONLY for lighting.