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EWorkshop1708 15th March 2008 05:50 AM

Simple 12V SMPS for small powered devices
I was thinking of a super-simple way to run low power stuff from 12V, without needing TL494, SG3525 or similar IC's. Just a simple oscillator made from common parts.

The idea was to take an Astable Multivibrator circuit similar to below but with different timing resistors and capacitors for 30khz or so.

Then take each NPN output and directly drive a MOSFET, or a Totem Pole+MOSFET from each side for push-pull. Even could be modified for Push-pull with bipolars.

Example Oscillator Type (Ignore the component values)

I was thinking of taking a toroid inductor already wrapped with lots of turns I already have, add on a push-pull primary with a low turn count on it or so, and use the "secondary" to drive a fluorescent tube.

Another idea for audio use, was to make a simple split supply for preamp.

Any input on this?

DigitalJunkie 15th March 2008 07:07 AM

It should work okay..

I have an older car amplifier that uses a similar setup for the SMPS,no 494/3525,etc. Just a couple transistors,and some Mosfets. ;)

ph_christensen 15th March 2008 08:17 AM

I have built one like this before, But I had a lot of problems with it. It was not efficent at all, like 50 to 60 %.

luka 15th March 2008 08:28 AM


Go for it, it will work great

EWorkshop1708 16th March 2008 07:59 AM


theChris 16th March 2008 09:51 PM

you might also try royer push-pull type designs, though these need more care to the ferrite. such designs self-oscillate, and rely on transformer saturation to cause the switching. low power for this reason.

you must ensure you have a very symmetric oscialltor as the core can saturate if one switch is on consistantly longer then the other in a push pull. in which case you are relying on the source impedance to balence the volt-seconds.

megajocke 16th March 2008 10:11 PM

For really low powers an SG3525 can drive the transformer directly. :hypno2::whip:

EWorkshop1708 16th March 2008 11:20 PM

Toroid Transformer
I just wound the Toroid. I used a 1.5 inch toroid that already had about 100-120 turns of green 24 gauge wire wrapped on it.

I just wrapped 7+7 turns of thick red 15 gauge wire for a primary side.

I'm wanting 170-200V of HF AC from this thing.

Any idea of how to limit current and power to 80-100W in case the lamp is too bright? I was going to use a power resistor, but I would like to limit it with the osc directly for less heat. 120V CFL uses small inductor like a ballast to limit current, but I want to avoid having to make another inductor if possible.

I want to use SMP60N06 MOSFETS with a small heatsink. I wonder if PWM IC is a better choice for CFL Lamp......

EWorkshop1708 16th March 2008 11:21 PM


Originally posted by megajocke
For really low powers an SG3525 can drive the transformer directly. :hypno2::whip:
True, but not a 100W load. Even if so, I'd rather blow up some transistors during a fault, instead of the IC chip.

EWorkshop1708 17th March 2008 04:06 AM

HOT Transformer
I hooked up the transformer, and 60A MOSFETS, 4700uf cap to filter 12V in, and used a TL494 driver board I already have to drive the MOSFETS.

When making it run, it was getting a little over 11.5V from a computer PSU I used to test with. The MOSFETS ran cool. It wasn't loading down the PSU with no load. Acts like it runs fine.

However, with NO load, the transformer is getting pretty warm, , not HOT, but pretty close. I'm wondering what I can do about it. Primary 7+7 turns of single solid #15 coil wire. Frequency 93khz.

I did rectify with a capacitor and measured 217V!!! after the diode, so I am getting HV out. I'm going to try with a CFL tube tomorrow, but I want to know if the transformer is supposed to get warm first.

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