piclist 2001\07\26\072342a >
Thread: Transformerless PSU
www.piclist.com/techref/power.htm?key=transformer
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face BY : mikeKILLspam@whitewing.co.uk



On Wed, 25 Jul 2001 21:52:50 -0300, you wrote:

>        Dear guys,
>
>        I think it's a basic one, but maybe fits the list
>
>        I designed a circuit with a PIC (12C508) that turns on and off a relay, controlled by an external source. The power to this circuit (around 120mA) is a small transformer. I'd like to build a transformerless PSU into this circuit. But EVERY scheme I found on the internet was unusable. I saw something on the Embeeded Control Handbook from Microchip, but it only goes to (if I'm not mistaken) 45 mA. Maybe someone can help me to develop (or at least learn to) a simple transformerless PSU that can give me 12vcc / 120 mA???
>
For that current, a transformer will be the cheapest, and probably not
far off the smallest solution.
Can you change the relay? - some types of relay are readily
availalable with 48v coils, so you immediately reduce your current
requirement by a factor of four, making a transformerless solution
more practical.
Another possibility is to use a mains-coil relay, and a small (TO92)
triac, or MOC3020 opto-triac to drive it. The PIC can be run from a
small transformerless 5V supply  
Of course if you REALLY wanted to do it as small as possible, and have
a strong nerve, you could use the PIC to PWM or phase-control the
relay current to get 12v avarage from the mains, using the relay's
inductance to limit peak current, but make sure think very carefully
about what would happen if the software crashed!

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