5V Transformerless PSU Review

transformerless 5v power supply

Hi all, this is my first post at this forum!

Anyway...

I'm building a small PSU of no more than 50mA at 5V, driven from 220V mains, without a transformer, based on Microchip's AN954.

I am looking for suggestions and advices regarding my current design, as I'm afraid I'm not confident enough about my knowledge to feel safe about it, yet.

This is what I have so far:
**broken link removed**

Thanks!

transformerless psu

Just make sure C1 is non-polarized paper capacitor. When I was in school, I made a mistake of using electrolytic cap and there was a big-boom!!

transformerless 5v supply

is the value for R6(1M) not much? I was wondering if 330K won't be enough?

transformerless

I am regularely using this type of power supply with my AC operrated microcontroller designs which require below 50Ma. But do rememeber the circuit will be live when connected, and also seriously consider post from CMOS.

brasky said:

is the value for R6(1M) not much? I was wondering if 330K won't be enough?

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The 1M is just used to discharge the o.47 Mfd capacitor. 470K is optimum. 330K is oK. Good luck

transformerless power supply for 5v

Thanks everyone for the feedback!

pranam77, what do you mean by "post from CMOS" ?

R6 is really just to discharge C1, so the PSU will be safe to handle once disconnected. I chose 1M because I wanted it to be 1/4W resister. Peak voltage is about 315V, so at 470K the power dissipated will be around 0.21W, which seems to me marginally close to the 0.25W rating.

I consider adding an isolation transformer (yeah, I know it won't be a transformerless PSU afterward ) for safety (the application might have a trimmer that will be adjusted with a screwdriver while the circuit is "hot").
The main consideration is space - the application should have a small footprint. So I'm also looking for suggestions for small isolation solution...

I also consider splitting R5 to two 220Ohm resistors, to lower their power rating (although I'm sure its worth it).

transformerless bridge rectifier power circuit

z9u2k said:

Thanks everyone for the feedback!

pranam77, what do you mean by "post from CMOS" ?

Click to expand...

It means the asnwer/words written by Memebr "CMOS" before my answer. Good luck

transformerless 5v

pranam77 said:

z9u2k said:

Thanks everyone for the feedback!

pranam77, what do you mean by "post from CMOS" ?

Click to expand...

It means the asnwer/words written by Memebr "CMOS" before my answer. Good luck

Click to expand...

hehe... yeah - sorry 'bout that

Anyways, the schematics above will not work properly. I've just prototyped it, and the voltage across the Zener started to climb, until it was burnt. Now I have to buy a new one...

Having another look at the AN, I notices their 470uF cap is after the Zener. Relocating C2 to between D6 and C3 solved that problem - voltage is fixed and Zener won't die.

I ran a quick test (for a short time) in the "old" configuration, and found out the voltage across climbs, but the current through is fixed, so I'm a little stumbled.

What went on in there, that caused my Zener to burn?

building 5v paper capacitor

z9u2k said:

Relocating C2 to between D6 and C3 solved that problem - voltage is fixed and Zener won't die.

Click to expand...

This should not matter at all since all 3 parts are in parallel ! There must be something wrong with the old zener that burnt out or C1 is leaky causing more current to pass through !

microchip transformerless 220v

Yes...i had similar problem but when i was using bridge rectifier....AAAahhh yes one thing i spoted in your schematic is that 1 microfard capacitor which is too much for the purpose and naturally the zener will blow off. Try with 0.33 microfard capacitor or 0.47 microfard capacitor with 440V AC rating. Hope this will work fine. I have practically worked with the given cicruit but the capacitor was as mentioned and too i had not used the zener. Good luck. Hope it helps you.