Comments on Isolated Transformerless PSU design.

[3v0]

I need ~50mA ~5V PSU's and found this isolated transformerless design on the net.



C1 and C2 are PolyPropylene which are the only expensive components (found these).

Obviously we need a fuse on the hot side. One thing that worries me is what happens when the zener fails. If it goes open circuit we are back to 120V.






**broken link removed**

 

[JimB]

Unless the device which is supplied with power from this PSU is fully isolated from the user, this PSU is a VERY bad idea.

At best the design is marginal, at worst criminally irresponsible.

JimB

 

[Diver300]

It's not isolated.

If you earth the terminal marked earth, it will trip the ELCB/RCD/GFI and if you don't, the voltage and current on that pin will be enough to kill you. (Ok, less likely to kill you if the supply is 120 V, but it will hurt)

Also, 250 V capacitors on 230 V mains, is asking for problems. And there is no inrush protection.

The page you linked to is written by another armchair idiot, who has obviously never made the circuits and touched the output. (Well, if he has done so since writing the article, he won't be writing any more)

 

[Diver300]

swagatam from Find Health, Education, Science & Technology Articles, Reviews, How-To and Tech Tips At Bright Hub - Apply To Be A Writer Today! has published loads of designs, none of which look as though they would work in real life situations.

"Build a 250 to 5000 watts PWM DC/AC 220V Power Inverter" is one of them. I don't know much about inverter design, but using darlingtons so that about 1.5 V is lost from a 12 V system, adding 600 W of losses, and paralleling darlingtons, just don't seem to me like good ideas.

 

[geko]

This might be a useful for a reference design

https://www.electro-tech-online.com/custompdfs/2011/05/00954A.pdf

 

[KMoffett]

Why not just get a old 5v cellphone charger wall wart. $0.50 at a thrift store. Or, check with your friends. Almost everyone has them laying around in a junk drawer. If you want the power supply inside another device, you can remove the PCB from the wall wart case. It will be as small as the transformerless design and is isolated for safety. Plus, most of them have a regulated 5V output, with a much higher current capacity than you can ever get from the transformerless design.
https://www.electro-tech-online.com/threads/not-a-transformerless-power-supply.93884/

Ken

 

[3v0]

I did not understand how the author could claim it was isolated.

I was looking at this because I need low current 5V PSU's inside lights and ceiling fans where I will already be switching the AC line. Not much room. The 5V is not critical as it is the input to another regulator that has an input of 6V max.

@KMoffett I am using wallwarts for other network nodes. Opening the cases had crossed my mind at one time but got lost in the shuffle (duh). I expect I can find enough in the lot that can be opened. My wife harvests them for me at the local thrift store and I not looked at the pile in some time.

@ Diver300. I agree regarding the caps. I was looking at using caps with 2x my 110VAC input.

Thanks all.

 

[cachehiker]

I've seen similar circuits used in motor controller designs where "isolated" does not mean low voltage and safe to touch. I wouldn't classify this as low voltage or safe to touch. On the other hand, if you need a low power +X volt supply riding above the +Y volt supply rail to power a fet driver or a -X volt supply riding below the -Y volt supply rail to power an inverting amplifier connected to a shunt, circuits like this can be a viable option.

 

[crutschow]

The bottom line is that a capacitor can not be used to isolate AC mains, only a transformer can do that.