Mains Ac to Reg DC Cheep Chinese Way

[minime72706]

I think you're right, PSI. They test them at thousands of volts apparently!

[SeanB]

Just remember that with time X2 capacitors degrade, and they can fail as a short circuit as well ( allowed as the spec calls for them to be used across the lines, not between line and PE) so they need to have a fuse in the line to them. I have seen many fail after a few years of use.

[minime72706]

Suppression capacitors are supposed to last quite a while. It should last longer if you protect the input with something other than the capacitor itself. A MOV or TVS may prevent surges from hitting the capacitor full-force, increasing the lifetime. Definitely include a thoughtfully-chosen fuse in the circuit.

EDIT: Here's a couple of pictures of random stuff I have laying around.



Picked the big gray one below up at the MIT Swap Fest (flea market) on Sunday. Got like 12 for a buck.

[Sirus]

Thanks for all your help guys, I have gained a lot from this. Now I know what these PSU's are called I
have been doing a bit of research.

After a google of capacitive reactance, I found a free usefull and small program/calculator that does
this and some other handy stuff, sorry if you all know about it. Electronics Assistant v4.31.

Sorry not noting anyones name, I am using notepad to type this out as forums usualy drop out on me when
I submit

It just so happens, that one of the failed units was a radio controled mains switch with a manual button
on the plug pack. I have 4 of them. They have loads of room for my custom boards in my simple projects
below. I looked a little bit deeper on the board and found that they are using the 2 stage method some
one mentioned, They are using a double drop type of zener regulation, at first I thought they where
using a half wave rectifier, but it is full wave, but 2 diodes are 24v zeners.I thought that was quite
smart as that should double your power rating also, then for the logic a 5v Zener after that.

I striped the dead one for parts and tested them against a working one. I couldn't test the 24v Zeners
in reverse as my power supply only goes up to 20v but they test good forward. I also checked the mains
cap esr's, they where quite close. so I just swapped the mains cap and 24v zeners on the working one, and
it worked fine. So I baged up the PSU parts, labeled it Tranformerless kit 240ac to 24v, 35ma and put it
in the new draw in the part bin .

I think the fail must have, as some one mentioned, transients ripping the logic or a transistor on the
board.

As these are going to be usefull as project cases I didn't dig any further, I am using one for my desk
light, but I may kill it as its a nice case, lol.

The projects I was hoping to build from this type of psu are..

Timed once only Mains switch--
This is to turn my 5 & 7 year old daughters bed blankets off after 30mins, so I don't cook them. I
forget to set my alarm 50% of the time, don't worry, waterproof sheets just to be on the safe side. They
haven't done that since they where 3. Might even make another 2 for our bed, as we often forget due to
drifting orf to nodland.

Plug in the wall Power failure lighting--
We have had a dodgy mains feed to our street for around 2 years now. So if the power goes, we are
plunged into darkness, my youngest get quite distraught because of that bang of silence and sudden
darkness. And I get really peeved while finding the ladder in the dark. Using some rechargeables and
bright leds in one wallwart type box, I might even get flash and do it all with diodes, resistors and
the mains and smoothing cap. I am making a heavier duty 12v backup system for the fridge and freezer, I
was also thinking of rerouting the house lighting circuit through a UPS, anyhow that's all another story.

DIY suse vide controller--
My only concern on this would be the temperature sensor, it is waterproof but I was thinking of earth
grounding the metal tube it is epoxyed in for belt and braces.
Using an arduino, been trying to make one for 2 years now. In the beginning I was trying to figure out
thermal mass and the delays involved to make some kinda formula, then around a 3 months after I found
out about PID control, HALLELUIAH, learned all of that then discovered adafruit do a kit based on
some bodys library's 3 months later, using a long timebase PWM. I am already to go on this using just the
library's the fella made, my program and hardware.

TTFN

[Sirus]

Here is a few pics of the "new" project boxes. Now I have figured out photobucket again.
http://s245.photobucket.com/user/Mr_Sirus/media/Wallprojectbox1_zps8de9c3e4.jpg.html
http://s245.photobucket.com/user/Mr_Sirus/media/Wallprojectbox2_zpsc3bb82f8.jpg.html
There's lots of room in them and if you don't need to control the socket you can just jump it, so you don't lose the use of the wall socket. The in built push button directly hits the board to a pcb micro switch.
Does anyone know where this type of case could be purchased? I think this is a custom package.

IRC I bought 4 of these 12 years ago at £15 a pop. they also had a 4 button remote with each one to control up to 4 of the plug packs.

For £15 you couldn't buy such a versitile case in my opinion, even back then.

TTFN