To avoid switching noise near the work bench, I wanted to drive my LED strips from a plain ol’ transformer -> bridge -> capacitor supply. It would have to supply about 2A @ 12V for my LEDs. Here’s the setup:
Ignore the diodes at the top, these were 1N4004 diodes which weren’t quite up to the task. The ones at the bottom right are Schottky diodes for a lower voltage drop, rated at 5A. The capacitor is 2200 µF, but ripple isn’t an issue.
Alas, the heat from this thing is excessive. I even went for an artsy swiss-cheese design:
Trouble is the transformer (and surprisingly not the diodes!) – I measured them both, with everything closed:
Diodes heated up to 75°C – but the transformer went all the way up to well over 90°C within the hour! AC mains consumption is 50W, so this silly thing is eating up half the power while trying to behave like a stove…
The transformer is a cheap 2x6V @ 2.5A unit I had lying around (draws 3.7W, even without any load). I’ll keep this as a high-ripple 12..20 VDC power brick for testing – but it definitely can’t be used continuously @ 2 Amps!
PS. If you’d like to meet up – I’ll be presenting JeeStuff at HackersNL coming Thursday evening in Utrecht.
Is your presentation going to be recorded?
Not that I know (and it’d be in Dutch).
Did you tie it to one 115V end or did you place them in series? From the pictures it looks like you’ve only used one of the 115V inputs (and you’re on a 230V net). If this is the case, this may be the cause of all the heat you get. Place the two 115V in series and do the same with the two 6V channels and see what happens. It might just make the difference your looking for.
That’s not it – 115V primaries and 6V secondaries are in series, and using proper phase. It’s just a very bad transformer, I think. I’ll re-check, though.
Using a choke input to the bridge rectifiers can reduce the peak current loading on the transformer; reducing the approach to saturation will at least reduce the “iron” losses. A simple toroid core wound with fairly heavy enamel wire will do the job. Looks as though there is room to squeeze in and save the swiss cheese box.
The penalty is a lower output voltage – instead of Vsec1.41, the output is ~ Vsec0.90, perhaps closer to the 12 Vdc required. Further benefits are better power factor/reduced mains EMI.
I don’t understand – 2200uF is too low at 2A load – you should be seeing heavy ripple (7 to 8 volts)
Yes, there is heavy ripple. But for white LED lighting it doesn’t really matter. No visible flicker.