Hi All,

I've come to a bit of a weird situation... but first some explanations. My background in electronics is minimal (first year electrical engineering) so please bear with me with the stupid questions/statements.

I fly r/c planes and helicopters, and for that I need a power supply for battery chargers. Currently I have an old ATX power supply im using, but it can barely do 7A on 12v, and im after something bigger. I need something in the range of 12-18v, and as many amps as i can manage.
So i found an old computer UPS, which has a pretty hefty transformer in it. I peg it at 3-400 watts or so.
So i plugged this in, and I ended up with 16.5V AC out. perfect. I bought myself a 40A rectifier and a couple capacitors and i figured i'd be all set.

So i soldered everything up... around 16.5v dc rectified, 17v with the caps in place (2x 1000uf). Life was good. I added a cooling fan and a heatsink to the mix. thats when things went awry.
The output is now 13.5v. still within spec. But... i reconnected my capacitors again. First in series, which got me 6v at the output and the caps went to 20v. weird but ok. after some searching i found that they should be connected in parallel with the load. So i switched that around. Now, if the caps are unplugged (they are attached via a deans connector to the outputs) i get 13.5v, and life is good. When i plug in the caps, the voltage jumps to 21v and the fan greatly increases in rpm (its a 12v fan with 2x 47ohm 5w resistors in parallel).

so my question... where is this extra voltage coming from? do i need a regulator or something in it? should i just forget about the caps?
Any input is greatly appreciated. Sorry for the bit of a long winded post, but i figured i should try to explain stuff as best i can.

Thanks
NIck

 

Can you post up a schematic?

So i plugged this in, and I ended up with 16.5V AC out

Plugged which side of the transformer into what?

 

The filter capacitors are charging to the peak of the rectified AC, causing the apparent voltage to be ~1.4 times the AC Vrms of the secondary winding.

When you measure across the rectifier output WITH NO CAPACITOR, you see the effective value of the full wave rectified DC. When the capacitor is added, the capacitor charges to near the peak of the waveform.

Look at these two circuits, which are identical except for the addition of the filter capacitor. The RMS (effective value) of the left is 12.8V while the right is 17.7V. Your meter probably reads the RMS value.

 

Heres the schematic I came up with... I dont really have one drawn out completely.

When i plug the transformer in with no load, I get 16.5v AC out.
When i attach the rectifier with no load, I get 16.5v DC
When i attach the fan, I get 14v DC
when i attach a load to v out (battery charger, not charging) it jumps up to 21v.
When i connect the caps, I get 21v.
I've also tried connecting the caps in series with Vout, which got me 20v across the caps and 6v at the outputs.

 

The filter capacitors are charging to the peak of the rectified AC, causing the apparent voltage to be ~1.4 times the AC Vrms of the secondary winding.

When you measure across the rectifier output WITH NO CAPACITOR, you see the effective value of the full wave rectified DC. When the capacitor is added, the capacitor charges to near the peak of the waveform.

Look at these two circuits, which are identical except for the addition of the filter capacitor. The RMS (effective value) of the left is 12.8V while the right is 17.7V. Your meter probably reads the RMS value.

I had thought that is whats happening, but why would the addition of the fan lower the voltage? Also, is it possible to regulate that voltage down to something lower? I tried plugging directly into the output of the rectifier, but I dont think my chargers like the 60hz pulsating because they didnt work...

Thanks for your help.

 

Your recifier diodes are connected wrong.

 

Just my careless drawing. Its a rectifier bridge from NTE