PWR supply help

[DSL PWR]

I have a 120vac to 80vac transformer. I've also aquired a 600vac 25a bridge rectifier. I have 2 85v cap.

When I put the cap across the dc side of the bridge the cap gets very warm. I disconect the power before things get out of hand. It's been a while since I built a supply in highschool, please tell me what I'm doing wrong.


Do I need a load resistor?

[smarbaga]

kaaaaaaaa - booooooommm......
.... where's the fans ..... peeee uuuuu
those caps are going to blow up
and they can hurt real bad
you have about 110 volts dc off of the rectifier bridge and your caps can only handle 85v ( new ones)
ac to dc multiply the ac by 1.414 this will give you the dc voltage output
always use at least the next highest voltage rated caps.

[DSL PWR]

Please read the post again. I have 80vac off my transformer. When you go from ac to dc, you derate the power by 1.414, so you divide, not multiply. When derated the voltage is under 60v, the 85v caps should be fine.

[Al_The_Man]

Originally Posted by DSL PWR I have a 120vac to 80vac transformer. I've also aquired a 600vac 25a bridge rectifier. I have 2 85v cap.

80VAC translates to 80x1.414 = 113VDC minus around 1.5v for the bridge diodes thats 111.6VDC The caps are probabally getting warm at that much over the rating.
The 80vac is an rms value, the 113vdc is the peak.
Al.

[DACMACHINE]

DSL

smarbaga is right

80vac at the rectifier = 80vdc
connected to the capacitor 80v * 1.414 = 113.12vdc
cap not rated high enough


check it youself with a meter

[2muchstuff]

Calculate for maximum smoke. Even Gecko's won't take that much voltage.

[DSL PWR]

As I pull my foot out of my mouth, I'm pretty sure these caps were on this transformer before. Is it possible they were in series? All this stuff came off of an old esab control, so it all worked before. How did this setup work on my 80vdc servos before?

[DSL PWR]

I also checked the voltage across my bridge, and it's under 80vdc on my meter. And yes my meter is accurate, it is calibrated yearly. I'm probably just missing something.

[smarbaga]

without a filter cap on the dc side of the bridge you will only see pulsating ac whick you cannot measure with a dc meter.
half the ac signwave is flipped over to a plus side thus making the ac dc, but not pure dc.
if this transformer has a center tap perhaps perhaps it was used with just 2 diodes and the center tap wired as common ( negative )
or maybe the caps were in series ( doubt it, but maybe)
glad you didn't leave the caps on too long.

[CJL5585]

Only one comment:
If you plan on using the capacitors to do further tests, then put on a suit of ARMOR and definately wear safety glasses so you will be protected as much as possible when the capacitors blow, because they will eventually. You do not want to be around when they do go.

[Al_The_Man]

Originally Posted by DSL PWR I also checked the voltage across my bridge, and it's under 80vdc on my meter. And yes my meter is accurate, it is calibrated yearly. I'm probably just missing something.

Just out of curiosity I check my bench supply, with and without caps connected, No load.
Quality Fluke Multimeter
AC secondary ~35vac
Bridge output - No Capacitor - DC range = 56vdc
Bridge output - Capacitor connected = 48DC
The DC with cap connected would include the voltage drop for the bridge.
I would have expected actually to have had less than 48dc with no cap.
I assume the strangely high reading to be due to the ripple content in the DC causing this. A moving coil meter would probabally give different results.
This points to something I found out rapidly when I originally changed over to an electronic meter from a moving coil, is to always take into consideration the signal content or the type of source of the reading as sometimes to the un-initiated things can be a bit puzzling. As for instance obtaing readings due to SSR leakage etc.
Al.

[DSL PWR]

I put the 2 caps in series, and no smoke, or bangs. I've got 100vdc at the caps,no load. What is the best way to regulate this down to 80vdc, without getting a new transformer?

Can I just go across 1 cap and get about 50 vdc?
Could anything blow up?