is it safe for the caps...

Thomas Giz · 5th April 2004, 01:18 PM

is it safe for the caps rated 1000uF/50V to work on 48V. I have transformer with center tapped 30V secondaries 0.5 A. Right after the diode bridge I have about 32V but after connecting caps voltage goes to about 48V. I tried it once. No big boom, smoke or anything. Caps were sitting intact but it worked maybe a ten seconds and unloaded so can't tell more. It's supposed to be a part of power supply for tube in valve buffered LM3875 amp.
Is it gonna work or I have to order different caps ?
Anyway it would be cheaper than new transformer.

thanks

analog_sa · 5th April 2004, 01:30 PM

It will probably work fine for months but it's not a good long term solution. If the mains fluctuates 10% you'll exceed the caps rating. They will certainly fail faster, especially if the the ambient temperature is on the warm side.

Matt Neilson · 5th April 2004, 01:39 PM

i'm not pretending to be a professional on the matter but i did actually ask iee teacher in elec engineering about something similar, and he said it's fine to run the capacitors within 10% over their limit but you are taking a risk of increased heat and hence shortened lifetime, just my understanding

Thomas Giz · 5th April 2004, 02:00 PM

any ways to lower the voltage to be safe for the caps
what about resistor across the cap ?

HDTVman · 5th April 2004, 02:07 PM

Should not be a problem. As soon as you put a load on that PS the voltage will drop a little. It depends on the load current but right now the caps are charged to peak and nothing is discharging them between peaks of the rectified AC. Most of the time short term overvoltage by 10% or so is not a problem as caps are designed to work at rated voltage.

Later BZ

Thomas Giz · 5th April 2004, 02:11 PM

thanks guys

It makes me feel better and my caps too.

ClassD · 5th April 2004, 02:31 PM

Running a cap that close to it's rating is not a very good idea. It would be much safer if you payed the extra couple of dollars for 63V caps. This is from one of National's datasheets:

Quote:

The capacitor’s voltage rating should be at last 1.5
times greater than the output voltage. For a 10V regulator,
a rating of at least 15V or more is recommended.

This is probably taking it a bit far, but I think 63V caps would be the way to go.

dhaen · 7th April 2004, 08:51 AM

The ambient temperature is a major factor, since if the cap fails at all, it will be through leakage and overheating.
In other words, you can get away with more in a cold place.

Jennice · 7th April 2004, 10:16 AM

classD,

I they refer to a regulator with e 10V output. However, if they mean that the regulator is SUPPLIED from 10VDC, there's no point in this margin.

Regards,
Jennice

ClassD · 7th April 2004, 11:40 AM

But there is no guarentee that the mains voltage will be nice and constant. Watching the voltage history on the UPS systems at work there is quite a large variation. The caps won't fail straight away, but as dhaen says - running them at high voltage does derate the lifespan.

5th April 2004, 01:18 PM	#1
Thomas Giz diyAudio Member Join Date: Feb 2003 Location: CA	is it safe for the caps... is it safe for the caps rated 1000uF/50V to work on 48V. I have transformer with center tapped 30V secondaries 0.5 A. Right after the diode bridge I have about 32V but after connecting caps voltage goes to about 48V. I tried it once. No big boom, smoke or anything. Caps were sitting intact but it worked maybe a ten seconds and unloaded so can't tell more. It's supposed to be a part of power supply for tube in valve buffered LM3875 amp. Is it gonna work or I have to order different caps ? Anyway it would be cheaper than new transformer. thanks

5th April 2004, 02:07 PM	#5
HDTVman diyAudio Member Join Date: Aug 2001 Location: Chicago area	Should not be a problem. As soon as you put a load on that PS the voltage will drop a little. It depends on the load current but right now the caps are charged to peak and nothing is discharging them between peaks of the rectified AC. Most of the time short term overvoltage by 10% or so is not a problem as caps are designed to work at rated voltage. Later BZ __________________ What ever makes the tunes flow

7th April 2004, 10:16 AM	#9
Jennice diyAudio Member Join Date: Jun 2003 Location: Planet Earth	classD, I they refer to a regulator with e 10V output. However, if they mean that the regulator is SUPPLIED from 10VDC, there's no point in this margin. Regards, Jennice __________________ I get paid to break stuff. My g/f gets paid to play with children. Life is good.

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5th April 2004, 01:30 PM	#2
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	It will probably work fine for months but it's not a good long term solution. If the mains fluctuates 10% you'll exceed the caps rating. They will certainly fail faster, especially if the the ambient temperature is on the warm side.

5th April 2004, 01:39 PM	#3
Matt Neilson diyAudio Member Join Date: Oct 2003 Location: Sydney	i'm not pretending to be a professional on the matter but i did actually ask iee teacher in elec engineering about something similar, and he said it's fine to run the capacitors within 10% over their limit but you are taking a risk of increased heat and hence shortened lifetime, just my understanding

5th April 2004, 02:00 PM	#4
Thomas Giz diyAudio Member Join Date: Feb 2003 Location: CA	any ways to lower the voltage to be safe for the caps what about resistor across the cap ?

5th April 2004, 02:11 PM	#6
Thomas Giz diyAudio Member Join Date: Feb 2003 Location: CA	thanks guys It makes me feel better and my caps too.

7th April 2004, 08:51 AM	#8
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	The ambient temperature is a major factor, since if the cap fails at all, it will be through leakage and overheating. In other words, you can get away with more in a cold place.

7th April 2004, 11:40 AM	#10
ClassD diyAudio Member Join Date: Apr 2004 Location: Brisbane	But there is no guarentee that the mains voltage will be nice and constant. Watching the voltage history on the UPS systems at work there is quite a large variation. The caps won't fail straight away, but as dhaen says - running them at high voltage does derate the lifespan.

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