I have plenty 10uf 450v caps.
How much trouble am I asking for using them in a power supply portion of a radio spec'd for 475v caps?
xeric

xeric

Need to check the radio circuit. DC output from a rectifier can be 1.414 times AC input with no load. A 5Y3, 80 or other filament type rectifier will bring up DC voltage before other tubes start drawing current.

Hi Norm,
Circuit uses 5X4 recitfier w/ cap & choke design.
xeric

We would need to know the AC voltage going into the rectifier...

Rather than trying to relate to the original specs, I'd focus on the margin over the actual circuit voltage. For example, if the set is designed for 110 volts AC and is now running on 125, then all the voltages will be higher than the designer planned for.

If in doubt, always replace capacitors with a higher voltage rating than what the schematic says.

pixellany,
Voltage would be approx 705v at 117v.
I was just looking at the math that +25v additional, is only 6% more than 450v rating.

xeric

Usually 475v caps were specified over 450v for a reason. But...I think modern caps are more durable and forgiving to start up voltage spikes.

Depending on how close you are, 6% is certainly enough to put you over the limit.

I assume you mean 705 RMS, center-tapped.....So--one side is 353 RMS = ~ 500peak

5X4 tube drop 33 volts @100mA-----it looks like you could easily have ~470 to 480 volts peak--especially if the line goes up to 120-125.

The steady-state DC voltage will be close to the average of the sine wave (average = .637 of peak, RMS = .707 of peak, so maybe 480*(.637/.707) = ~430 volts

I'd be using 500-volt parts......

Thanks for the info pixellany!
I'll get the 500v variety
xeric

475-volt caps were sometimes spec'd for better reliability. Less internal leakage current, so they run cooler, and therefore last longer.

What is the actual measured voltage on the caps with the radio operating? If it is as high as 420-430 volts, you can use modern 450-volt electrolytics, but they will probably only last about a thousand hours or so in service before they fail. That may be fine if you are only going to play the radio for a couple of hours every Christmas Eve. But if you want a longer lasting repair, you need to use higher voltage caps, or double-up two in series.

Don't worry about surge ratings too much. Modern electrolytic caps can withstand 10% over-voltage conditions for brief periods of time, so a 450-volt cap will be okay up to 495 volts while the tubes warm up.

xeric,

What radio are we talking about? Are the original caps really specified for a working voltage of 475V, or is this a surge rating? Many filter caps used in old radios had both a 450VDC working rating and a 475V surge rating.

Present-day electrolytics often lack a surge voltage rating. I suspect that they will survive moderate overvoltage for a short time, but it would be nice to have some assurance from the manufacturer about the extent that this is true. Has anyone seen any recent information about this?

Remember, if you decide that you really need a rating higher than 450V, two caps in series usually cost less than a single part with the higher rating. Usually the single part is made from two caps in series anyway. If you want to use the parts on hand, and be very safe, you could put four of the parts in a series-parallel arraignment for the first filter cap. Two 22uF parts in series would be a bit easier, if you have that value.

I imagine that extra voltage rating margin would be most helpful for a radio that sits for long periods between use. This might help reduce excess leakage current as the parts form back up. Here, the two caps in series scheme will also help.

If you are buying electrolytics, there is no excuse not to purchase part rated for long life at 105C.

These do cost more than 2000hr @ 85C units, but not so much. For instance a Panasonic EEU-EE2W470S, 47uF 450V 10,000hr / 105C part runs $1.59 to $2.45 ea. for one piece. Avnet Express seems to have the best deal at the moment.


Chris,

You said:

"What is the actual measured voltage on the caps with the radio operating? If it is as high as 420-430 volts, you can use modern 450-volt electrolytics, but they will probably only last about a thousand hours or so in service before they fail. "

Can you point us to your source of information? I would expect that a part operated within its ratings to last much longer. Would a part rated for a 10,000 hour life at 105C fail this soon? When operated at a fraction of its rated ripple current and rated temperature?

At the rated maximum working voltage, the maximum rated leakage current would produce less heating than the heating you will get due to the ripple current, even in a small radio. I'd worry more about proximity to something else nearby that would heat the cap, such as a tube or power resistor.

Ted

Thanks very much for the information Chris108 and Usually Lurking
I went ahead and bought 105c 500v caps.
xeric

If you go to the websites of the big electronics houses like Mouser, Digi-Key, Allied Electronics, and Newark Electronics, and look up actual capacitors, they usually have links to the manufacturers' detailed data sheets available for downloading. There you will find the average life expectancies and derating factors for temperature and voltage, and in some cases ripple.

Surprisingly enough, for more than a few general purpose replacement grade electrolytics, 1,000 hours is the normal life expectancy if they are operated at maximum voltage and maximum temperature. Of course, they last a lot longer if you run them at lower temperatures and voltages.

The factor which most determines the life expectancy of an electrolytic cap, or a tantalum for that matter, is the internal temperature. This is certainly influenced by the ambient temperature externally, but also by leakage current. Bear in mind that leakage current tends to increase as a capacitor ages, so it has to be considered over the life of the cap, not just when it is new.

I never go below specified voltage with any caps in any context, especially lytics. That's a crapshoot with loaded dice. Been there, seen them blow....

Larry

And ripple current times ESR produces more heat.

Just out of curiosity, how much ripple current would you expect in a typical table radio with a full wave rectifier and perhaps 60 mA of B+ load?

I would expect more ripple current in the input filter cap, on the rectifier cathode.
After the choke the second filter has a much easier life.

Pete

No idea. If you assume 30 ohms impedance at 120Hz for the input cap, and maybe 30V of ripple impressed, that's an ampere. But the rectifier adds some impedance.

Really old consumer grade radios and such never needed over 450 volt capacitors. If you want to tame today's higher wall voltages, do it up front with a bucker or resistor or variac and spare the filaments as well. Any doubt make sets of twice the capacitanse in series to double working voltage. Use bleeders to stop it down after power off. Plenty of stuff here on how to do that, don't want to repeat.