can a 3 Ufd ac start capacitor be replaced with a 3 Ufd 600 volt dc capacitor

No. A DC capacitor will explode on AC.

Look for film capacitors intended for use as motor start or in this case run capacitors. These are easily obtained.

Look in places like Automation Direct and DigiKey.

Joe Gwinn

If the new cap is electrolytic, no. If its a film type, yes certainly.

If you're thinking of putting the new cap in the old can, be aware that oiled paper caps were still in use then, and the 'oil' was nasty toxic stuff. Keep it off your skin.

NT

Other places to look are W.W. Grainger and Herbach and Rademan.

Dan

Thanks everyone for the fast reply I found this site by accident looking for info on capacitors The cap is a run cap in an old emerson fan. It is marked 2.9 uf , 250 volts and in a can that looks looks like a sardine can. I subbed a 3 uf /600 vdc and the fan runs. This was just for test. I will get the proper type cap for permanent placement. It is an old fan that was in the attic approx 60 years old. Thanks everybody. I will be monitoring this site and inputing where I can. Have 60 years in various electronics fields. Retired now but still into electronics/ Again Best Regards to you all. Ben

"Joe Gwinn"

.... Phil

Lots of film/polystyrene AC capacitors are available, cheap and very reliable in our experience. DC capacitors, especially polarized ones, would lead to an electrical fire. Often the AC polystyrene capacitors come in small capacitance values, but connecting several in parallel, works fine.

Either polypropylene ( PP) or polyester (PET) films are used.

AC rated types use a special winding technique that results in two or more caps in series.

DC rated, single wound types fail prematurely when subjected to AC voltages of more than 100V rms.

.... Phil

The latter is a puzzling thing to say. I've used plenty of dc rated film caps at >100v ac with an excellent reliability record.

NT

Phil already mentioned the importance of mains rated caps (marked as X1 or X2). Smart people listen to Phil. *When* that DC cap fails it will do so catastrophically with smoke, noise, fire and possibly attorneys involved.

He is using at least three level of false logic.

1. That my generalisation about DC rated caps must apply to all examples.
2. Since it does not comply with the secret example he alludes to, it must be false.
3. That arguing from the particular to the general is not a fallacy.

FYI:

In the USA, makers have been getting away with fitting 600V DC rated film caps in 120V AC circuits for a long time - they sometimes even connect them from active to chassis in guitar amps. Such capacitors are known by guitarists as "death caps". The evil practice was discontinued about 25 year ago.

A given 600 or 1000V DC rated cap may well be series wound, impregnated with oil or just use unusually thick film and so last quite well. But the vast majority of DC rated caps last only a short time when used with a 240V supply - then typically explode violently. Failure is due to internal corona discharge across trapped air pockets inside the cap eating away the insulation.

Corona simply does not occur in wound film capacitors at 100VAC, that is why I mentioned the number.

.... Phil

To repeat the question that has not been addressed, what is the basis for this claim that a 600v cap on mains will fail?

NT

Try not to lose it again. Just stick to the relevant issue here.

Same here, for decades until X & Y caps came along. I've not found any reliability problem with them

Thats not whats being discussed here.

FWIW its now common practice here to address RFI with class Y caps from L to chassis - and the way its done here its safe.

Either a cap is rated to take mains peak v plus transients or it isnt. If it is, and isnt electrolytic, it works, and keeps working. What type of caps are you describing that have this dismal record?

I've dealt with & used 400v & 600v caps for decades, and (other than lytics & obsolete oiled paper) they've had an excellent reliability record.

NT

Well surge tests go up to 4kV don't they?

He is using at least three level of false logic.

1. That my generalisation about DC rated caps must apply to all examples.

1. Since it does not comply with the secret example he alludes to, it must be false.

2. That arguing from the particular to the general is not a fallacy.

FYI:

In the USA, makers have been getting away with fitting 600V DC rated film caps in 120V AC circuits for a long time - they sometimes even connect them from active to chassis in guitar amps. Such capacitors are known by guitarists as "death caps". The evil practice was discontinued about 25 year ago.

A given 600 or 1000V DC rated cap may well be series wound, impregnated with oil or just use unusually thick film and so last quite well. But the vast majority of DC rated caps last only a short time when used with a 240V supply - then typically explode violently. Failure is due to internal corona discharge across trapped air pockets inside the cap eating away the insulation.

Corona simply does not occur in wound film capacitors at 100VAC, that is why I mentioned the number.

.... Phil

However , it's the *continuous AC rms voltage* rating is crucial for long term survival in AC mains applications.

This is what class X and class Y ratings are about.

Class X types ( active to neutral) may fail short eventually, while class Y types ( active to exposed metal) should never fail short since they are safety critical parts.

.... Phil

I'm still waiting for you to tell us why you think 600v rated caps cant handle 110 or 240v ac.

NT

I thought someone already did that - voltage spikes and surges. And for

-------------------------------------------

He is using at least three level of false logic.

1. That my generalisation about DC rated caps must apply to all examples.

1. Since it does not comply with the secret example he alludes to, it must be false.

2. That arguing from the particular to the general is not a fallacy.

FYI:

In the USA, makers have been getting away with fitting 600V DC rated film caps in 120V AC circuits for a long time - they sometimes even connect them from active to chassis in guitar amps. Such capacitors are known by guitarists as "death caps". The evil practice was discontinued about 25 year ago.

A given 600 or 1000V DC rated cap may well be series wound, impregnated with oil or just use unusually thick film and so last quite well. But the vast majority of DC rated caps last only a short time when used with a 240V supply - then typically explode violently. Failure is due to internal corona discharge across trapped air pockets inside the cap eating away the insulation.

Corona simply does not occur in wound film capacitors at 100VAC, that is why I mentioned the number.

.... Phil

I presume you mean due to inductive spikes. What I dont see is how you get >600v spikes by disconnecting a max 13A inductive load from a low impedance supply with several amps times many houses of load on it. The numbers just dont add up.

NT