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Old 6th Apr 2018, 7:12 pm   #1
Vakito227
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Default 600V AC Output Transformer Capacitor

Hi everyone, in the midst of restuffing paper caps on an Ekco television I bought, I accidentally gutted the 0.01uf cap across the audio output transformer thinking it was rated at 600V DC when it was actually rated at 600V AC. I want to avoid spending big on getting a proper 600V AC cap if at all possible and was wondering if the manufacturer was just massively overcompensating since I'm fairly sure the 8" 3 ohm speaker can't take a full 600V let alone a fraction of that. I did some research and found that the 600V DC caps I originally planned to fit are good for 220V AC. Is it okay to fit this instead?

Peter
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Old 6th Apr 2018, 7:32 pm   #2
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Default Re: 600V AC Output Transformer Capacitor

Fit a 1000V DC cap and it will be ok, there can be high transients across the primary of the winding.
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Old 6th Apr 2018, 9:28 pm   #3
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Default Re: 600V AC Output Transformer Capacitor

Use a polypropylene cap, they withstand transients better.
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Old 7th Apr 2018, 2:12 am   #4
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Default Re: 600V AC Output Transformer Capacitor

Thanks for the advice, I got a 1000V poly cap on its way
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Old 7th Apr 2018, 6:34 am   #5
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Default Re: 600V AC Output Transformer Capacitor

Your OPT "generates" high voltages like any inductor, due to back EMF, see - https://electronics.stackexchange.co...n-a-dc-circuit

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Old 7th Apr 2018, 11:57 am   #6
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Default Re: 600V AC Output Transformer Capacitor

The presence of the cap will actually prevent very high voltages from developing.
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Old 7th Apr 2018, 12:35 pm   #7
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Default Re: 600V AC Output Transformer Capacitor

How does that work Dave, by preventing or smoothing changes in current?

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Old 7th Apr 2018, 12:49 pm   #8
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Default Re: 600V AC Output Transformer Capacitor

Or use 2 double the value in series for complete peace of mind.
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Old 7th Apr 2018, 9:34 pm   #9
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Default Re: 600V AC Output Transformer Capacitor

On a related note, is a 1000V DC capacitor good for the mains filter capacitor? The old capacitor was rated at 500DC and 300AC and I later found out that the 630V DC cap I intended to replace it with was again, only good for 220V AC.
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Old 7th Apr 2018, 10:19 pm   #10
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Default Re: 600V AC Output Transformer Capacitor

You should really only use 'X' caps across the mains....they are designed to fail safely (at least reputable brands are). They are also designed to cope with transients (spikes) which can be quite high. A normal 1000V DC cap is probably good for about 300V AC BUT it would not cope well with transients and could eventually fail catastrophically (as typically happens to the old waxey types). X caps are cheap and usually can be had for nothing if you strip down an old computer power supply. You could even do away with it altogether if there are no modulation hum problems.
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Old 8th Apr 2018, 11:49 am   #11
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Default Re: 600V AC Output Transformer Capacitor

Oh fair enough, I've ordered an X2 capacitor made by RIFA
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Old 8th Apr 2018, 9:28 pm   #12
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Default Re: 600V AC Output Transformer Capacitor

Please tell me you didn't order a PAPER X2 capacitor from Rifa. In that case, any polypropylene 1000VDC will outlast it by far. The problem isn't in the handling of transients by the way, but in that the X rating guarantees that it will fail relatively safely (open or short, not half open and burning) given the large available amount of energy that the mains supply might pump in to it. The Rifa paper cap will fail half open and smoking anyway, the 1000VDC MKP will probably never fail.
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Old 8th Apr 2018, 10:01 pm   #13
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Default Re: 600V AC Output Transformer Capacitor

Uhhh I might have done! I've cancelled that order then and instead bought a 275V AC X2 poly capacitor with a peak voltage of 2.5kV
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Old 8th Apr 2018, 10:20 pm   #14
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Default Re: 600V AC Output Transformer Capacitor

I'm sure if you do a search (up top) on Rifa, you'll find plenty of info.....
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Old 8th Apr 2018, 10:45 pm   #15
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Default Re: 600V AC Output Transformer Capacitor

Oh I see, RIFA seem to have a bad rep on here as a phoney brand. Possibly dodged a bullet there
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Old 8th Apr 2018, 11:04 pm   #16
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Default Re: 600V AC Output Transformer Capacitor

They're not phoney...they've been around for half a century or more. They just don't seem to be much good.....
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Old 9th Apr 2018, 2:45 pm   #17
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Default Re: 600V AC Output Transformer Capacitor

The back emf you get from interrupting the current through an inductance can be estimated as follows:
Energy stored in inductor = 0.5 L I^2
When the circuit is broken this kinetic energy has to go somewhere and much of it ends up as potential energy across whatever capacitance is present - if nothing put there then it just uses stray capacitance.
Energy stored in capacitance = 0.5 C V^2

Equate the two formulas to get L I^2 = C V^2, so V = sqrt(L/C) x I

Increasing C thus reduces V.
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Old 9th Apr 2018, 6:49 pm   #18
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Default Re: 600V AC Output Transformer Capacitor

Quote:
Originally Posted by Vakito227 View Post
Oh I see, RIFA seem to have a bad rep on here as a phoney brand. Possibly dodged a bullet there
Woah... steady on a moment! What was formerly EVOX-RIFA (Kemet, distributor) make many, many products.

Some of the old RIFA brand is much, much more discussed here than others - notably, those in an X1 or X2 role, line-to-line, or line-to-ground. Easily the most-replaced component in all vintage equipment, along with 'that cap.' That's a historic issue. It's pretty amazing that many have survived as long as they have, given the incredible stress/distress they get in this role.

The old Rifa's may not be everyone's favourite, but the company in its current form make, for example, excellent (IMO) high pulse capacity electrolytics, for example, which are robust and great for decoupling DC power rails.

I can't see why their polypropelene X1 and X2 rated caps in their current form are to be avoided, but I'd be interested to hear if anyone has reason to suspect them?

I think as with all things, it's best not to generalise and instead to stick to a specific example.
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Old 9th Apr 2018, 9:21 pm   #19
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Default Re: 600V AC Output Transformer Capacitor

Quote:
Originally Posted by Vakito227 View Post
Oh I see, RIFA seem to have a bad rep on here as a phoney brand.
Back in the '90s we replaced all the smoothing electrolytics in the 1200V and 200V supplies of our modulator valve switching decks with hi-temp EVOX-RIFA caps (white cases, plastic).

Never had a problem with them.
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Old 10th Apr 2018, 1:41 pm   #20
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Default Re: 600V AC Output Transformer Capacitor

Sounds like a pretty solid recommendation!
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