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Old 29th Jul 2019, 8:57 am   #1
deliverance
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Default Capacitor question

Hi all. I am checking out the seperate power supply for the Redifon R50M. On the secondary side of the mains transformer there is a 375v 0 375v source and connected across Both 375v terminals are two caps TCC types which are 0.5 MFD 600 volt AC can I use DC caps or not . Any help would be appreciated cheers Chris .
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Old 29th Jul 2019, 9:21 am   #2
snowman_al
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Default Re: Capacitor question

Do you mean use a 600 volt DC rated cap? NO.
Very simply each 375 volt secondary 'swings' through the full sine wave so the cap would need to be at least 2 x 375 volts DC rated.
Better use a cap that has a 600 volt AC rating.
Alan
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Old 29th Jul 2019, 9:31 am   #3
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Default Re: Capacitor question

Hi Alan. Thanks for your reply yes I did mean a 600 volt DC rated cap do you know the best place to buy AC rated caps ? Cheers Chris .
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Old 29th Jul 2019, 9:34 am   #4
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Default Re: Capacitor question

Edit: Crossed post!

I get my capacitors from either Farnell/RS or Cricklewood Electronics.

Alan - if it's between the 375-0 winding, won't it see 375 x sqrt2, or nominally 530VDC?
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Old 29th Jul 2019, 9:43 am   #5
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Default Re: Capacitor question

The two caps are between the 375 v and the 375 v both the caps are connected together not to the 0 if that makes sense .
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Old 29th Jul 2019, 9:57 am   #6
deliverance
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Default Re: Capacitor question

A picture should help.
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Old 29th Jul 2019, 10:07 am   #7
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Default Re: Capacitor question

Looks like they're wired in series.
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Old 29th Jul 2019, 10:11 am   #8
deliverance
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Default Re: Capacitor question

Yes Graham that's it I tried to put that part of the schematic on the thread but I couldn't do it .
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Old 29th Jul 2019, 10:14 am   #9
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Default Re: Capacitor question

It's difficult to be absolutely certain but the capacitor in the picture appears to be 0.05uF, rather than 0.5uF. 0.05uF seems more likely, and 0.047uF will be cheaper and smaller than 0.47uF!
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Old 29th Jul 2019, 10:20 am   #10
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Default Re: Capacitor question

Your right the value is 0.05 uf.
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Old 29th Jul 2019, 10:24 am   #11
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Default Re: Capacitor question

That matches the component list. Total value 0.025uF.
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Old 29th Jul 2019, 10:38 am   #12
David G4EBT
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Default Re: Capacitor question

There are lots of suppliers of tubular axial polyester capacitors (including the BVWS to members), though some are of questionable origin, best avoided. They will of course have to be 0.047 uF, which is the nearest current value to 0.05uF. Cricklewood Electronics, a well established well regarded supplier, stock MKT branded caps, 630V 0.47uF:

https://www.ebay.co.uk/itm/Polyester...ujsVzpx1D6mSrQ

Presumably the reason they used two 0.05 uF in series rather one 0.025 uF was to share the 2 x 375V AC load?

Hope that helps.
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Old 29th Jul 2019, 11:17 am   #13
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Default Re: Capacitor question

Would it be better to use one 1kV 27nF?
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Old 29th Jul 2019, 11:44 am   #14
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Default Re: Capacitor question

750VAC (or 730VAC) x1.41 would be pushing it for a 1kV capacitor- plus there's pre-warm up and mains regulation to consider. For a single capacitor, I'd be looking for a 1.6kV or higher voltage component.
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Old 29th Jul 2019, 11:48 am   #15
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Default Re: Capacitor question

For film capacitors it is not generally safe to assume that a dc rating equal to √2 times the ac voltage will be adequate. Vishay MKT1813 rated at both 630Vdc and 1000Vdc are rated 220Vac so would be prone to early failure if used at more than 220Vac. The Technical Datahsheet can be found here:
https://uk.farnell.com/vishay/mkt181...pet/dp/1166874

For this application it would be best to select a capacitor with a suitable ac voltage rating.

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Old 29th Jul 2019, 12:18 pm   #16
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Default Re: Capacitor question

Thanks for your input guy's would I be right in saying I can use a single 2Kv 27nf cap instead of using two 0.05 uf caps.
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Old 29th Jul 2019, 1:22 pm   #17
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Default Re: Capacitor question

Hi,
The Redifon R50M was primarily intended as a ship's main receiver and dates from the late 40's to early 50's. AC mains on a ship would be derived from a motor generator of some sort and so prone to surging, bangs and bumps of all sorts.
The R50 power supply has two "snubber" networks to help protect the rectifier against over-voltage spikes, C1 and C2 on the transformer primary and C3 and C4 on the secondary. The idea is that the capacitors have relatively high impedance at 50Hz and so pass little current, but look like a much lower impedance to a fast spike coming in on the AC mains and so divert the fast spike current away from the rectifier.
The original capacitors have a combined voltage rating of 1200v AC equivalent to a rating of nearly 1700v DC, reflecting the need for them to withstand spike voltages appaearing at the input superimposed on top of the normal 230v AC mains supply. The capacitors are also physically large as they would be rated to carry significant current during the fast spike event - and they are 1940's design capacitors .
So a replacement capacitor(s) should probably have a combined DC voltage rating of at least 2kV and be large enough to handle significant current surges without failure - this will rule out most of the smaller high voltage caps intended for use in high impedance circuits where currents are low.
On the other hand, if the power supply is to be used on a modern domestic AC mains supply the chances of the kind of voltage spikes and variations which would have been common on a ship supply are very low, and I am sure the power supply would operate correctly long term if these capacitors were not fitted at all. There is still the C1/C2 network on the primary after all.
Why are you looking to change the capacitors?
cheers
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Old 29th Jul 2019, 2:02 pm   #18
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Default Re: Capacitor question

I see your reasoning, Peter- a typical IEC C14 integrated filter or similar would suit domestic mains usage, plus a suitable MOV on the primary side would be my approach.

I think Chris (deliverance) is wise to think of evicting the capacitors under question though, whether or not they are replaced- they look just like the sort of waxy that most of us here would be instinctively and rightly wary of, especially where they could fuse the fine HT winding of an expensive-to-replace transformer,

Colin
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Old 29th Jul 2019, 2:04 pm   #19
deliverance
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Default Re: Capacitor question

Hi Peter. Thanks for your informative reply , one of the caps had left a pool of wax in the bottom of the case hence my questions I have removed the caps and the power supply is ticking over nicely and the receiver is working after a slow wake with the Variac . So you think it's okay to leave the caps out running the set from domestic mains ?
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Old 29th Jul 2019, 6:51 pm   #20
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Default Re: Capacitor question

Hi deliverance as an aside,
I guess the blue wire in the picture has found a home?
Alan
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