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Old 12th Apr 2021, 4:41 pm   #1
The_Archiver
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Default Capacitor voltage ratings: AC / DC / dual

Hello, all. This query is related to ongoing efforts to repair a Philips N1500 (please see separate thread if interested).

The attachments which are from a section of that VCRís Service Manual show that the polyester caps are rated at 250V, but is that a reference to AC or DC? As there are no value markings on the existing polyester caps, only colour coding, and not wanting to order new for old replacements of the wrong rating and possibly cause irreparable damage, can anyone please advise as to what ratings should be ordered?

Having looked on the RS Components website, almost all of the capacitance values needed are available, but even some of the voltage ratings on the polyester caps there are confusing me further: some have AC only rating, some DC only rating, and some dual rating. If the general rule (correct?) is that DC peak voltage is AC*sqrt2, should a polyester cap rated at a DC maximum voltage of 250V suffice? Similar solution for the ceramic caps rated at 500V(DC?)...?

This may be an easy concept to grasp for some, but am having difficulty understanding even basic information nowadays for whatever reason/s.

Thanks in advance.

Philip
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Old 12th Apr 2021, 6:21 pm   #2
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Default Re: Capacitor voltage ratings: AC / DC / dual

That will be the DC value in the schematic snippet, 250 volts DC is normally Red for the 5th band:

https://www.petervis.com/electronics...lour-code.html

Lawrence.
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Old 12th Apr 2021, 6:42 pm   #3
Lucien Nunes
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Default Re: Capacitor voltage ratings: AC / DC / dual

By default, voltage ratings quoted for film capacitors for electronic applications or in an electronic context will be DC, while those for electrical machinery (e.g. motor run caps) are normally AC. The sum of the DC and any superimposed AC peak should not exceed the DC rating.

Additional ratings and parameters come into play with capacitors intended for severe duty such as fast-risetime impulses. It is not then safe to assume that the AC and DC ratings will be in the ratio of sqrt(2) as the dielectric losses and dV/dt may be limiting factors.
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Old 12th Apr 2021, 7:16 pm   #4
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Default Re: Capacitor voltage ratings: AC / DC / dual

If you are referring to the polyester caps that are flat with coloured bands, they are very reliable and would not normally need replacing. They can be damaged mechanically quite easily by pulling or tugging on the leads when they can become detached. Electrically, I've never had a faulty one.

They do have values....the colour code is the same as the resistor code with the exception of the last band which is either red or brown...red = 250Vdc, brown is 100Vdc.

The first three colours indicate the value (example brown, black, orange = 1 0,000 = 10,000pF or .01uF. The fourth band is the tolerance (white I think is 10%, black is 20% but I could be corrected on this). The fifth band is the voltage rating as mentioned above.
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Old 12th Apr 2021, 9:28 pm   #5
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Default Re: Capacitor voltage ratings: AC / DC / dual

To Lawrence, Lucien, and Sideband,

Many thanks for all of the information, which has explained so quickly what Iíve been trying to find for days.

Whilst the attached photo doesnít show it very well, almost all the polyester caps in question from a specific suspect board have hairline or worse cracks in their casings and have still to be checked with ESR meter, but itís good to know that they are considered very reliable.

One step closer to a possible repair...

Philip
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Old 12th Apr 2021, 11:40 pm   #6
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Default Re: Capacitor voltage ratings: AC / DC / dual

Yes, the outer casings on these Mullard C280 Metallised polyester types would crack / get bits chipped-off, but they still seem to work OK, providing a leg at one side hasn't got pulled away. They were replaced by the all-orange epoxy (with text markings) type, that the outer casing material is much more robust.
(Although it seems the Audiofools are convinced the C280 ones sound better and pay a lot for these)

I remember the polyester ones sold in Maplin that didn't have any outer protective casing, plus had short straight leads that could easily pull-off the conductive material sides (but could often solder back-on if done quickly to not damage this).

And agree, capacitors are DC Working Voltage rated, unless marked otherwise. I don't think mains safety Class X & Y capacitors existed back then, and mains-filter ones were always DC voltage marked (but could sometime also have an AC voltage rating on the Polypropylene etc. ones). However, the originally used ones were a bit marginal at 400Vdc, and often failed s/c, so were sometimes changed to 630Vdc or 1000Vdc ones.
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Old 13th Apr 2021, 1:37 am   #7
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Default Re: Capacitor voltage ratings: AC / DC / dual

Philips always used 800V, 1000V or 1300V DC-rated capacitors on the mains, and for a good reason. I'd say 400V DC rated capacitors would only have been used at 110V mains.

Rule of thumb (when a datasheet cannot be found) is that the AC rating is a factor 2.8 smaller than the DC rating. This corresponds to the full Vpp of the AC signal rather than the Vp. Imagine the scenario where you unplug a set that is switched off then replug it in a socket with line and neutral reversed while the capacitor is still charged at Vp. Or just a capacitor that gets "tired" of the charge constantly being reversed.

On some MKP capacitors there's indeed an AC rating given as well, for example 250VAC for a 630VDC WIMA type. The datasheet explicitly mentions that it isn't suited for connecting across the mains.
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Old 13th Apr 2021, 6:28 am   #8
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Default Re: Capacitor voltage ratings: AC / DC / dual

Quote:
Imagine the scenario where you unplug a set that is switched off then replug it in a socket with line and neutral reversed
which is why your supposed to discharge the coupling cap in your scope after every reading by switching to "gnd". A practice I admit to not doing often after blowing the front end of a scope whilst switching to gnd with the probe still connected to some bit of HV gear.

Andy.
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