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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Mar 2009, 11:09 am   #1
bosscock
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Default Capacitor voltage VAC/DC ?

Hello,

Please can somebody explain the difference between Vac and DC markings on a capacitor.

I have a Ambassador coronet with all of the waxed caps rated at 1000v. Which seems high..

on several sites i have seen the axial polyprop cap (yellow body ones) with two ratings example 450 Vac/ 1.5kV(dc)

Is it because Vac is AC and V DC. Why would the dc one be three times bigger in voltage...

Sorry for this noob question

Neill
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Old 13th Mar 2009, 11:42 am   #2
paulsherwin
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Default Re: Capacitor voltage VAC/DC ?

The AC and DC ratings of a capacitor will be different, and the DC rating will always be higher. This is because of the greater stress applied to the dielectric insulation by AC voltage.

1kV is a high rating for domestic valve circuits. Manufacturers sometimes had access to stocks of high voltage caps and used them up in this way. As a general rule, 400V caps will be fine in all positions except the tone correction capacitor attached to the output valve anode.

Capacitors should be rated at the maximum voltage to which they can be exposed, including when a fault is present elsewhere in the circuit.

Paul
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Old 13th Mar 2009, 11:55 am   #3
bosscock
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Default Re: Capacitor voltage VAC/DC ?

Thanks Paul,

From reading on the forum I did get the feel that 400-650v was the normal, just got a little spooked with 'all' the wax capacitors including hunts being rated at 1000v. will go with the 400v.

thanks again
neill
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Old 13th Mar 2009, 12:25 pm   #4
ppppenguin
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Default Re: Capacitor voltage VAC/DC ?

At best, the AC rating will be SQRT(2) times less than the DC rating. This is simply because the peak voltage of AC is this much higher than the RMS voltage. In practice the AC rating is even lower because of factors such as dielectric losses.

The critical positions in an ordinary radio are:

Across the mains: Usually rated at 1000VDC, now always use X or X2 rated parts which are specified for this application.

Tone correction: Sometimes a capacitor connected to the primary of the output transformer can be subject to unusually high voltages. Often rated at 750VDC or higher.

Otherwise, as Paul said, 400V parts are adequate. In many cases lower voltage parts (250V) are OK too but you need to assess each case on its merits.
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Old 13th Mar 2009, 12:42 pm   #5
newlite4
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Default Re: Capacitor voltage VAC/DC ?

Tone correction: Sometimes a capacitor connected to the primary of the output transformer can be subject to unusually high voltages. Often rated at 750VDC or higher.

Ooops!
Some major refitting of components is due methinks
Neil
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Old 13th Mar 2009, 1:25 pm   #6
paulsherwin
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Default Re: Capacitor voltage VAC/DC ?

Quote:
Originally Posted by newlite4 View Post
Ooops!
Some major refitting of components is due methinks
Neil
This is less critical with modern components. The original wax caps would fail short circuit and this could have serious results for the output stage, damaging the output valve and/or transformer, so the caps were rated conservatively. Modern plastic film caps normally self heal when the dielectric is bridged so shouldn't fail short circuit. This means that a lower rated cap can be used. However, it's still good practice to use a 1kV rated cap in the tone correction position.

You can sometimes find suitable caps in scrap switch mode PSUs or monitors.

Paul
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Old 13th Mar 2009, 1:26 pm   #7
XTC
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Default Re: Capacitor voltage VAC/DC ?

Quote:
Originally Posted by newlite4 View Post

Ooops!
Some major refitting of components is due methinks
Neil
I think you'll get away with it until something happens such as the speaker being disconnected.

1000V rating makes sense for tone correction caps and they're not that dear.

Pete.
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Old 13th Mar 2009, 8:17 pm   #8
Nickthedentist
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Default Re: Capacitor voltage VAC/DC ?

Physical size is also a factor.

I often end up using components with a higher voltage rating than's required if that's all I have to hand. It saves having to stock loads of different ratings.

However, there are many occasions where space is very limited, in which case a cap with a voltage rating similar to the original is all that can be squeezed in.

Conversely, modern components of the same rating are typically smaller than the vintage component that they're replacing. Sometimes, these can look so odd, that fitting a higher-rated cap is a better solution aesthetically.

Nick.
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