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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 24th Oct 2020, 12:11 pm   #1
murphyv310
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Default AC capacitor ratings.

Hi.
In a recent thread we discussed the voltages present over a half wave rectifier and the use of a 250v ac rated capacitor.
Prompted by another thread on replacing a blown Rifa safety capacitor I saw a link to a suitable replacement. Looking at the data sheet for this 68nf 275v ac capacitor one of the tests is to apply 1333v dc to the terminals for 60 seconds without failure. Another cap I looked at, the type I have been using a 250v ac X type has the same test at 2120v dc without failure for a 60 second test.

Although my theory got a bit muddled up I'd say that this type of cap would be safe across a half wave rectifier diode in an AC DC radio. What do others think?

https://www.mouser.co.uk/datasheet/2...58-1222124.pdf
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Old 24th Oct 2020, 2:42 pm   #2
Maarten
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Default Re: AC capacitor ratings.

A test voltage that can be withstood for a minute, is not the same as a continuous DC volts rating. As a rule of thumb for any non polarised capacitor, the continuous DC rating is 2.8 times the continuous AC rating (and the continuous AC rating is the continuous DC rating divided by 2.8) unless the datasheet specifies otherwise.

For use across a 1kV rated rectifier, a 1kVDC rated ceramic would generally be the best fit. Both are 'jelly bean' components, often cheaper than their 600V/630V counterparts and suffciently dimensioned to never worry about, while a 250/275VAC rated foil capacitor of the Y2/X2 variety, may fail open some day, rendering it useless (a similarly rated paper capacitor will probably fail short, with additional damage as a result).
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Old 24th Oct 2020, 3:13 pm   #3
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Default Re: AC capacitor ratings.

Forgot to write in between the alineas: Also, across a diode a rather high dV/dt can result, so polyester isn't suitable. Polypropylene and ceramic are better at withstanding pulses.
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Old 24th Oct 2020, 5:16 pm   #4
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Default Re: AC capacitor ratings.

Thanks for the answer.
So a Y2/X2 in parallel with the diode could fail O/C that may go unnoticed whereas a paper cap could short, hopefully blowing the HT fuse if fitted.
A small value fusible resistor placed in series with the cap of any type woukd add protection especially on sets with no HT fuse.
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Old 24th Oct 2020, 6:35 pm   #5
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Default Re: AC capacitor ratings.

A paper cap (which can also be Y2/X2 rated) can go leaky and burn up before it either shorts or goes open, so a fuse only gives limited protection. A ceramic cap can short as well but will mostly do so in a more defined way, I forgot to mention that. However this is quite rare when it's dimensioned correctly. If you put in a resistor, though, you'd probably be better off using a fusible resistor as you say.
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