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 Post subject: Someone enlighten me to using a cap for voltage dropping....
PostPosted: Jan Wed 03, 2018 9:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1205
Location: Tucson AZ
O.k., I read this, online of course, and have never heard of such a thing. It goes like this:

AA5 AC set which has a resistance cord. Owner wants to replace cord with a power resistor, but someone makes the suggestion to use a capacitor to drop the excess voltage because that would be better since caps do not generate heat. It was implied that a motor start capacitor would be appropriate.

I don't know everything, but I've never heard of using capacitors in place of a resistor to drop voltage in this fashion, so teach me something new.


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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Wed 03, 2018 9:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 27316
Location: Detroit, MI USA
Capacitors have reactance as AC is applied, which can be calculated based upon the frequency, and the value of capacitance.

For the purpose of dropping voltage in a filament string, reactance is just as good as resistance and as you noted, it creates no heat if the right type of capacitor is used.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Wed 03, 2018 10:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37225
Location: Livermore, CA
This site explains different ways of dropping filament voltage.

https://www.vintage-radio.com/repair-re ... calcs.html

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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Wed 03, 2018 10:07 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1205
Location: Tucson AZ
Norm Leal wrote:
This site explains different ways of dropping filament voltage.

https://www.vintage-radio.com/repair-re ... calcs.html


Thank you for that link Norm.

Talk about feeling like a dummy for not ever have used a cap specifically for the function of dropping voltage.


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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Thu 04, 2018 1:32 am 
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Joined: Jun Wed 14, 2006 11:49 pm
Posts: 7357
Location: Leo, IN or Zellwood, FL
Not sure what value capacitor your looking for but this link is to an 8 Mfd non polarized capacitor.
https://www.digikey.com/product-detail/en/EXH2E805HRPT/493-13831-ND/4694508

Or select this address And then select the Mfd you want and the AC voltage of 250 should narrow down to 1 or 2 available.
https://www.digikey.com/products/en/capacitors/film-capacitors/62

John k9uwa

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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Thu 04, 2018 3:05 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10900
Location: Powell River BC Canada
If a capacitor is in series with a resistave load, the voltage across the two
parts summed, will exceed the line voltage by a maximum of 41 %.

This may cause confusion is some circumstances.

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Steve Dow
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 Post subject: Re: Someone enlighten me to using a cap for voltage dropping
PostPosted: Jan Thu 04, 2018 3:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37225
Location: Livermore, CA
Don't use a capacitor in series with an inductive load. Voltage will greatly increase.

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