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 Post subject: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 12:59 am 
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Joined: Sep Wed 13, 2017 11:55 pm
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Location: Lambertville, Michigan
Hi all,
I’m restoring a Kadette Jewel model 41 with the “curtain burner” resistance line cord which I want to replace using a capacitor for the voltage drop. I know that the cap has to be AC and somewhere between 1-10 uF (to be determined) and rated about 400 V or so. I’ve tried to find these caps and when I go on various supplier websites I become immediately lost and confused with the myriad of types and choices. Can someone please direct me to a source and explain exactly what I need to look for? Any help greatly appreciated.
Thanks,
Mike


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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 2:51 am 
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A film capacitor with those ratings could work. There are various types of film, you should make sure they can withstand high ac current. There are types made for "snubber" use that would be good. Probably only a few types of film are made with so large a value, so that would narrow it down, as well as cost.

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 3:02 am 
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Look at "motor run capacitors". They are in this range and can handle the current.

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 3:42 am 
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Location: Massachusetts
you will want a capacitor rated at 250VAC


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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 3:46 am 
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SHenion wrote:
Look at "motor run capacitors". They are in this range and can handle the current.

You might be able to find what you need at Home Depot of a similar place. Capacitors like this are common in ceiling fans.

Here is a graph to help you get closer to the value you need: http://americanradiohistory.com/Archive-Poptronics/50s/57/Pop-1957-06.pdf page 66. This is for a line voltage of 117 so you may need to tweak a little.

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 8:00 pm 
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Location: Lexington, KY USA
You can parallel capacitors to get the value you need.

Many reccomend that you use a cap that's a little larger, then add a small resistance in series to help limit the surge at turn-on. The resistor can be used to fine tune the heater voltage. ( The series capacitor provides a soft start for the heaters, except for a very brief transient that depends on the phasing of the AC line vs. the moment of switch-on.)

1000 Bulbs sell several values of lamp ballast capacitors at low prices. Sometimes you can find the value you want.

Kemet make a series of stud mount plastic can caps that are inexpensive. See Mouser.

If you need a smaller value for a parallel combination, safety-rated caps are available and bullet-proof in this application. Class X2, Y2, whatever.

There is a really nice site online that helps you figure out what capacitor value you need. Is that what's linked above? If not, perhaps someone has the link handy to share.

Ted


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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 8:43 pm 
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Look for polypropylene film caps. They don't tend to go up in value with temperature the way polyester/mylar caps do. In addition to motor run capacitors, there are AC snubber caps that are pretty good.

Consider putting a large value resistor, like 27-k ohms, in parallel with the capacitor. The reason is, if the power switch is turned off right at one of the peaks of an AC cycle, the capacitor will store a charge, and modern plastic film caps can hold it a long time. The resistor provides a discharge path so you don't while you are working on the set. A 27-k resistor will dissipate very little power at the voltages being dropped, so you can use a half-watter or whatever you have handy.

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Feb Fri 23, 2018 11:04 pm 
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If anyone cares, the formulas for calculating the required capacitance.
I = current used at rated voltage
Er = rated voltage
Es = line voltage
Ec = voltage drop on the capacitor
Xc = reactance of the capacitor

find Ec: Ec = sqrt (Es^2 - Er^2)

Find Xc = Ec / I

Find C = 1 / (377 * Xc) ( 2*pi*f = 377 for 60Hz line voltage)

As suggested, make the cap slighty larger (lower impedance), and then "dial it in" with a series resistor.

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Aug Mon 20, 2018 3:07 pm 
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There is a link on this site that will take you to a program that will calculate the value for you:

http://www.pavekmuseum.org/curtainburner.html

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 Post subject: Re: Capacitor Help, Please.
PostPosted: Aug Mon 20, 2018 6:36 pm 
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This puppy ?

http://www.rfcafe.com/references/radio- ... -craft.htm

If so, 300mA string. The sum resistance, hot, is 84 ohms for the 3 tubes. A 6.8uf cap has a reactance of 390 ohms at 60Hz. Vector summed with the 84 ohms gives you close to 400 ohms, which is what you want (120V/400 ohms = 300mA). 6.8uf is a standard value, readily found. 400VDC (250VAC) rated.

Only polypropylene types show up for this value, at the rate voltage, from Mouser:

https://www.mouser.com/Passive-Componen ... =Pricing|0

So there goes the polypropylene vs polyester issue. Polypropylene is the better dielectric, for most applications.

I would change the parallel resistor to 27K 2W, or 56K 1W. When I was a kid, the old crusties, had a rule of thumb, about resistor derating. In an enclosed environment, limit the power actually dissipated to 1/4 the rated value. A 27K will draw 0.5W, so use a 2W minimum. Resistors are cheap.

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