What size capacitor to couple Radio Shack FM Intercom?

I am attempting to use a pair of Radio Shack FM intercoms between the 2 sides of my house wiring.
I understand that a high pass coupling capacitor would allow the FM to cross between the lines.
What size AC capacitor would do the job?
What would the negative effect be if a large capacitor such as one used to start a 220 VAC motor is used?
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On Wed, 25 Feb 2009 16:44:29 -0800 (PST), snipped-for-privacy@gmail.com wrote:

a good start.
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On 2/25/2009 4:44 PM snipped-for-privacy@gmail.com spake thus:

I think you're confusing physical size with electronic size. You actually need a fairly small capacitor, somewhere in the 0.01 uf range, I'm guessing (someone else here can supply a more accurate figure). It needs to be rated for at least the voltage applied to it, 120 volts in this case: probably a 200 volt capacitor would be good. But you definitely *don't* want one of those big honking motor-start caps here.
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spake thus:

You are correct on using a capacitor of .01 or so, but way too low on the voltage. The peak voltage of a 120 line is about 170 volts. Any small spike will exceed this. The capacitor should be rated at 600 volts or more when used on a 120 volt AC line.
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spake thus:

hot sides of the panel. This would make it 240V rms or about 340V peak I think. A 600 volt capacitor should be adequate, a 1000V one couldn't hurt.
Don Young
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If a 220 VAC appliance is on, say an oven, should the FM intercoms on opposite sides operate?
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If a 220 VAC appliance is on, say an oven, should the FM intercoms on opposite sides operate? I don't know but I suspect that most 240V appliances do not provide very good paths between their terminals for the radio frequencies used in these intercoms. It is a valid idea, though, and it could very well work, it seems to me.
Don Young
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Ralph Mowery wrote:

P-P is ~240V, not 170V. 600V rated dksc cap is quite common.
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That's 240 volt RMS (Root mean square) that means peak voltage between A and B legs is of order of 340 volts.( 240 x 1.414 = 340 ) On other hand the peak voltage between either leg and neutral/ground is of order of 170 volts. ( 120 x 1.414 = 170 ) Not including surges, spikes, transients etc. Believing that that those intercoms work at FM type frequencies (30 megahertz plus etc.) we are talking capacitors in the 0.1 microfarad (mfd.) or 0.01 microfarad range. So 600 volts or better, 1000 volts, DC rated caps are suggested. Would see no point whatever using an AC rated motor starting capacitor in the size range of 3, 10 or 40 mfd.
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wrote:

P-P is 340V.
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On Feb 25, 6:44pm, snipped-for-privacy@gmail.com wrote:

I agree with others (.01mfd) but would up the voltage greatly, use . 01mfd ceramic disc rated 1000 volts just to be safe. The cost for 1000v capacitor will be about the same anyway.
Or just get an X10 phase coupler, they are already packaged as a device that will mount in a std junction box.
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wrote:

For the benefit of those considering this.....
In the 1990s, I installed a 600 WVDC high quality .01 uF capacitor from Cornell Dubilier, an excellent part. It solved the coupling problem beautifully. However, as a result of line surges, transient spikes, etc., the 600V part broke down and failed within a few months. I replaced it with another 600V part and it too failed within months. The 1000 working volt part I eventually bought (Cornel Dubilier also) has been installed for at least 10 years now with no problems.
My point is that a 240V RMS source across the capacitor can occasionally encounter spikes which will cause the 600WVDC part to fail. The 1000 WVDC part is a much better choice.
Smarty
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wrote:

I agree. DONT use a 600v WVDC capacitor!
On a 220vac line the peak to peak voltage is 2 x 1.414 x 220v 622vpp.
It's not occasional transients that destroys the cap, its that its working voltage is being exceeded whenever power is applied.
Use at least a 1000v cap to be safe.
-dickm
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wrote:

They (the opposite hot wires) are 1.414 * 220V apart. The extra doubling would be appropriate for 440v systems.

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wrote:

If you are seeing those kind of transients on your power lines you need some surge protection. These days I believe everyone needs surge protection. Electronics are not as hardy as the old tube or discrete transistor stuff was. You are also interconnecting cable, phone and power. It is not a question of if you will get blown up, only when.
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Thanks for the excellent suggestion. I actually did install a "whole house" surge suppressor in the entrance panel, and it hopefully will provide the required protection.
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On 2/26/2009 6:56 AM RickH spake thus:

Probably a good idea. Here's how I would do it:
Get the capacitor (like this one: http://www.allelectronics.com/make-a-store/item/332D1K/.0033-UF-1KV-DISC-CAPACITOR/-/1.html ). (Of course, since the cap is only a buck for 10, you'll probably need to order some other stuff from them as well.) Also get a 4x4 square box and cable to connect it to the panel.
What I would do is to solder 2 leads to the capacitor. The leads should be the proper gauge of wire sized to the breaker (#12 for 20 amps, #14 for 15 amps). To protect the capacitor and wiring, either cover the whole thing in several layers of heat shrink tubing, or put it inside a length of clear plastic tubing large enough to cover the whole deal (the capacitor's pretty small). Wire the capacitor between two "hots" on opposite sides of the panel and you're done.
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