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Old 1st March 2008, 04:17 PM   #1
pfcs49 is offline pfcs49  United States
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Default how to create a system pwr relay?

Now that I have a huge amp that draws 60W in standby mode, I'm wondering about creating some kind of switching system contolled by a low voltage power supply (battery eliminator) plugged into the acc AC outlet of my preamp that would control the 20A power circuit to the Krell FPB 300. Any thoughts? Am I reinventing the wheel-is something already available? phil
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Old 1st March 2008, 11:14 PM   #2
star882 is offline star882  United States
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You can just use a power relay with a mains voltage coil. They should be available from HVAC parts stores for very reasonable prices.
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Old 4th March 2008, 06:39 AM   #3
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U have to use a relay of the required low dc voltage coil and the contacts should have the load current rating.

Gajanan Phadte
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Old 5th March 2008, 11:47 PM   #4
pfcs49 is offline pfcs49  United States
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I have several push-in 9mm legged automotive power relays, 20-70A.
They have low current 12V control coils, but because they are in a plastic base, and because I've seen so many overheat and the components/contacts move, and because 12/2 mains cables look so much more robust than the contacts and contact arms look so wimpy, I'm not disposed to using them. There's also the issue of mounting and wiring them in an approved box.
Isn't there something available like this?
Re: 120v relay: there's something that scares me about the possibility of crossing 2 domestic circuits (of having the on/off 120v "signal" emanating from a 15A branch/preamp outlet controlling a 20A circuit's outlet. If they are of opposite phase, there would be 220v potential difference. phil
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Old 6th March 2008, 12:08 AM   #5
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DPDT realy with 12v coil (with contacts rated for your load of course). Simple. The second pole can control a standby LED when inactive and an "on" LED when the relay is activated or a SPST if you merely want to switch power with no indication that it has indeed been "switched", which is the case when running an LED off of a second pole on the power/standby switch itself.
If you choose to use a mains coil relay (why? I dunno) you can always use the same circuit for coil and contacts... no worries there.


7/10
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Old 6th March 2008, 04:32 AM   #6
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There is something readily available, the Niles AC-3. A device could also be built very easily using a relay and an AC plug and socket (see attached schematic). When finished, connect a 12VDC AC adapter (wall wart) to the switched outlet of the preamp/receiver. When the preamp/receiver turns on, the relay is activated and the Amplifier turns on.

Richard
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Old 7th March 2008, 12:37 PM   #7
oshifis is offline oshifis  Hungary
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I used Solid-State Relays for this purpose. An SSR has an opto-coupler rated for at least 1000V isolation, a zero-cross switch and an internal triac. It just needs a few mA DC control.

But since I was worried that silicons spoil the sound, I replaced them to ordinary 24V DC relays in my tube amplifier. The heating and bias voltage is permanently on, the relay is powered from the bias voltage through a contact switched by the preamplifier.
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