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Old 15th July 2012, 11:08 PM   #1
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Question Relay or switch mains

Hi all,
I am working on a amplifier build, it's still in the cerebral stage. I had the idea to put a relay in the back of the chassis for mains switching. I like the idea that 240 ac doesn't travel to the front of the box for noise reasons. What do you think?

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Old 15th July 2012, 11:16 PM   #2
DF96 is offline DF96  England
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A relay won't do any harm if the right relay is used in the right way, but a front-panel mains switch is the usual way of doing it. Just keep the mains wire away from signal circuitry.
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Old 16th July 2012, 12:04 AM   #3
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Peavey uses a TO3 package triac on the main hot with the heat sink screwed to the case. Legs are .250" spade pointed in. Front panel switch is light duty. Jameco.com sells the triacs in that case but they don't stock in Aust. Peavey might stock in Aust., one amp using it is the PV1.3k. It is not on the schematic so I don't have a part number. Don't forget an EMI snubber on the gate and MOS transient supressor clamps on the main line. .
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Old 16th July 2012, 09:43 AM   #4
DF96 is offline DF96  England
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If you use a triac you add switching transients which you then have to get rid of again. Transients are much worse than plain mains AC. Guitar or PA is less worried about a bit of noise, though.
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Old 16th July 2012, 10:11 AM   #5
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Or use a triac and a relay in parallel with a delay, the relay avoids the triac misbehaviour at the crossover points, the triac saves the relay from wearing down.
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Old 16th July 2012, 10:16 AM   #6
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surely the transients are only when you turn it on or off?
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Old 16th July 2012, 10:22 AM   #7
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It's AC, goes through the zero crossing twice every cycle, makes 100-120 times a second for a triac.
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Old 16th July 2012, 10:31 AM   #8
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good to know!
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Old 16th July 2012, 11:05 AM   #9
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sorry if i seem a bit dim here but this knowledge also applies in another part of my life,
I assumed a Triac is on or off and not switching each cycle.I also assumed a relay would be good for 1000's of switches? As i hope I imply,I don't know and it would helpful with a non audio applicatition,a packaging machine that needs to switch "silently"
Thanks
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Old 16th July 2012, 11:32 AM   #10
Mooly is offline Mooly  United Kingdom
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There are many ways to drive a triac. It's best to trigger the device constantly using a "high" frequency gate drive.

Eva has a brilliant example of this here,
Soft-start circuit with no relays and no aux. transformers

I think this was the original diagram as the image seems to have disappeared,
Attached Images
File Type: jpg Eva Soft Start.jpg (33.5 KB, 142 views)
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