Relay to turn on amp using 12v trigger - diyAudio
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Old 27th June 2012, 12:58 PM   #1
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Default Relay to turn on amp using 12v trigger

I have (will shortly have in truth) a 12V output on my preamp and want to use it to turn on a power amp I am working on.

I guess that I should set the power supply up as follows: IEC socket > Fuse > Manual Switch > Relay > Transformer > ...

However, this will mean that the amp will be off without 12V. I do not want to set things up so the manual switch can be off but the amp will be on due to the relay (it would be dangerous in my opinion). I have been thinking I need a second manual switch that 'bypasses' the relay.

Can anyone say how this is 'normally' done?

Also I would assume it is best to switch the active and neutral at all switch points (relay included) too. Finally, I propose to put a power socket on the back of the Amp to power my crossover. It would go in at the same point in the circuit as the transformer. Comments?
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Old 27th June 2012, 06:07 PM   #2
macboy is offline macboy  Canada
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Consider a dual throw (SPDT or DPDT) switch: ON-OFF-ON. Center is off, up would be "on", bypassing the relay, down would be "automatic" mode, going through the relay and requiring the 12V trigger.
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Old 27th June 2012, 09:39 PM   #3
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I could mark the switch clearly as 'On-On' I suppose. I have an IEC socket with integrated fuse holder and switch which would ensure (for example) that no-one would start trying to change a fuse while the amp was on.

I believe the standard connection for a 12V trigger wire in hifi is a mono 3.5mm headphone style socket. Is this right? I have for example checked out a Rotel manual that says:
TRIGGER OUT jacks to the 12 volt trigger input of a Rotel amplifier, using a cable with mono 3.5 mm mini-plugs on both ends. The +12 VDC signal appears at the “tip” connector.
I assume this is the headphone plug we know and love. A Rotel Amp manual says:
With the switch in the 12V TRIG. position, the amplifier is turned on automatically when a 12 volt trigger signal is applied to the 3.5mm Jack labeled IN. The front panel POWER SWITCH overrides this function. It must be ON for the +12V trigger to work. Turning the switch OFF cuts power to the amplifier, regardless of whether or not a trigger signal is present.
I would prefer not to use a front panel switch though (I am containing all the 240V wiring in a metal enclosure inside my amplifier enclosure which is likely to be wood). I suppose I could, to increase notification of the amp's power on status, add power LEDs front and back.

Last edited by Mickatroids; 27th June 2012 at 09:42 PM. Reason: Formatting
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Old 28th June 2012, 12:31 PM   #4
macboy is offline macboy  Canada
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Quote:
Originally Posted by Mickatroids View Post
I could mark the switch clearly as 'On-On' I suppose. I have an IEC socket with integrated fuse holder and switch which would ensure (for example) that no-one would start trying to change a fuse while the amp was on.

I believe the standard connection for a 12V trigger wire in hifi is a mono 3.5mm headphone style socket. Is this right? ...
Many IEC sockets with fuse holders that I have seen have a lockout that prevents the fuse from being changed if there is a cord in the socket, like a sliding cover that can only slide away when the cord is not there.

I think that the 3.5 mm jack is very common for a trigger, with +12 V on the tip. Try to isolate the jack's sleeve/ground from the chassis or other grounds in your system (to help avoid ground loops).

A "power" LED is a good idea, and you could add a "standby" LED too. That would indicate that AC is present but the amp is off due to the trigger state. Consider powering the "standby" LED from the normally closed contact on the relay, and the "power" LED from the overall switched power (i.e. the normally open contact of the relay).
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Old 28th June 2012, 11:08 PM   #5
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Thanks macboy

My fuse holder is of the type you describe. However the integrated switch is DPST so I am going to need a second switch after all.

My plan now is to make it clear that the switch on the IEC/switch/fuse combo is the master switch and to include a green LED that is on whenever that master switch is on. I will then create two 'paths' from the switch to the transformer. One via a relay and one via a second switch.

It does occur to me though that with the second switch and relay being DPDT there is the possibility of unused terminals becoming live. For example if the trigger bypass is engaged (so the amp is always on) and the relay is off the unused terminals of the relay will be live. I will insulate them and the switch terminals accordingly!

Edit: thanks for the tip on ground loops!
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