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On/Off switch using momentary push button
On/Off switch using momentary push button
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Old 10th November 2011, 09:09 AM   #1
polsol is offline polsol  South Africa
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Default On/Off switch using momentary push button

I am in the midst of designing something of a 'universal power supply' for audio power amplifiers in a seperate enclosure.
Most electronics these days seem to incorporate a 'push on, push off' momentary switch design rather than a latching switch.
Any references of how to design this?

The switch I am considering using (latching) is a 3 amp/250V coupled to a 300 VA 25-0-25 torroidal. However, would prefer the 'push on, push off' scenario. I guess there should be no problem with the latching switch handling the 1.25 Amps @ 240V nominal current but am a little concerned about surge currents at switch on so using a relay might be the better option - I mention this as possibly a relay would be an integral part of the first question requirements.

Your thoughts would be appreciated.
Tony M
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Old 10th November 2011, 09:43 AM   #2
DF96 is offline DF96  England
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You can get mechanical switches which incorporate a 'push to change state' operation, but whether these are available in panel mounting form I don't know. Single-pole versions are sometimes used for lighting, but you really need double-pole for safety. You basically need a flip-flop. The advantage of doing this mechanically is that Off really does mean Off; an electrical flip-flop needs a power supply.

Personally I prefer the certainty of a proper toggle-type switch, whatever external shape it has.
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Old 10th November 2011, 01:06 PM   #3
lcsaszar is offline lcsaszar  Hungary
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I use a RAFI illuminated panel mount switch for exactly the same purpose on my preamplifier.
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Old 10th November 2011, 04:59 PM   #4
AndrewT is offline AndrewT  Scotland
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Use relays to achieve the latch ON latch OFF action.

There are circuits on the Forum.
I seem to recall they use three ordinary (cheap) relays to achieve latching with a momentary contact switch.

All the switching and relays can use low voltage DC with just the final Mains relay at high voltage.

Is this the mechanical equivalent of a bi-stable?
regards Andrew T.

Last edited by AndrewT; 10th November 2011 at 05:12 PM.
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Old 10th November 2011, 05:09 PM   #5
theAnonymous1 is offline theAnonymous1  United States
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A latching relay would work a charm. Too bad they're so expensive.
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Old 11th November 2011, 05:12 AM   #6
polsol is offline polsol  South Africa
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Thanks all for the input.
Ultimately I guess it's easier just to use a DPST pushbutton so will order a Schurter MSM 19 LA. Bit of an overkill - and expensive - but it does handle 12A @ 250V on the contacts.
Just rather keep to the KISS principle and avoid over complexity.
Tony M
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Old 11th November 2011, 05:40 AM   #7
CBS240 is offline CBS240  United States
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Along with the flip flop you need to be sure it is de-bounce or you have to add a small cap across the switch. A one shot can be an advantage. But then you have more complexity. But then it is easier to interface protection detection circuits for auto shut-down.
All the trouble I've ever been in started out as fun......
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Old 11th November 2011, 08:20 AM   #8
Elvee is offline Elvee  Belgium
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This circuit includes the debouncing, and you can make make six of them out of a single IC.
It was made for automotive applications, but can work from 3 to 15V if you remove the supply filter.
You can use a smaller MOS or a bjt to drive a relay.
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Old 11th November 2011, 08:25 AM   #9
Boscoe is offline Boscoe  United Kingdom
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Jk flip-flop also works well driving a relay, your momentry button has to be two way though as you need to send the input high then down to ground.
Audio projects and more: gswdh.co.uk
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Old 15th November 2011, 08:22 PM   #10
TechGuy is offline TechGuy  United States
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Perhaps you're looking for something like this (using D Flip-Flop)
Single-Pushbutton ON/OFF Power Control - Maxim

other debouncing circuits using logic gates.
Switch Debouncing - The Lab Book Pages

You also need an High side mosfet driver with built in charge pump to retain the boostrap voltage. Something like this:
LM9061 - Power MOSFET Driver with Lossless Protection

You'll need a small aux DC power supply to power the logic gates and high side driver.

A small saturable inductor could be used in serious with the high side switch mosfet to limit initial current rush. Alternative you could couple the power switch with AC zero crossing so that the mosfet only turns on just after the zero crossing for a slow start. Or you can use a high value gate resistor (perhaps 100 to 1K ohms) but use a parallel diode to speed up gate turn off.
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