Power a relay with a zener diode

[CommanderLake]

I've been designing a 7.1 amplifier board (7 amps and sub out) with built in volume control and I want to add some protection with a DPDT relay that switches the + and - of the smoothing caps to a large resistor to discharge them, the supply will be effectively 61v from + to - so if I use a 47v zener in series with a 12v relay it will only turn on when both capacitors are charged, this should protect against power on transients if the charge of the capacitors is uneven and any DC at the outputs after power off and if one of the fuses blow on either supply rail the caps will be cut off.

I will need a relay thats not center off but looking at THESE on Farnell Its not clear if they are center off or not, take THIS for example I cant find anything in the datasheet (link on product page) that says if its center off?

So what do you think of the zener idea?

 

[(*steve*)]

Well, no relay is centre off, and a zener doesn't power anything.

It sounds like you want to know if they're break before make, and use a zener to drop 47 volts or so.

The zener will do the trick, but it will probably draw almost as much power as your resistors would if always connected, so you could probably save some effort and just connect the resistors across the caps.

I'm not sure that it matters if the relays are break before make because it seems to me that there is no real risk if they don't.

I think your proposed circuit would be OK. The relay would turn on at before Vz +12 (because the relay probably turns on with 8 volts or so, and would turn back off at Vz+5 or so volts because relays will stay pulled in at a lower voltage. Neither of these is a concern.

Make sure the zener is appropriately rated for the power it must dissipate (because if it fails it is likely to fail short circuit -- and the relay isn't going to like that.)

Also make sure the relay contacts are OK for the peak currents demanded by the amplifier (and these could be very high).

It would also be worth testing that applying power this way doesn't cause a thump form the speakers.

 

[CommanderLake]

Sorry that sounded stupid I am fully aware what a zener does and does not do, I'm improving on an amplifier I have recently built by combining everything into a single board, with my current amplifier I get a pop at power on and when powered off the negative cap discharges slightly quicker causing DC on the outputs which causes a much louder pop if I turn it back on again without waiting a while, my speakers have crossovers that protect from DC but still this is something I dont want.

The reason for switching the caps to a resistor is simply so they discharge when the relay disconnects them from the amps, I'm using the Falstad circuit simulator and it shows the zener will consume 1.65w and the relay 0.45w which is fine as I will get at least a 3w zener.

 

[(*steve*)]

Yeah, I figured out what you meant in the end.

Initially I wasn't sure if you meant using a zener to regulate a voltage, or to drop some voltage -- you meant the latter.

You should also discharge the capacitors to ground rather than place a resistor across the + and - of the (presumably split) power supply.

3W for the zener sounds fine. But go for a 5W one

 

[CommanderLake]

Yes wise to discharge to ground.

 

[CommanderLake]

Just to be sure,
http://www.farnell.com/datasheets/419217.pdf
on the 5th page numbered 207 on the datasheet the Terminal Arrangement drawing for G2RL-2 (bottom most) shows pins 2 and 7 are connected I'm assuming this is the normal powered off state yes?

 

[(*steve*)]

Just to be sure,
http://www.farnell.com/datasheets/419217.pdf
on the 5th page numbered 207 on the datasheet the Terminal Arrangement drawing for G2RL-2 (bottom most) shows pins 2 and 7 are connected I'm assuming this is the normal powered off state yes?

do you mean this one?

Pins 2 and 7 are NEVER connected together.

pin 3 is connected to pin 2, and pin 6 is connected to pin 7 when power is removed from the relay

Pin 3 is connected to pin 4, and pin 6 is connected to pin 5 when power is applied.

However, you would be wise to verify this with a multimeter.

 

[CommanderLake]

No I didnt mean they are connected together I mean does the relay switch to them when its not powered.

 

[CommanderLake]

Actually, I'm using a Picaxe for the PGA4311 volume control IC's so I think I will use a 5v relay and a darlington transistor to control it with the Picaxe instead, I'm always having great ideas!

 

[(*steve*)]

The diagram should show as closed the normally closed connections, i.e. the ones which are connected when the relay coil is not powered.