Power supply for input selector relay.

[plasnu]

This is my first question in this forum, a question about relay input selector power supply. The preamp will have +15 and -15 rail for the chips. I found a schematics (attached) which seems to be OK for the +12V relay power, but I'm not really sure that if it's OK just to draw some current from +15V line.

Any ideas?
Thanks!

[indianajo]

Relays produce a lot of nasty high frequency noise (sharp edges) that couples into everything. This is true even if you use a back diode. People that design pro things like packaging machines put the computer and instrumentation on +5, +-15 supplies, and use an entirely different supply with a different transformer for the relays. Then they put the devices in separate steel boxes, and separate the two with optoisolators to drive the relays. You should at least use a separate power transformer and rectifier for the relay. Don't see a relay in your schematic. People that design SS front ends for Leslie motorized speakers go through this all the time. Cheap causes problems. A cheap source of separate transformer for the op-amp circuits is wall transformers from the charity resale shop. A suplus HP printer 30V @ 4A wall transformer puts the nasty transformer outside your box. The relay and its 12 or 24v supply should have its own steel box. Don't know if the LM317 works okay without a center tap, I used two 1n5344 zener diodes producing a fake "ground' in the middle of 18 VDC for the four op amps in my disco mixer.

[Mooly]

Originally Posted by ayumi

This is my first question in this forum, a question about relay input selector power supply. The preamp will have +15 and -15 rail for the chips. I found a schematics (attached) which seems to be OK for the +12V relay power, but I'm not really sure that if it's OK just to draw some current from +15V line.

Any ideas?
Thanks!

I think it will be fine... just be sensible how much current you draw (which applies no matter what it feeds). I assume you are talking about small signal relays.

Make sure the "zero" return for the relay coils goes back to the PSU and not anywhere that current spikes could contaminate a signal ground. In other words couple to TP14 as shown in your diagram.

[boyofwax]

You can use the supply as shown above. A 3V dropdown (15V to 12V) can simply be achieved by using a resistor (calculate or measure). It is only for 1 relay at a time I presume? An 7812 or 7912 can also be used to make a separate supply from the rectifier part.

What indianajo writes is true, but I would not be so concerned of noise switching for a small relay in the preamp.

[Nico Ras]

Why don't you draw the 12V regulator to feed the relays from both the -+ power supply, then they remain loaded the same and your noise remains symetrical to both supplies.

[Miniwatt]

Originally Posted by indianajo

Relays produce a lot of nasty high frequency noise (sharp edges) that couples into everything. This is true even if you use a back diode. People that design pro things like packaging machines put the computer and instrumentation on +5, +-15 supplies, and use an entirely different supply with a different transformer for the relays. Then they put the devices in separate steel boxes, and separate the two with optoisolators to drive the relays. You should at least use a separate power transformer and rectifier for the relay. Don't see a relay in your schematic. People that design SS front ends for Leslie motorized speakers go through this all the time. Cheap causes problems. A cheap source of separate transformer for the op-amp circuits is wall transformers from the charity resale shop. A suplus HP printer 30V @ 4A wall transformer puts the nasty transformer outside your box. The relay and its 12 or 24v supply should have its own steel box. Don't know if the LM317 works okay without a center tap, I used two 1n5344 zener diodes producing a fake "ground' in the middle of 18 VDC for the four op amps in my disco mixer.

I never heard this, but it might explain why I didn't like the relay attenuator I used for a while, although it only switched when changing the volume.

[Nico Ras]

Better still, add another bridge and draw it from the transformer directly, so each power supply comes from the transformer and the switching noises will be reduced significantly

[plasnu]

Thank you for the quick replies. I'm really surprised so many different opinions about this. I expected there is some kind of common way for the relay PSU, but it seem like there isn't.

The unit will have 5 relays daisy chained for balanced 5.1. Each relay will have a back diode.

Would it be better taking 30V directly from 2x15V transformer tap (1 and 4 in the picture) , then bridge and +24V regulator to drive 24V relay? The relay coil is not grounded, though. 24V relay seems to draw less current then 12V, but it may do not matter at all...

The unit will have 5V PSU with separate transformer and regulator for digital control chips, so I don't really want to see another transformer in the unit. I can order custom wound transformer (5V 12V 15Vx2), but I'm afraid if it would be overkill.

[indianajo]

JimmyWilliams just went through this on a Leslie interface, see Op-Amp question Separate transformers means to me a transformer with a different core and different steel box for the nasty relays and fans. I'm building a SS organ power amp, preamp power (100 VDC @ 100 ma) with stop switching relays and fans. I'm putting the unshielded 100 VDC transformer and the 24VCT transformer in a PCAT box this week. The three surplus steel encased power amp transformers, and something to produce +-8V for op amps, will be going in an aluminum master chassis, since I have about 10X as many aluminum and stainless steel boxes as ferous steel. 24 VCT transformers are easy to come by in factory surplus, since it is the standard DC relay power supply for factory equipment. +5 +-15 packaged supplies like from Power1 are standard for factory instrumentation like scales, but usually leave the factory blown up, so I'm buying power transformers surplus. Not compact, but more so than the +450 VDC @ 2A power supply I'm replacing.

[Rundmaus]

... let me ask a n evil, evil, really evil question:

Who the **** cares about switching noise "coupling into audio circuits" from a relay that switches only to select a different source?

To be honest, I do not really judge the sound quality of my setup in the instant it switches to another source....

Use a separate transformer if you have a small one available, use the existing +-15V if not.

Greetings,
Rundmaus

EDIT: I just read that you have a +5V supply for digital circuitry already planned. Why don't you use this for relays with a 5V coil, as they are readily available?