12v transformer for relays

[lowell]

hey guys,
got a 12v transformer (which is actually 14vac), 2 1n4007 (1 on each secondary wire) with anode towards transformer, then into 50uf 50v electrolytic. I'm only getting a few hundred mv on the cap..?? Yes the cap has correct polarity. I then ditched the cap to see if it was the problem and it wasn't. The primaries are on the hot and neutral mains IEC receptacle fyi. I checked for ac before the diodes and I was getting 7vac on both anodes but now am on only one anode and the other is only 1.4vac now. I'm confused. Maybe I'm missing something stupid.

[lowell]

perhaps it's because the secondary has no center tap?

[Enzo]

You need to either use a bridge rectifier or four diodes, or you can go half wave with one diode. It is just a relay supply, nothing to do with the signal path, right?

[lowell]

right.

[lowell]

ok converted it to full wave bridge... now it's 18vdc...? will check data sheet to make sure that's cool w/ the relays.

[Enzo]

Are they 12v relays?

If so, slap a 7812 voltage regulator in there. Make sure to heat sink it.

7812 data sheet will have typical circuit.

[cbarrow7625]

18VDC unloaded, right? What is the voltage with a simulated load similar to the load that the relay coils will place on it? You can't go wrong with a 7812, just making sure you have to use it.

[lowell]

it's a DS2Y relay and its max switching voltage is 220vdc. guess the 18vdc will be fine and leave out the regulator... right?

[cbarrow7625]

220vdc? Are you sure that is not what the contacts are rated for? I doubt the coil will be happy with that.

The coil usually has a max voltage & dropout voltage rating (minimum voltage required for turn on). For a 12VDC coil I would expect something like 9-15VDC to be the specification, not 220VDC.

Like an LED, relay coils are driven by current not voltage. A simple series resistor with the appropriate dissipation can step it down for you to the proper current (i.e., so you measure 9-15VDC across the relay coil when energized in all situations). This would be the "simple & inexpensive" route.

[lowell]

ok totally, yeah that makes sense that that's the contact rating. thanks! max allowable voltage is 24! good there I guess.

[lowell]

Just thought I'd post in case anyone uses relays and runs into this issue. I had some pretty bad hum in my OD channel. Turns out the source is the relay supply. I traced the hum w/ my scope and it went all the way back to the output of the first relay I have. I alligator clipped in a 510ohm and an extra 50uf and 50v cap to form a pi filter for the relay supply and nipped that problem in the bud. All I hear w/ the OD channel now is that hiss. I'm thinking it's from the plate resistors and tubes and cannot really be rid of. I may try 1watt plate resistors. At this point I'm extremely impressed at how quiet my new build is. And proud to say it's my first design! Pi filtering the bias supply, and raising the heater center tap were also VERY KEY to the quietness of this amp.

[cbarrow7625]

Agreed. I usually go for a PI bias filter as well. Good point about filtering the relay supply well. I'll bet a lot of folks overlook that.

Here is a trick I use: turn the relays off to turn the channel on. Why send 12VDC straight at the channel you are going to be listening to? Use DPDT or SPDT relays and you can send the 12VDC to the channels that you are MUTING. That way, all the DC ripple noise (and relay coil field noise) can be muted as well - none gets injected into the active signal.

You still want to filter the DC pretty good though. You still don't want stray interference from any layout issues.

[Enzo]

What you particularly don;t want is the ground return of the relay sharing any circuit path with the ground of the audio stuff. That adds ripple currents from the relay to the system.

[lowell]

Enzo I have the relay coil grounded at the input with all preamp grounds on a lug right under the input jack. The relay 14vdc supply filters are grounded near an output tube socket w/ the HT CT....?? Good? Bad? I'm not currently bothered by any switching noise.

[Enzo]

If it works it works, or if it ain;t broke don;t fix it.

I was worried more over hum/buzz when the relay was energized rather than switching noise per se. But grounding the coil at the input jack and the supply ground at the other end of the amp means that the relay coil current flows through the same ground return path as the preamp grounds. That would be poor practice. At least potentially. The coil is not in the signal path in any way, so there is no point in having it grounded to the signal path. I would have sent the coil ground down a wire to its poweer supply ground.

I have had to repair buzzing amplifiers for this exact reason.

But "would have" doesn;t mean to do so now if it works well for you as is.