Q: Correct voltage drop resistor for 12v relay voltage (SilentArts PSU)

Hi friends, I'm planning to power a single 1272 using this PSU schematic courtesy of Silent Arts. One rail for phantom, one for +24v, and the fixed regulator for 4 relays for phantom, phase, etc. I'm trying to find the value of R3. Can you tell me if I'm off in my calculations?



The relays are Omron G5V-2-DC12 which draw 500mW.
I *think* thats 41.6mA per relay, or .166A for all four.

My transformer secondary is 22v which I figure becomes 31v after the bridge.

After a page full of scribbles, I came up with 186Ω to give 15v before the fixed regulator.
Am I close at all?

Your contributions are appreciated,  Matthew

What happens if you make R3 zero?

> 31v after the bridge

Regulator takes 31V and wastes 21V to make 12V.

Load is zero to 0.17 Amps.

Regulator would dissipate zero to 3.57 Watts.

The TO220 parts will dissipate 4 Watts with a few square inches of thick metal.

R3 is not absolutely needed.

> 186 ohms to give 15v before the fixed regulator.

Yes, that nearly works. Regulator dissipation runs from zero, to 0.87W on two relays, to 0.5W on 4 relays. Now the reg does not need a heatsink (though it will run HOT naked); but the resistor has to dissipate 3.06W (not the "2W" shown).

3V is the reg's drop-out. And 0.166A is nominal... bad day in the factory, the relays might pull a bit more. Also what if you have to work where there is only 108V in the wall, your "31V" something less?

Since it will work with no resistor, or up to 186 ohms (with no margin), I'd pick something in the middle. 100 and 150 are standard sizes. I'd do math on 150r against a +20% relay load and a -10% wall voltage. OTOH, if the reg input sags to 13V and the "regulated 12V" sags to 10V, the relays will hold-in and the few-Volt ripple "probably" does no harm. (None to the relays; but if low-level signals run nearby the ripple could sneak into the audio.)

And consider worst-cases. Someone drops a screwdriver (or coin) on the relay board. With no resistor, the reg will max-out over 1 Amp, dissipate nearly 30 Watts. Unless you use a motorcycle head for heatsink it will soon shut-down harmlessly (if it don't shut-down soon, you should ask if your transformer can stand an extra 1+A sustained). If you use a 100r resistor you have about 31V-3V= 28V across the resistor, 8 Watts dissipated though only 0.28 Amps flowing. Plus 21% when wall-voltage is 10% higher than nominal. A 10W part is OK for quick accidents. If it may be left shorted while you look everywhere else for the problem, a 20W resistor is needed for long-term abuse resistance.

Not only was that helpful, but I laughed my ass off at the end. - at the thought of a full abuse 20w resistor.

Thanks for helping me think through that, and considering the margins. I should definitely give the regulator a little more headroom to work with in case of house voltage sag. I'll have to see what I have in stock to fit the bill.

PRR, you're the man.

Hi,

I would like to understand the meaning/ function of R4 and C16. Anybody? Why is it drawn separate from the rest of the stuff?
Its connected from ground to.... what?

Between chassis ground and PCB ground.

It's a ground loop breaker