relay power supply questions

[maxkracht]

I’m making a capacitor coupled power supply PCB for relay switching, curious if anyone has incite on best practices. I’m copying a valve wizard schematic that uses the heater supply and puts out 6-7v unregulated. (Tried this on the breadboard and got close to 8v with line fluctuation) I read a couple relay data sheets that want +-10% for the coil, so it seems like a good idea to slap on a voltage regulator. Not much voltage to regulate, and I don’t want it to drop out, so I found a LDO regulator that seems to fit the bill. https://www.mouser.com/ProductDetail/511-L4931CZ50-AP I’m guessing I don’t need near the 1000uf filter merlin was using if I have the regulator, but if there’s a chance voltage is going too low to regulate is it worth having the big cap anyway? Is it silly to go to all the trouble to keep the coil voltage close to spec, or better to drop a bit of voltage another way to get closer to my 5v target?

[nuke]

The LDO should work.

The power supply seems kind of complicated. Usually, the 6.3vac filament winding will be center-tap ground, or virtual center ground with resistors, which is the reason for all of the extra capacitors and diodes.

A simpler solution might be to just not ground the relay supply to the chassis. Then you only need the full-wave-bridge on there, skipping the diode/capacitor pairs. Isolate the relay power and coil circuit from the chassis ground and use a 5v or 6v LDO depending on your relay coil voltage.

[maxkracht]

However unlikely it is to accidentally ground a footswitch jack, i've seen a lot of dumb while doing amp repairs. Four extra parts and a bit of space seems like a fair tradeoff to me.

[Kagliostro]

https://el34world.com/projects/relay_sw ... e_vignette

https://el34world.com/Forum/index.php?topic=14468.0

Franco

[maxkracht]

Already have boards mostly designed, just trying to decide if I did anything too dumb. Definitely a bit overcomplicated but might as well make it nice if i'm already going to the trouble. I'm guessing extra cap on the regulator isn't necessary, but might as well have a pad, and the 1000uf filter off the rectifier could be smaller.

[R.G.]

Check the pull-in and drop-out voltages on those relays. It is common for relays to specify a pull-in voltage of 75% of the nominal coil rating. You might be able to get by with just a resistor in series. Relays are much less fussy about voltages than many other things.

And capacitor-fed-bridges are less "stiff" than you would think. Was your 8V on that breadboard loaded with a resistor equal to the coil resistance of one of your possible relays?

Finally, the proposed circuit bothers me a bit. I suspect that the two caps each build up a DC offset that keeps it running. It's been a while since I messed with one of these.

[maxkracht]

Yes, relay datasheet says it needs 75% to operate. Recommended +- 5% coil voltage but maximum allowable voltage is 150% so it should tolerate 6-7v.



I’m getting around 7v when the relay is on, 8v was without a load. Only dropped another .1v with a second relay.



I don’t have a LDO regulator to test, but I was assuming the 2.2uf stability cap at the output was necessary, the .1uf cap on the input was less necessary if I already have a larger cap in relatively close proximity on the board. Are you suggesting these values might be more critical, opposed to just having something large enough?

[R.G.]

Could you just use 6Vdc or 9Vdc relays? That might just make the issue go away. Mouser says they're the same price.

[maxkracht]

You're right, the relay datasheet suggesting tighter tolerances got in my head. Will just leave a place for a resistor or diode to drop a bit of voltage and call it good enough and/or choose a different relay. Thanks for talking me out of the added complexity.

[Kagliostro]

Each diode you put in series will lower the the voltage of around 0.7V

You can lower the voltage also with an in series resistor, the difference is that the use of a resitor implies a bit of math using the consumption of the relay, this math is not necessary if you use the diode trick

Franco