[tommyc]

I made some edits above as I realized my napkin math was for a bridge rectifier instead of a full wave. The theoretical output current is higher, so the B+ supply could in theory run the coils. But I still don't like it for reasons I outlined above (namely, putting more work on the tube rectifier to supply the current for the coils). I think it makes more sense to use a low voltage winding to power the coils, and I would design around using the excess current available on the 6SL7 heater winding.

So.. I breadboarded the circuit and with 4 1N4007 and a 220uf cap rectifying a 5VAC source(i don't have a 6VAC supply), the 6 relays plus the bridge takes about 130mA when on and 5mA idling. With the 5vac source, rectified dc drop to 4.3v under load(pretty much just the voltage drop of the diode). I would imagine having a 6vac source, the dc under load would fall somewhere in the 5v range. Since the G6K is rated for 150% Vmax, this simple little bridge should be OK?.....

 

[L0rdGwyn]

So.. I breadboarded the circuit and with 4 1N4007 and a 220uf cap rectifying a 5VAC source(i don't have a 6VAC supply), the 6 relays plus the bridge takes about 130mA when on and 5mA idling. With the 5vac source, rectified dc drop to 4.3v under load(pretty much just the voltage drop of the diode). I would imagine having a 6vac source, the dc under load would fall somewhere in the 5v range. Since the G6K is rated for 150% Vmax, this simple little bridge should be OK?.....

Sorry I'm working night shift this weekend so not really able to respond quickly.

That might work, but you're going to have a ton of ripple. My opinion is to use a 3.3V LDO regulator. There are lots of options and they can be very cheap, a simple 3-pin regulator in a TO-220 package, a bridge rectifier and two capacitors and you will get a perfect 3.3V output no matter what the fluctuation in your wall AC or the number of coils active (keep in mind current draw on the supply will change depending on which output is selected).

Fixed output LDO regulators are very easy to implement, you just need to read the datasheet and application notes.

Here is an example using an LT1085 3.3V regulator from Analog Devices. This one is on the more expensive side, around $10, but most from Texas Instruments, Onsemi etc. are around $2, this is just the one I found the LTSpice model for quickly.



LTSpice operates in peak voltages, so the input is 8.48Vpeak which is 6VAC RMS. I used 1N4007 since that's what you have on hand, but you could also use a all-in-one bridge rectifier, or even Schottky diodes which will have a lower Vf. The green trace is the input voltage at C1. The dropout voltage for this particular regulator is actually on the higher side, around 1.3V. That means the voltage drop across the regulator must be at least 1.3V or the regulator will stop regulating properly. That means the trough of the ripple at the input (green trace) must not be less than 3.3V + 1.3V = 4.6V. For that reason, C1 must be relatively large to keep the ripple low, the reason I used 1000uF. The 150uF output cap is per the LT1085 datasheet to ensure output stability.

You can see the output trace, in blue, is very low ripple, a perfect 3.3V and it will be 3.3V no matter how many coils are being used as long as the input voltage never drops below 4.6V, which it should not even with a sag in your wall AC of as high as 10%. In this example, the regulator is dissipating around 0.5W, which is fine for a TO-220 package, might add a little clip on heat sink.

So anyway, that's my $0.02 on how at a high level I would implement a 3V supply for the coils of a relay switch.

 

[funch]

This might be an option:
https://www.amb.org/audio/sigma25/

 

[L0rdGwyn]

This might be an option:
https://www.amb.org/audio/sigma25/

Yeah same idea. I had thought to add a load resistor for when no coils are connected (similar to R1 in that schematic) but figured we'd discuss that if this is something @tommyc wants to pursue. At some point I might just put together something like this myself, just don't know when as I have a lot going on at the moment.

 

[tommyc]

Sorry I'm working night shift this weekend so not really able to respond quickly.

That might work, but you're going to have a ton of ripple. My opinion is to use a 3.3V LDO regulator. There are lots of options and they can be very cheap, a simple 3-pin regulator in a TO-220 package, a bridge rectifier and two capacitors and you will get a perfect 3.3V output no matter what the fluctuation in your wall AC or the number of coils active (keep in mind current draw on the supply will change depending on which output is selected).

Fixed output LDO regulators are very easy to implement, you just need to read the datasheet and application notes.

Here is an example using an LT1085 3.3V regulator from Analog Devices. This one is on the more expensive side, around $10, but most from Texas Instruments, Onsemi etc. are around $2, this is just the one I found the LTSpice model for quickly.



LTSpice operates in peak voltages, so the input is 8.48Vpeak which is 6VAC RMS. I used 1N4007 since that's what you have on hand, but you could also use a all-in-one bridge rectifier, or even Schottky diodes which will have a lower Vf. The green trace is the input voltage at C1. The dropout voltage for this particular regulator is actually on the higher side, around 1.3V. That means the voltage drop across the regulator must be at least 1.3V or the regulator will stop regulating properly. That means the trough of the ripple at the input (green trace) must not be less than 3.3V + 1.3V = 4.6V. For that reason, C1 must be relatively large to keep the ripple low, the reason I used 1000uF. The 150uF output cap is per the LT1085 datasheet to ensure output stability.

You can see the output trace, in blue, is very low ripple, a perfect 3.3V and it will be 3.3V no matter how many coils are being used as long as the input voltage never drops below 4.6V, which it should not even with a sag in your wall AC of even 10%. In this example, the regulator is dissipating around 0.5W, which is fine for a TO-220 package, might add a little clip on heat sink.

So anyway, that's my $0.02 on how at a high level I would implement a 3V supply for the coils of a relay switch.

Yeah... after playing with the bridge a little more this morning, i wasn't very happy with the ripple either... I've ordered a few MAX882 to play with, it only has 200mA at 3.3v, but it is the fastest LDO i can get my hands on and should be enough for 6 relays. Also ordered some 3V G6K relays. Will breadboard it in a couple of days when they come in. I actually had a load resistor and led on the breadboard when i was setting it, accidentally pulled it off with the MM's alligator clip .

@L0rdGwyn, Thanks for spending time helping me out. Will report back once the LDOs come in.

 

[tommyc]

Yeah same idea. I had thought to add a load resistor for when no coils are connected (similar to R1 in that schematic) but figured we'd discuss that if this is something @tommyc wants to pursue. At some point I might just put together something like this myself, just don't know when as I have a lot going on at the moment.

This is kind of crazy.. out of curiosity I went and check how much LT1085 are over here. Apparently, I can get them for about $0.8 each and will arrive in 3 days... so 10 of them will be arriving Tuesday... even before the max882..

 

[L0rdGwyn]

This is kind of crazy.. out of curiosity I went and check how much LT1085 are over here. Apparently, I can get them for about $0.8 each and will arrive in 3 days... so 10 of them will be arriving Tuesday... even before the max882..

That's convenient! Just make sure they are TO-220 package and 3.3V model.

 

[cats4cans]

This is kind of crazy.. out of curiosity I went and check how much LT1085 are over here. Apparently, I can get them for about $0.8 each and will arrive in 3 days... so 10 of them will be arriving Tuesday... even before the max882..

I’m starting to get the idea I may not be cut out for DIY. I’m still in the daydreaming stage, but was thinking to do something similar. First I thought about adding an additional deck to the Elma switch and extending the PCB. But of course all that fiddly soldering was the kind of thing the dexterity challenged like me want to avoid. Then since I’d be the only one using it, I’ve been trying to convince myself that a series of toggle switches would be just fine. Of course I think there could be a big downside in case of operator error. Sometimes the idiot you need to design around is yourself when you’re distracted.

Relays seem like a much more robust design and ignorance resistant to boot. Interested to hear how it turns out.