RELAY POWER SUPPLY

[ViperDoc]

I see Hoffman has a design for a relay power supply board that converts 6.3 V heaters to 5V for the relay. Can you power a relay directly with a 5V individual transformer?

[martin manning]

Of course you can use a small auxiliary transformer, but it needs to be 6.3VAC to produce enough rectified DC for the regulator to function properly.

[ViperDoc]

Excellent. Thanks!

[studiodunn]

Of course you can use a small auxiliary transformer, but it needs to be 6.3VAC to produce enough rectified DC for the regulator to function properly.

Could this transformer be fed from the AC to a 5v regulator for relays?
https://www.mouser.com/ProductDetail/546-166MS

[bal704]

From my experience, relays are pretty tolerant of voltage. A 5 volt relay will work just fine off 9 volts or 12 volts. I'd just get a small cheap 5V supply like this one:

https://www.jameco.com/z/RS-15-5-MEAN-W ... 19043.html

AC to regulated DC in a small package.

[martin manning]

Of course you can use a small auxiliary transformer, but it needs to be 6.3VAC to produce enough rectified DC for the regulator to function properly.

Could this transformer be fed from the AC to a 5v regulator for relays?
https://www.mouser.com/ProductDetail/546-166MS

Too much current, not enough voltage. I like this one better: https://www.mouser.com/ProductDetail/Ha ... 8YHDpOI%3D

[martin manning]

From my experience, relays are pretty tolerant of voltage. A 5 volt relay will work just fine off 9 volts or 12 volts.

I recommend reading the data sheet for the relays you are using. The popular Omron G5V-2 5V relays are rated at 75% to 120% of nominal (3.75 to 6V) for the standard type, 75% to 180% (3.75 to 9V) for the high sensitivity type.

[studiodunn]

Of course you can use a small auxiliary transformer, but it needs to be 6.3VAC to produce enough rectified DC for the regulator to function properly.

Could this transformer be fed from the AC to a 5v regulator for relays?
https://www.mouser.com/ProductDetail/546-166MS

Too much current, not enough voltage. I like this one better: https://www.mouser.com/ProductDetail/Ha ... 8YHDpOI%3D

I'm not sure I understand Martin. If i were to spec the PT to have a 5V tap like this one - http://www.classictone.net/40-18094.pdf - 5v @3 amps would seem correct.

This is the Regulator circuit I am feeding. This is then driving 5 relays and 5- 5m LEDs

MTL V reg.PNG

[martin manning]

Are you planning on getting the AC from a PT with a 5V rectifier winding or using a separate transformer?

[studiodunn]

Are you planning on getting the AC from a PT with a 5V rectifier winding or using a separate transformer?

I need the separate transformer as my PT doesn't have the 5v winding and I don't have enough 6.3v current available to tap off of.

[sluckey]

The input voltage must remain typically 2.0V above the output voltage even during the low point on the input
ripple voltage.

A 5VAC winding and FWB rectifier will produce 7VDC unloaded and unregulated. That's the problem. 7VDC is just on the threshold for a LM7805 to regulate properly. That 7VDC will be bouncing around a bit due to line fluctuations an/or load changes. The LM7805 will be constantly dropping out of regulation and probably putting all kinds of hash on the output.

A 6.3VAC winding will produce 8.9VDC unloaded and unregulated. Now the LM7805 has enough input voltage to put out a solid/clean regulated 5VDC. So, if you must use a 5V regulator chip, give it enough input dc voltage to do it's job correctly.

Having said all that, there is no reason to regulate the voltage to a relay coil. You can use a 5VAC winding with a FWB and a couple 1N4001s to produce an adequate supply voltage for a 5V relay. No regulator chip required.

[martin manning]

Ok, voltage wise you need about 2V of headroom for the regulator, so if you use a bridge rectifier you need a 6.3V filament transformer: 6.3 * 1.414 = 8.9, but then you'll lose two diode junction drops, leaving 7.5, which should be fine. Current wise, each relay needs 30 mA (high sensitivity type), and each LED needs about 10 mA. Assuming they are all on at the same time, that's 40 mA * 5 = 200 mA. I'd choose a transformer rated for 3x that current, or 600 mA.

[studiodunn]

Ok, voltage wise you need about 2V of headroom for the regulator, so if you use a bridge rectifier you need a 6.3V filament transformer: 6.3 * 1.414 = 8.9, but then you'll lose two diode junction drops, leaving 7.5, which should be fine. Current wise, each relay needs 30 mA (high sensitivity type), and each LED needs about 10 mA. Assuming they are all on at the same time, that's 40 mA * 5 = 200 mA. I'd choose a transformer rated for 3x that current, or 600 mA.

Thank you Martin, that makes sense. So it sounds like the transformer you suggested is what I’m after.

[bal704]

I use 12V relays typically, and power them with anywhere from 5 to 12 volts with no issues. I've also used the 5V variety, and had no problem using them with 12 volts. It is possible there could be long term issues, but I've been using them for several years now with no issues.

From my experience, relays are pretty tolerant of voltage. A 5 volt relay will work just fine off 9 volts or 12 volts.

I recommend reading the data sheet for the relays you are using. The popular Omron G5V-2 5V relays are rated at 75% to 120% of nominal (3.75 to 6V) for the standard type, 75% to 180% (3.75 to 9V) for the high sensitivity type.

[SoulFetish]

one thing to consider if your thinking about using the 6.3V heater supply as an auxiliary low voltage supply. A fairly common mode of failure in power tubes is a plate to heater short. If that happens, then it can toast your low voltage supply/components. I've seen this happen in some Marshall amps which use this type of LV supply.