Mojotone 5v Relay Supply

[lowell]

Anyone tried implementing this kit? Mojotone Channel / Component Switcher Kit

I drew a schematic using their online PCB board images. Please let me know if I made a mistake and/or if the schematic works theoretically. Not sure if the heater winding should be center tapped and grounded, or not.

[lowell]

And why not just do this? Relay power supply information

[jpfamps]

I have admit I'm not a fan if deriving a DC supply from the filament supply if the power valve filaments are attached to the same winding.

If the power valves short to the cathode then then filament/ cathode insulation is often comprised and the HT ends up on the filament supply, which will cause significant collateral damage.

This does happen in practice: a very high proportion of later SF amps have a burnt out hum balance pot.

A small inexpensive auxiliary transformer can usually be fitted in most chassis.

[loudthud]

Your schematic is not correct. I love Doug, but he kind of missed the boat on this. If you ground the centertap or use a virtual center tap then you can't get enough Voltage out of a bridge rectifier unless the DC is floating because it has AC riding on it. This is OK except you can't ground any part of the DC circuit including the footswitch. You could let the AC float and ground the DC but then you have noise on the AC that can get into the preamp tubes. One soultion to that is to use a large capacitor connected to the center tap which will float to about 4VDC.

The solution posted below must use a center tapped transformer winding. It generated about 12 or 13VDC which the regulator will knock down to 5V for relays. Depending on the relay, the regulator may need a heatsink. You can mount the regulator on the chassis without an insulating washer and the ground loop probably won't cause a problem. Or use a small heatsink.

[lowell]

Thanks LT. What is the purpose of the parallel diode/cap? Seems like there may be some (purposeful) phase cancelling going on between the 2 secondaries... but I can't wrap my brain around it all.

JPFamps I also have an unused 5v rectifier winding that I can use which will eliminate the possibility of the shorted power tube/high DC voltage issue.

[loudthud]

On the first half cycle, when all the caps are discharged to around zero Volts, one side of the 6.3V winding goes positive and the other goes negative. The side that goes positive will apply a reverse Voltage to the cap that is in series with that side. The diode just limits the reverse Voltage to 0.6V. After the second half of the cycle, the cap should be charged up to around 8V and the diode won't conduct. An arguement can be made that those two diodes aren't needed, but that arguement depends on unpublished specs for the capacitors. I included the diodes just to be on the safe side.

You didn't specify the relay you are using, and there is a chance that the capacitors might not be big enough to supply enough current. If you find that the 5V drops when the relay is on, you should increase the capacitance to the 470uF caps. Try 1000uF.

If you look at a relay spec sheet, many relays have a polarity marked for the coil. The relays actually work better if you use that polarity. They require less current to "pull in" at that polarity as if there is a magnetic bias that means less magnetic flux from the coil is required. I suspect that the magnetic core of the relay is a soft steel that retains some magnetism after the relay is tested. Famous words: "If you can't fix it, feature it."

It would be easier to use the 5V winding. Just use a bridge rectifier, 1000uF filter cap to make about 7 to 8 Volts. You don't need a regulator chip, just use a resistor in series with the coil so that the coil gets 5V when the relay is on.