12v relays

[WhopperPlate]

Hello everyone, I have a stash of old 12v relays that I'd like to use for my Dumble build. I wanted to use 3 12v relays for OD, PAB, and MID boost. I'm shopping at CEDist I'm still trying to wrap my mind around what transformer and rectifier setup to use with this configuration.

I've tried reading all the old posts but I'm still not catching on. I'm always trying to learn more and understand the basics, but rectifiers have always confused myself. Any help would be appreciated.

Thanks for the time.

[martin manning]

This circuit, clipped from icracer's HRM schematic is all you need. A Radio Shack transformer is the traditional choice, diodes are 1N4007. Regulated 12VDC is at pin 3 of the LM7812, and goes to the relay coils. The 1K resistors are current limiters for two LED's, but some people would rather put those in the foot switch box so that a single conductor in the cable can carry the 12V supply to all of the indicators. Check the current draw of your relays and add 12mA per LED to make sure you aren't overloading anything.

[M Fowler]

Martin, great information for us relay noobs I really appreciate the information.

[WhopperPlate]

Thank you for the advice.

What would change if I used a center tapped 12v tranny, would I just not hook up the center tap?

[ChrisM]

Yes, your voltage would be higher (double) though, so keep that in mind.

[WhopperPlate]

So I could use a 6v center tap to get the 12v, or would the 12v center tap be preferable. All the transformers I see on CE are center tapped is why I ask and am trying to make a one stop shop. It's really the rectifier part that really gets me.

[WhopperPlate]

delete

[martin manning]

What would change if I used a center tapped 12v tranny, would I just not hook up the center tap?

The transformer shown in the schematic is a 12.6V CT (same as 6.3-0-6.3), and note that the CT is marked "N.C." for no connection. With 12.6V rms across the outer leads on the secondary, the rectified voltage will be 12.6 * 1.4, minus two junction drops for the full-wave bridge rectifier, so about 16VDC. The 12V regulator needs this excess voltage to work properly.

[WhopperPlate]

Thanks for helping me learn.