Relay power supply

[jeff]

I have a 12V relay(I know I should have bought a 5V from Doug but I have these lying around) and a 12V CT transformer. How do I get 12V DC from the 12V AC transformer? I imagine I just build a rect and use a big cap, then a dropping resistor then another cap. But here's my question. If I just do that when the cap has no load the resistor won't drop and isn't the voltage going to shoot up to 16.8V? So when I do connect the relay that will excede the max rating. Do I need a voltage divider? Do I need to use a voltage regulator? Is there any way to do it without the regulator?

Schematics please if you got em.

Thanks
  Jeff

[SILVERGUN]

Hey Jeff,
It's just as easy as you said it is....
Intended use of a voltage regulator: (why not use one?)  

[sluckey]

All you need is a bridge rectifier  and ONE big cap. Then you put one resistor in series with the relay coil. The voltage rating of the relay coil only applies when current flows thru the coil. IOW, you could put 50V on one side of the relay IF the other side is open due to a switch.

What size resistor? Well, in your case the unloaded power supply will be 17 volts. The relay wants 12 volts so the resistor must drop 5 volts. Now you need to know the current rating of the relay coil. If it's written on the relay, great. If not, measure the resistance of the coil. Now divide 12V by the measured coil resistance. This is the operating current for the relay coil. Let's just pretend that current is 80mA. That 80mA must also flow thru your dropping resistor, so now you can calculate the resistance value. R = E/I = 5/.08 = 62.5Ω. Power rating would be P = E*I = 5*.08 = .4W. Double that for safety, so use a 1 watter.

[kagliostro]

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

K

[jeff]

Thanks Sluckey. That makes perfect sense.

[PRR]

And steady 17V won't hurt most "12V" relays. Nearly all will take 1.5X rated voltage for a long long time.

You only need enough filtering so that the relay power leads don't spray buzz all over the audio signals on the relay contacts. Bring the coil leads away from the contcat leads, maybe inside shield, you can use moderately dirty DC.

[jeff]

Darn, Just realized I have a 12V CT transformer. That means 6V-0-6V. If I ground the CT and just use the two diodes I won't have a high enough voltage for my 12V relay(6 X 1.4)

What if I use a full wave bridge rect(4 diodes) and not use the CT?
 
 If the transformer is rated 450mA how much current can I pull with the full wave CT VS the full wave bridge rects?(I hope I'm calling them by the right name)

[sluckey]

see reply 2

[SILVERGUN]

Sorry for my inaccurate drawing, as well...
You had mentioned in your original post that you had a 12V CT xfmr,,,,,and I hope that it didn't lead to your confusion...

I was so busy trying to convince you to just go ahead and use a voltage regulator, that I rushed my schematic drawing and didn't really think it through

 
Dave