relay power supply question

[Tonegeek]

Using Duncans PS designer, i built a simple full wave PS, containing 1 12.6v tranny with Center tap, 2 diodes, and 1 3300 uf cap. It only measures about 9.1 volts DC with no load. I expected it to be over 12 V DC. What did I do wrong? I checked the diodes, AC sec. volts (14.4) and swapped caps. Will a bridge recto give me higher volts than the 2 diode arrangement?
thanks

[Bob-I]

Using Duncans PS designer, i built a simple full wave PS, containing 1 12.6v tranny with Center tap, 2 diodes, and 1 3300 uf cap. It only measures about 9.1 volts DC with no load. I expected it to be over 12 V DC. What did I do wrong? I checked the diodes, AC sec. volts (14.4) and swapped caps. Will a bridge recto give me higher volts than the 2 diode arrangement?
thanks

It shouldn't. I don't know what's wrong but that just doesn't make since. You should be seeing closer to 14V, the PS designer is usually very accurate.

[Normster]

Do you have your diodes wired for half or full wave? (Maybe post a drawing.)

[Tonegeek]

You should be seeing closer to 14V, the PS designer is usually very accurate.

I agree. I even thought my meter was busted, but it read 8.9 volts on a 9 volt battery so I figured it is OK. Here's the schematic. BTW, the relays all work with this voltage (LED's are a little dim but useable). but I am afraid that with a cable run to the footswitch it will drop enough voltage that the relay wont work.

[oldhousescott]

Sounds about right. 12.6VAC center-tapped is 6.3VAC per winding to ground or about 9V P-P. Throw away a volt for diode drop and, depending on wall voltage, you should be around 8 to 9 VDC. If you used a bridge rectifier setup you would get around 17 to 18VDC. If you do switch to a bridge, you may want to go ahead and add a 7812 regulator.

[Tonegeek]

Sounds about right. 12.6VAC center-tapped is 6.3VAC per winding to ground or about 9V P-P.

Maybe I am not using the Duncan designer correctly because I get almost the same value whether I am using a bridge or full wave rectifier.

Here is a good tutorial on PS design which sort of validates what you are saying: http://my.integritynet.com.au/purdic/power1.html. I read somewhere else that fullwave and bridge rectification produce the same voltage which I now suspect, is an incorrect statement.

I shall prototype a bridge tonight and see what that does for me. I have a 12V zener if I need to bring it down. BTW, I chose the relay based on it's coil resistance because for one it is a low power consumer and two I can directly run an LED indicator in series with no current limiting resistor. Only drawback is these things are tiny, but I came up with an elegant (although labor intensive) solution for integrating them in a tube design and will post a pic sometime for all to see.
thanks,
whit

[oldhousescott]

I read somewhere else that fullwave and bridge rectification produce the same voltage which I now suspect, is an incorrect statement.

Not so much incorrect as incomplete. PT secondaries designed to feed a full-wave rectifier will generally be center-tapped with each winding having the desired AC voltage. Across the full winding you will have double the AC voltage. PT secondaries designed to feed a full-wave bridge rectifier will generally have no center-tap and the full winding will have the desired AC voltage.

In your case, if you lift the center-tap ground and then feed the AC into a full-wave bridge rectifier you should get about 18 VDC.

[Tonegeek]

PT secondaries designed to feed a full-wave rectifier will generally be center-tapped with each winding having the desired AC voltage. Across the full winding you will have double the AC voltage. PT secondaries designed to feed a full-wave bridge rectifier will generally have no center-tap and the full winding will have the desired AC voltage.

That clears up the mystery with Duncans PS designer. When I went back and put in my tranny as 6.3v (instead of 12.6) it gave me a reading more like what I am seeing on my PS. I just need to add a couple of diodes and the appropriate limiting resistance and the 12v zener and will be good to go. Thanks much! I should be able to cruise on my build over the weekend, having solved this issue.
whit