# Another transformer with 2 secondaries question

Joined Oct 16, 2008
3
Howdy all! First post yay.

Okay. So I bought a small 3VA PCB mount power supply that steps down 240V to 2x6V. It has two secondaries and I'd want to hook it up as if it was a single secondary with a center tap. That way I can get dual rails(?) for a simple dual 5V DC supply.
Thing is, I have no diagram of the windings and I'm unsure which way the two should be connected. On the front it merely says [ 0 - 6V 0 - 6V ] roughly over the pins for the secondaries.
What could happen if I connect the two secondaries up so its not in phase?

As a side question: would I be able to get ±5V DC from two regulators (7805/7905) with just 6V AC input? I'm unsure what the minimum "input" voltage for these regulators should be above the required output voltage.

#### Wendy

Joined Mar 24, 2008
23,320
Lets assume you have this transformer pegged, you should be able to jumper 2 of the secondaries together and measure the other pair. If you measure less than 6 VAC you have to move the jumper on one of the secondaries, if you got it right you should have around 12VAC.

Be carefull, this is some dangerous voltages you're dealing with on the 220AC side.

#### mik3

Joined Feb 4, 2008
4,843
By just estimating, if you use a large filter capacitor and draw currents less than 250mA out of the regulator then it would be ok. I suggest you do buy a 9-0-9 volts center tap transformer to use it with a 7805/7905.

Joined Oct 16, 2008
3
Ah, thank you for the advice.
I've bridged the two center pins and measure 18VAC straight of the other two pins.

#### studiot

Joined Nov 9, 2007
4,998
You are measuring 18 Vac off load. This will reduce to 12v or less as you load it.

#### SgtWookie

Joined Jul 17, 2007
22,230
A bridge rectifier will cause you to lose around 0.8v on both the positive and negative rails.

78xx and 79xx have a fixed dropout voltage of 2v. That is, in order to get 5v out, you need to have 7v in. If you put in 6v, you won't get more than 4v out.

So, between the bridge rectifier and the regulators alone, you will have about 2.8v lost on both rails, for a total of around 5.6v lost. That's with no load. When you start loading the supply, you'll lose more in the rectifiers.