How can I replace a transformer for batteries?? There is a third wire I dont know what to do with it!!?

I'm working on a proyect and need to take off this transformer and change it for batteries. The proplem is that I don't know where to conect the wires of the batteries, I have solder and tester everything...

thanks for your help

btw, New instructable if you help me with this issue!!

Picture of How can I replace a transformer for batteries?? There is a third wire I dont know what to do with it!!?
sort by: active | newest | oldest
Re-design7 years ago
The red wire is the center tap on the transformer.  The yellow wires are the power wires.  THey all lead to the bridge rectifier.  That's those four black diodes with a red stripe around the end.  The out put of those should go to a capacitor of capacitors.  That is where you need to connect your batteries.  The pos and neg poles of the capacitor are the power output that supplies the device.

Measure the voltage there at the capacitors not at the transformer to make sure that you don't over or under power the device.
juanvi (author)  Re-design7 years ago
I think i know what you mean...look at the photo...
I see the four black diodes, but they have a geay stripe. And  yes i think they go to that big capacitor. I measured the voltage there and it said 13 V (DC). In the pic you can see its writen 16v in the capacitor..

so i have to solder the wires from the batteries on the pins of the capacitor, right??
thank you so much!!
jeff-o juanvi7 years ago
Yeah, pretty much.  The four diodes are probably configured as a "bridge rectifier" which, along with the capacitor, form a simple DC power source for the rest of the clock.  It'll probably run on anything from 9 to 12V.
juanvi (author)  jeff-o7 years ago
ok, i have another proble...hehe.
i conect the batteries to the capacitor ans yes i works, but it only works the radio, the LCD screen doent light up. I've tried with more voltage, but it doesnt work...i'm trying right now another aproach...
jeff-o juanvi7 years ago
Ah, weird!  I wonder if the LCD requires a clock of some sort, and the clock is derived from AC...  Or maybe it's something else.  Hard to say without more testing!
jeff-o jeff-o7 years ago
Aha, it does need AC.  See this post on another forum.

I wonder if you could rig up a 555 timer to provide the needed AC?  Hard to know without knowing what LCD driver IC is in there.
Its  a SPLIT supply, the LCD needs a negative bias voltage
juanvi (author)  jeff-o7 years ago
oh, thanks!
 I was about to llok it up on google. So now i need more time for testing and trying to fix this...thanks anyway, i'll pm you if  have more doubst
jeff-o juanvi7 years ago
np, and good luck!
Yes and get the polarity right our things gonna splode!
This is a split rail supply. You CAN replace it with a battery pack, but you will need to divide the pack into two halves. The +ve terminal of the first half goes to the common cathode connection  of the diodes, the -ve terminal of the first half AND the +ve terminal of the second half go to the COM terminal on your board. The -ve terminal of the second half goes to the common anode connection on the board.

jeff-o7 years ago
That's a center-tapped transformer, which could mean the circuit requires a split supply (positive and negative voltage, with the red wire as the 0V reference) - or maybe not.

What is the board?
juanvi (author)  jeff-o7 years ago
aha, i understand. I'm new to transformers...haha

its an old radio/alarm clock

thanks for your answer!!