# Combine dissimilar 12v transformers in parallel?

Discussion in 'General Electronics Chat' started by bhvm, Oct 7, 2014.

1. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
Hello friends,
I am working on an educational project that involves lighting up a bunch of 12v LEDs(modules). However I feel that the transformer i am using 12v 3A is not sufficient as LEDs light up dim. I am thinking in putting additional transformer in parallel to help the old one and bring up some voltage.

I do have a few 12v transformers of different Amps and would like to learn how can i combine them for greater output- (all have single 240v primary)

1)Some of them are just 2 wire secondary that needs full wave rectifier
2)some are 3 or 4 wire secondary that need half wave rectifier. Some are are bizarre 5 wire that has 1 black,2 green, 2 yellow wires.
3) Most transformers i have are like 3A, 2A, 1.5A, 0.5A (but all output 12v after rectifier)
4)There are going to be about 12 LED lamps in total rated for 12v at 0.5~1A each.

I read a few threads on this but I want more clarity,.Thanks

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
If they are not identical, I wouldn't.

It might be ok to have a full-wave rectifier bridge on each transformer, and then combine the outputs of the two bridges + to +, - to -, using just one filter capacitor.

3. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
no they aren't identical. but as you say. it's ok to combine after the diodes?
do I need to observe primary side Polarity?

Last edited: Oct 7, 2014
4. ### Papabravo Expert

Feb 24, 2006
11,131
2,172
LEDs work with current, not voltage. In particular, they work best with DC current, not AC current. Any LED or string of LEDs need something else, like a series resistor, to limit the current. How about a link to a datasheet for the LED and a schemastic diagram of your hookup. A picture is worth a couple thousand words.

5. ### alfacliff Well-Known Member

Dec 13, 2013
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429
the need for a half wave or full wave rectifier isnt determined by the number of wires on the secondary. only whether a 2 diode or 4 diode bridge is used, 4 diode on 2 wire, 2 on three wire secondsry, although a 4 diode bridge can be used on a 3 wire, if you dont use the center tap for ground.

6. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
Dear sir,
The LED we are using are not 1w Luxeons but rather simple 5050 or 3528 "strip" types. They are designed for direct 12v operation. Small strings of these will be cut and used as one module Glued to aluminium heatsink. Many such modules consist my project.

http://www.maxlit.co.in/led-strip.html
http://www.ledlightsworld.com/page.html?id=32

I do understand what you said. problem is,
Some transformers are 2 wires, which need 4 diodes.
Some are 3 wires which has 2 + and a center -. They need 2 diodes
some are 5 or 4 wire ones.
How to combine all of these for increased current?

i will try to get images tonight.

7. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
you will have to match the voltages on the secondary very well before you can paralel dissimilar transformers.

8. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,098
1,479
The chances are that the rectified voltages from the different sources will not be closely matched. As a result the source with the highest voltage will be supplying most of the total current and could well overheat .

9. ### crutschow Expert

Mar 14, 2008
16,513
4,444
If you have several strings then just connect one or a few of the strings to each transformer rectified output, depending upon the transformer ratings and the current draw from the strings. That way you don't have to parallel the transformer outputs (which likely won't work properly).

10. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
And how may I do that>?? The transformers are not adjustable

Hmm... I fear that. i guess I will simply have to try them out. I may put a Computer fan over them.
But it will be better than single overloaded tranformer supplying ALL the current.
I did think about that. However running a myriad of wires and maintaing separate circuits is nightmare.

What will happen if I parallel them after diodes and capacitor? will they still 'fight' each other?>

Sep 17, 2013
7,098
1,479
They will

12. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
Here are the transformers i was talking about.

File size:
1.5 MB
Views:
31
13. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
I did it.
I have combined 3 transformers in parallel. each one gets it's own diodes and cap. I used connectors to join the final output and supply to the load. the ocv of 3 transformers are-
28v
21v
24v.

after combining they all settle around 18v with the load running. I matched up my leds to the voltages.

thanks.

14. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,098
1,479
Great if it's working for you, but I suspect one of those transformers will be doing most of the donkey work. Check their temperature after they've been running for a few minutes. My guess is that one will be noticeably warmer than the others.

15. ### bhvm Thread Starter Active Member

Jul 16, 2009
53
2
I ran the setup overnight 13 hours. nothing blew or burnt up. the hottest part were actually the diodes. transformers were hot too, within a few degrees of each other. so it's not like only one is being used. thanks