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

http://forum.allaboutcircuits.com/t...ting-transformer-for-my-low-volt-rails.49719/

http://forum.allaboutcircuits.com/threads/multiple-transformer-in-parallel.55520/

 

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.

 

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?

 

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.

 

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.

 

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.

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

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.

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.

 

you will have to match the voltages on the secondary very well before you can paralel dissimilar transformers.

 

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 .

 

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).

 

you will have to match the voltages on the secondary very well before you can paralel dissimilar transformers.

And how may I do that>?? The transformers are not adjustable

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 .

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.

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).

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?>

 

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

They will

 

Here are the transformers i was talking about.

 

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.

 

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.

 

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