# AC-AC Tranformers in parallel to DC-DC series

Discussion in 'General Electronics Chat' started by dmachado, Feb 28, 2014.

1. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
Hello everyone,

Many times I have found answers to my questions in other members' posts, but not for this one...

I have two 200W 240VAC-12VAC transformers, which I want to connect to a bridge rectifier each, getting about 17VDC/200W per channel.

However, my application needs about 32V and I guess the booster efficiency will not be the best.

My question is: can I connect the bridge rectifiers DC output in series to get about 34V DC/400W from both units combined? And then just drop the 34V to 32V as needed?

Thanks for all the help.

Daniel

2. ### ericgibbs Moderator

Jan 29, 2010
7,383
1,494
hi Daniel,
Using a bridge rectifier will mean a voltage drop of 2*1V, so the Vpeak across the smoothing capacitor after the bridge will be only 10* 1.414 = 14.1V OFF Load.

As soon as you load the transformer/bridge and smoothing capacitor the voltage will be down to 10V, not 17Vdc as you expect.

E

3. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
Thanks for your reply.

So connecting both bridges in series (is this possible?) should put out about 20V under load?

Thank you.

4. ### ericgibbs Moderator

Jan 29, 2010
7,383
1,494
hi D,

You can connected identical transformer secondaries in series, providing that you ensure the phasing of the windings is correct.
ie: the End of one winding is connected to the Start of the second winding.
The output of 24Vrms will across the two free leads.

The primaries must be in parallel, with Start of one primary connected to the Start of the primary and of course the two free End's connected together.

If you do not follow that description, I will post a simple diagram if you wish.

Its important to note that transformer output voltages are specified at full load, so 200W/12V = ~ 16.6A.
Depending upon the construction of the transformer, the Off load voltage can be higher than 12Vrms

E

5. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
Thanks, but I meant each transformer to each bridge, then the DC from the bridges in series (to be tested outside ).

6. ### ericgibbs Moderator

Jan 29, 2010
7,383
1,494
If you have a separate bridge rectifier on each transformer secondary and connect the bridges in series, you will a double bridge diode voltage drop.!

Why not have one bridge rectifier across the series secondary windings as I suggested.?

E

7. ### inwo Well-Known Member

Nov 7, 2013
2,416
314
Yes you can.........................

8. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
Well, I'm not comfortable with opening the transformers and tapping the windings, what I have is two 200 AC-AC halogen lamp transformers.

I have to go with the bridges in series...

Thanks for all the help.

9. ### crutschow Expert

Mar 14, 2008
21,422
6,137
You don't have to tap the windings. If you have two transformers then then you just connect the output of the two windings in series with one bridge rectifier.

To determine proper phasing, connect the two outputs in series and measure the voltage. If it's not 24Vac then reverse one of the windings.

10. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
I got the transformer-bridge working, now I have the minimum load problem.

I want to power an Arduino from this DC (7805 regulated), but the load is not enough, and the transformer stays at 1VAC/ 1.5VDC approximately.

With a DC load above 1.5A-2A, everything works fine... It seems that the transformer "turns on" at about 1.5A load.

Of course, I don't want to leave a permanent load on, either on the AC side or the DC side - there's got to be a better way to get the "juice"...

Can this minimum load requirement be disabled? Any thoughts?

11. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,580
2,147
What are these "transformers"? Your description, and that part about having to open them up to bypass the bridge, makes them sound more like power supplies than transformers. A transformer as we are describing it is a chunk of iron with copper coils and either pins or leads as connection points. No diodes, capacitors, or anything else. Iron, copper, period. Is this what you have, and can ypu post a photo? Also, make/model/part number?

ak

12. ### dmachado Thread Starter New Member

Feb 28, 2014
17
0
Hello,

These are the transformers:

They are halogen light power supplies.

I gave up trying to convert them and went for a 12v switched power supply, however I learned a couple of things about rectifiers, etc, which is always nice.

I also learned there are components not to be messed with, power supplies being one of them. Not worth the risk/hassle.

Thanks for all the help.

Last edited: Apr 4, 2014