I have a transformer with two 0-12V secondaries rated at 10VA. The normal application for this would be a +/-12V supply with a capacity of about 0.8 amps. Is it possible to use both windings of the transformer to give a single 0-12V supply with a capacity of 1.6 amps?

 

hi,
Connect the secondaries in parallel, ensure that phasing of the two windings are the same sense.
E

 

Unless the voltages on each winding is the same, there will be circulating currents between the windings. Why not measure this, just to be sure?

 

Just to confirm, are there three secondary wires, or four?

ak

 

Hello,

As others have said you can usually connect the two in parallel to get more current. Some transformers are made just for this so they give the option of using two in series or two in parallel.
Also as others have noted, if the voltages are not exactly the same (12.0v and 12.1v) then the 12.1v will power the 12.0v winding slightly. The mount of no load internal current then depends on the resistance of the windings.
The other drawback is that for a given load, the two windings will not share the current as well because the 12.1v winding will try to power the load more than the 12.0v winding. That leads to some current difference in the windings.

The key point then is to get the windings oriented right first, then make some measuremnts with load. Since the winding resistance is low, it is probably not as good to use an ammeter to measure the current because it will only be placed in one winding at a time unless you have two identical ammeters. But often ammeters have a higher than desired resistance anyway. So the best bet is to connect small resistors in series with each winding and measure the voltage, then calculate the current using Ohm's Law.

You can also check the no load current through one winding when you connect them together to see if there is any internal current.

More than often this works though and you get roughly twice the current output.
The time when this works the best is when the two windings are wound bifilar, and not as good when one on top of the other.

 

If you haven't got much test equipment to check for circulating currents due to unequal voltages, a real world test will be to connect two 20 or 22 ohm 10 watt wire wound resistors to each winding without putting them in parallel, run it for a couple of hours and see how hot the transformer gets by putting a finger on it for about 5 seconds or so.
Then put the windings and the resistors all in parallel having checked you have them phased correctly, then run it and check every 15 mins for the same amount of time. (stop if you feel excessive heat) As long as the transformer is not significantly hotter, you should be good to go.
As I said, "a real world test" believe me, there are many fairly good transformers being used that have parallel windings that may not be perfectly matched, but due to all the losses and things like winding resistance variations that inevitably occur when windings are layer wound, still work within the manufactures tolerance limits. One of those being temperature of operation at specified load. Also remember that just because your transformer is now rated at 1.6 amps, you will only be able, with a good bridge rectifier and reservoir capacitor to draw considerably less DC current, (can't remember the ratio off the top of my head, Someone will give you that answer)
If you want DC then arrange the windings in series 12-0-12 and only use a pair of diodes. More efficient, and less voltage drop in rectification because only one diode Vf drop in each half cycle as against two in a full wave bridge.

 

Do you have the manufacturer's name and a part number for the transformer?
Do you have the transformer already? If so, can you post some pictures? - show us the windings and the pins as best you can

 

Are the windings each rated at 10VA or is it the core?
If it is the core, you cannot get more than 10VA safely out of the transformer no matter what the connection is.
Running the windings as 12-0-12 and using 2 diodes is probably the best. You can still run the same power out as both in parallel and a bridge but you gain an extra .7V as there is only 1 diode in line at a time.
But if you want 12V AC at higher current, connect the windings in parallel.
Hook them together at one end, and then measure the volts across the two remaining ends. If it is zero, then hook them together too.
Then you are ready to go.

 

Do you have the manufacturer's name and a part number for the transformer?
Do you have the transformer already? If so, can you post some pictures? - show us the windings and the pins as best you can

My question was more of idle curiosity after stumbling around on eBay. Maybe I should have said "I'm considering purchasing a transformer...", something like this: http://www.ebay.co.uk/itm/BVEI60110...956876?hash=item2ef500a94c:g:D1cAAOSwmkpZ9ou9 as searches for a 20VA 12V transformer most often seem to end up with a 12-0-12 dual voltage secondary with 10VA per winding. From the interesting discussion so far, it sounds like it will be simpler to just consider finding a transformer with a single 0-12V winding of the correct power rating.

 

Everybody in this thread, so far, made it way too hard.

1st, you may want to get DC out of it. if so, read this first: http://www.hammondmfg.com/pdf/5c007.pdf

Identical windings, you can parallel, so 0 connected to 0 and 12 connected to 12 Should be parallel.

if you don't know or are curious, you connect the windings in series first. You will get zero or 24 V.

Once you have them in series, you know the phase and can now connect them in parallel correctly the first time.

e.g. label the series connections 1,(2-3),4 with 24 VAC out between 1 and 4; 1 to 3 and 2 to 4 are the parallel connection.

 

hi kiss,
Not everybody, ref post #2
Eric

 

Many many smaller transformers are wound with double secondaries for the very purpose of giving the flexibility of double the voltage (secondaries in series) or double the current (secondaries in parallel).

Whilst the cautions of "different voltages ...circulating currents.." are theoretically sound...remember transformers are wound by machine and it's not real hard for a machine to ensure there are exactly equal numbers of turns. Unlike dudes winding trannys on a lathe, a factory coil winding machine does not lose count!

If the secondary turns are equal then so will the voltage be - certainly within any normal highly accurate measurements.

Worst that could happen is that due to the windings coming out at different spots there may be a one turn differential. Given there are likely to be some hundreds of turns on the secondaries you are looking at at 0.2% difference for a 500 turn secondary.. So don't worry at all about circulating currents and voltage differences.

All the other advice about measuring with resistors and similar is very sound but unlikely to show up any contra indications. The only advice that you need to heed is that about making sure the windings are connected in phase.

Do that by connecting one end of one secondary to either end of the other and measure for any voltage between the unjoined ends. If you get 0volts then it's safe to connect the two free ends as well, and you get double your current. If you read 24 volts then you have connected them in anti-phase so swap the connection and measure again.

 

My question was more of idle curiosity after stumbling around on eBay. Maybe I should have said "I'm considering purchasing a transformer...", something like this: http://www.ebay.co.uk/itm/BVEI60110...956876?hash=item2ef500a94c:g:D1cAAOSwmkpZ9ou9 as searches for a 20VA 12V transformer most often seem to end up with a 12-0-12 dual voltage secondary with 10VA per winding. From the interesting discussion so far, it sounds like it will be simpler to just consider finding a transformer with a single 0-12V winding of the correct power rating.

YES.

Like Eric said in post#2, connect your outputs in parallel make sure they are in the same phase, that will give you double the current rating at the same voltage.

If the phasing is wrong, you will get Zero Volts and a Hot Transformer !!

 

Without even looking at the data sheet, just the pic on ebay, I was pretty sure the transformer would have two completely separate secondary windings. It does. There are many many small transformers made that way and a large majority are designed so the secondaries and often the primaries can be connected either in series or parallel. I've designed a bunch of them into things.
I've also used large toroids in the hundreds of VA range the same way - primaries in series for 240 V, in parallel for 120 V. Bifilar windings are rare because magnet wire is considered to have insulation rated at about 30 volts for safety purposes. You simply can't series connect bifilar primaries wound with mag wire and get safety agency cert.

 

Connect it contra-serial, use middle point as common, and both ends for the diode, diodes are contra-pole. This is the center-tapped two wave rectifier. Important is to not overstep the average long-term current 0,8A. If the 10 msec it is 1,6A and next 10 msek is zero, means everything is OK.