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Transformers in parralel

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Hi again,


Well here is the way i see it...

If we had a 20v output transformer and a 10v output transformer and
each had rectifiers to convert the ac to dc and a filter cap to help
smooth out the dc, and we wired the two caps in parallel and connect
a load, the 20v output transformer would keep the output caps charged
up and supply most of the load while the 10v output sat there idle not conducting any (or very little current) because the higher voltage
output keeps the rectifier diodes of the 10v output reverse biased.

Thus, although it doesnt seem to hurt anything it also doesnt
help anything.
In that case the 20V transformer would only supply the load but I wasn't talking about using two transformers with different voltage outputs. I was talking about using two transformers with the same output voltage. Connecting up transformers with different output voltages in parallel is a silly idea, I wouldn't have even dreamed that you would have thought about doing it.
 
Hello again,


Hero:

That is interesting because i wouldnt have dreamed you would have
been talking about transformers that are exactly the same after this
question had been posted:

We derailed a bit in the thread, let's start over:

What will the effects be if two different transformers are placed in parallel with two different ratings (one has a bit more or a bit less winding ratio than the other)?


Freakazo:

The resistance of a transformer can be measured with an ohm meter
if the resistance is high enough, or with a current and use Ohms Law.
After measuring both primary and secondary the two resistances
can be added together after reflecting either the secondary resistance
to the primary or the primary to the secondary. The reflected resistance
is proportional to the square of the turns ratio.

For example,
We have a transformer with 4 ohm primary and 1 ohm secondary,
and the turns ratio is 1:0.5 (step down 2 to 1).
With no load and 10v on the primary, we get 5v on the secondary.
With load however, we have to take the resistances into account.
To reflect the primary resistance to the secondary, we take the
4 ohms and multiply by the square of the turns ratio. The turns
ratio would be 0.5 here so the square of that is 0.25, so we multiply
0.25 times 4 and get 1 ohm, so we add that to the 1 ohm secondary
and we get 2 ohms in the secondary. Thus, the transformer looks
like an ideal transformer with turns ratio 1:0.5 with 2 ohms in the
secondary.
If we now connect a load of 23 ohms on the secondary we get 200ma
to flow because we have to add that 2 ohms secondary resistance to
the 23 ohms. We get 25 ohms total and 5v/25ohms=200ma.

Reflecting the resistance this way helps to determine the effects
of connecting two different transformers in parallel.
In general though, if the transformers are not very close to being the
same it's not a good idea.
 
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Sorry I didn't notice that.
 
How would one measure series resistance?

I'd use an AC ohmmeter; that is, a doorbell transformer with the secondary in series with an ammeter, with a voltmeter across the device under test. Then Z = V/I. Connect it just long enough to take your readings.

You can also use a 7-1/2 w bulb as a current source; you put in series with the AC line and the transformer coil you want to test. Again, Z= V/I. You have to be more careful with this method.

If you also measure DC resistance of the coil then you can totally model the transformer coil impedance; Z = R + jXL
 
Last edited:
I'd use an AC ohmmeter; that is, a doorbell transformer with the secondary in series with an ammeter, with a voltmeter across the device under test. Then Z = V/I. Connect it just long enough to take your readings.

You can also use a 7-1/2 w bulb as a current source; you put in series with the AC line and the transformer coil you want to test. Again, Z= V/I. You have to be more careful with this method.

If you also measure DC resistance of the coil then you can totally model the transformer coil impedance; Z = R + jXL

That's true for 50 / 60 Hz transformers, but the OP is talking about switching power supplies (and their output thazformers).

If you want to model the transformer impedance you should use the same frecuency that the transformer would work at.
 
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