## Are transformers voltage dependant?

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Hi, I want to go from 400V to 7.2V (only have 400VAC to run my board off).
But I could only get 230V to 7.2V transformers.
So I figured no problem, as long as I draw less than the specified power at the secondary it will be ok. (ie Higher voltage less current).

So I calculated I need about 200mA at 5V (1VA).
I got a 230V to 7.2V, 5.5VA Transformer to account for losses and power supply drop.
So now I figured I have a 400V to 12.5V transformer. And since the output is 12.5 V instead of 7.2V, then I can only draw 5.5VA/ 14.5 = 440mA

Now the problem is the Transformer gets VERY hot when I am only drawing 34mA.

Test 1:
The primary draws 88mA at 400VAC,
The secondary draws 34mA at 14VAC
35W in, 0.5W out!! All the energy is going into the transformer itself???

Test 2:
If the primary is connected to 400VAC, and the secondary is not connected to anything, it still gets hot.

I have been reading a bit on transformer design and cant find anything about a transformer being designed to work at a certain voltage. Its always frequency, power out, and ratio of primary to seconday.

Are transformers voltage dependant?
The only fix I can think of is a big resistor in series on the primary, which is pretty stupid.
The circuit board is already made and everything mounted. I just couldnt test this power module part because I dont have 400VAC at home.

Thanks for any wisedom ! :oops:

Just a noob in this crazy world trying to get some electrons to obey me.

My explanation which may be wrong:

Bigger voltage than specified causes bigger magnetic flux than specified. Bigger magnetic flux than specified for the core material could cause the core material to be magnetically saturated, causing huge currents to flow.

Basically the same with an inductor, when voltage is applied, current rises linearly until core saturates and current is limited only by series resistance of the wire, not by inductive impedance.

Yes, you're saturating the core. You can run transformers at lower than rated voltage, but not higher for that reason. Perhaps the easiest way to lower the primary voltage to 230 would be to just put a resistor in series with the primary - edit - I see you mentioned that. If you don't want to waste power that way you'll need to either find the correct transformer or come up with another more efficient solution (bucking transformer? switching regulator?)

The solution might be a custom made transformer for 400V primary to 7.2V secondary.

Thanks for the reply Jepael. That does make good sense. So the only thing I could do to try reduce the magnetic field is gap the core? But it is in a plastic case filled with epoxy around the base. This is not going to be easy! As geoelec says, I think I will have to get one specially made. The person I was making this overheating thing for wont be happy about the wait!

Still going to try gaping it unless some1 says im wasting my time.

Just a noob in this crazy world trying to get some electrons to obey me.

How many are you making and can you get a 400-24V transformer? Might be easier/cheaper to add a regulator (perhaps switching to reduce losses) that to make a custom transformer.

Another option is to use a 400V to 230V transformer, but that is not what I would call an elegant solution :) but if you are in a hurry..

CountZero wrote:
How many are you making and can you get a 400-24V transformer? Might be easier/cheaper to add a regulator (perhaps switching to reduce losses) that to make a custom transformer.

Another option is to use a 400V to 230V transformer, but that is not what I would call an elegant solution :) but if you are in a hurry..

lol @ elegance part.
Hmm, I am actually liking that solution!

The 5V is actually made by a little LM2575T-5, so anything up to 40V is game on! I am probably more likely to find a 400 to 24V, good thinking batman!

It is late at night here, but tomorrow shopping I will go with a bag full of ideas thanks to you guys! *hug*

At the moment I am only making 2. But I hope he will buy more if he likes it.

What I have learnt is it is preferable to leave the Transformer off the board (adds flexability). I read in the "art of electronics" that you should try make your boards drop proof. And a honking big Transformer is not helping the cause. I think I shall do that in future.

Just a noob in this crazy world trying to get some electrons to obey me.

I don't think gapping the core would be the right thing to do here.

If you wish to modify the transformer, you need to reduce the number of primary turns, to get the same flux at 400VAC than it now has at 230VAC. I guess the ratio is 230/400=0.575.

This also means that you need to reduce the number of secondary turns too, to keep the voltage ratio same, or reduce any amount you wish to get suitable voltage.

You could use two identical transformers 230V to
some nice voltage and use them with primaries
in series, and secondary in series also.
You may also use parallel secondaries.

The only thing that worries me is that a 220V type transformer MAY NOT be a good isolator as things could break down with the higher voltages (400V +- 10% ??).

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

I was about to suggest taking TWO transformers, connect primaries in series and secondaries in parallel (watching out for phasing, of course). But, JS raised a very valid point that I overlooked - breakdown.

The quick and dirty "safe" solution is a 400V (maybe 440V or 480V) to 220V transformer, followed by the 220V to 7.2V transformer.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

But taking two 230VAC transformers, putting primary sides in series with 400VAC, will have only 200VAC per primary. I guess it would be good up to at least 460VAC, right? Quick and dirty solution for testing, much better than messing with the wiring or gap.

I guess secondaries would need to be run either parallel or serial, so you can't just use the secondary of one transformer. But for connecting parallel, the transformers really need to be identical.

Here is an example datasheet of a SMPS transformer.

http://www.tauscher-transformato...

The provide 4000V isolation. I think in general
mains transformers provide a very good isolation, due
to safety requirements. But the real
values are only in the datasheet or in the regulations.