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# Transformers!?

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#### hantto

##### Member
OK i have a 12V 6VA transformer, and doesn't that mean that the output is 6 / 12 = 0,5A?? And when i measured the output it was 3,someting ? So could someone explain this?

Yes, it means you can get up to 0.5A from it at 12v.

What were you measuring, and with what?

Ofcourse it means that you can pull out a max. of 0.5A at 12V but if your load is quite heavy (of low resistance) it will pull more current from the transformer which is beyond its capacity and may lead to burning of secondary coil. So 0.5A is the safe limit beyond which you should avoid pulling current out of it.

Ah.. ok that explains everything! I had only the multimeter in between

Thx kinjal

Never do that again . Never connect multimeter directly across power supply terminals without load in betwwen.

And then another question. The secondary coil is a 12V 0,5A (as i said), so does the primary coil then only consume 6 / 230 = 0,026A?

Considering that no device has 100% effeciency and if you are eating up 6VA on the secondary side, the primary winding would consume a bit more power than 6VA because of Copper and Iron losses in it.

And one last question. On which side is it better to put the fuse? Before or after the transformator?

Normally there are chances that the secondary winding getting overloaded if a unregulated power supply is used which does not provide any isolation between secondary and your load. Thus it better to have fuse in the secondary winding.

Ok thanks again!

kinjalgp said:
Normally there are chances that the secondary winding getting overloaded if a unregulated power supply is used which does not provide any isolation between secondary and your load. Thus it better to have fuse in the secondary winding.
I don't agree. Modern transformers can get very hot - often caused by the use of welded laminations instead of interleaved ones, and the resulting eddy currents.

This could lead to breakdown in insulation of the primary winding. If there is no fuse or a too-large one in the mains lead to protect it, there could be a fire! A secondary fuse would give no protection against this.

I've always found the best (and safest) solution to be an appropriate size fuse on the hot wire to the primary and a current limiter/regulator on the secondary for whatever current you wish to draw. that way there's no need for a fuse on the secondary and the mains is protected even if there's a malfunction/short in the transformer. To be on the safe side the output of the regulator should be in the range of 75-90% of the rated max output. (of course you run the risk of damaging the regulating circuitry in the event of overload, but it's better than a transformer fire)

as simple as can be. You would place it in the primary. The diodes in the secondary act as fuses. That is if it is for a power supply.

Also, charcoal, what do you mean by hot wire. In the primary? It's AC, either or could be the hot wire. But generally in schematics, they place the fuse on the top one.

Hot wire is the "L" (Live) wire of AC mains. Normally people connect fuse in hot wire but either ways its going to work.
For better safety you can put fuses on primary and secondary.

ok! thanks everyone!

well i think that the best way is to put fusess to th esecondary and a thermal fuse(if thats what is called, the type of fuse that burns at a given temperature and a given current also, they look something like capacitors, the olt type ones) in the contact with the primary, i usually use 125 or 150 degrees celsius fuses/1.....2.5 amps,
and use fuses for the secondary, to protect the seconary....but usually doesnt the primary burn down?
i mean for not such great power transformers, a few amps... :?:

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