# Transformer question

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#### Electronman

##### New Member
Hi,

I have a 220V to 12V transformer.
It was getting hot and the fired due to a short circuit in its secondary leads.
The first question is why a transformer gets hot while the SECONADRY coli become shorted for a while?

The other question is why the primary coil draws MORE current from the mains when the secondary coil is connected to a load (I.e. why the primary current changes with changes in secondary resistance?)

Thanks

#### chemelec

##### Well-Known Member
Hi,

I have a 220V to 12V transformer.
It was getting hot and the fired due to a short circuit in its secondary leads.
The first question is why a transformer gets hot while the SECONADRY coli become shorted for a while?

The other question is why the primary coil draws MORE current from the mains when the secondary coil is connected to a load (I.e. why the primary current changes with changes in secondary resistance?)

Thanks
VERY BASICALLY:
1) The Greater the Current, The Greater the Power Dissipation. (Thus Creates MORE HEAT.)
When you Exceed the rating of the Transformer, It will get VERY HOT, even Burn Out.

2) The Greater the Current Draw on the Output, The Greater the Input Current.

#### john1

##### Active Member
Hi,

These questions may seem basic, but i suspect that many people find them awkward to understand because the wires are not actually connected together, the windings are seperate and do not connect together. (normally)

Firstly, the transformer would become overheated if it were trying to deliver more power than it was made for.
Feeding into a short circuit is a case where excess current would be drawn, and cause overheating.
Spot welding transformers do this for short periods, not for 'a while'.

The other question is deeper, and requires an understanding of the way power is exchanged between windings.

Typically where two windings are on a transformer, the number of turns on each winding, forms the relationship between the windings.

Briefly, same number of turns means one-to-one, same out as in.
More turns on the secondary, higher voltage out, and draws more current from the primary.
Less turns on the secondary, lowers voltage out, and draws less current from the primary.

The windings have no electrical connection, the power is transferred by the magnetism (flux) operating within and around the windings, guided by the ferrous material.
Lower frequencies need more laminations or ferrous material,
Higher frequencies need less laminations or ferrous material.
Much higher frequencies often need very little or none.

The current draw in the primary is in proportion to its turns ratio with the secondary.
In this case, 220V primary - 12V secondary, the proportion is roughly eighteen to one, so the primary current draw would be one eighteenth of the current drawn by the secondary.

So a 12 volt lamp drawing about an ampere, would take approx 65 milliamperes from the primary winding.

The above is only a brief overview, and may not apply in all cases.
For a fuller understanding there are many in depth studies available on the net.
This is only to illustrate the mechanism between windings.

Regards, John

#### john1

##### Active Member
Hi Chemelec,

Took me ages to write my reply.
Kept on altering bits to try and make it simple.

Regards, John

#### MrAl

##### Well-Known Member
Hi,

This is a well known problem with transformers. The old doorbell
transformers used in somewhat older houses were known to start
many a fire and actually burn houses down.

From these lessons came the thermal fuse. The thermal fuse is
mounted right on the transformer so that it touches either the
core metal (and some thermal epoxy) or on the windings so that
it is close to the wires themselves.
This fuse is wired in series with the PRIMARY, so that in the event of
a heat up, the fuse blows and the entire transformer is disconnected.

The same problem began to occur with window fans. The wind would
blow and the fan would tip over, and still running with no where for
the air to go it would heat up and burn up. Again, enter the thermal
fuse. The thermal fuse is mounted right on the metal core part of
the motor with thermal epoxy. When it gets too hot, the thermal fuse
blows and disconnects the fan from the power. The fan is not usable
after that but at least there was no fire.

The current ratings vary for these thermal fuses depending on the
device it is used in. Common ratings are 2 amps and 10 amps, and
they have quite a range of temperature selections.

A relatively simple device that by now must have prevented thousands
of fires.

If you run a transformer 24/7 or unattended for any length of time,
do yourself a favor and wire in a thermal fuse. Mount it right on
the core metal inside where the windings are. Wire it in series with
the PRIMARY so that if it blows it disconnects the entire transformer
from the power line completely.

Good luck in the future with your transformers...

#### Electronman

##### New Member
Thanks for all inputs.

Well yea, actually I wanted to know how does the FLUX cause the transformer to get hot and then burns or melts? It seems to me that the interacting between primary and secondary fluxes cause that phenomenon?

#### RODALCO

##### Well-Known Member
In transformers there are mainly 3 types of losses.

1.-Hysteresis losses are losses in the B/H curve for magnetiseing the core back and forth 50 or 60 times a second. (steady losses)
2.-Eddycurrent losses are circulating currents within the TX steel where the insulation between the iron laminations is not perfect (steady losses)
3.-Copper losses are losses within the windings and vary with the load ( basically I²R losses)

In your case with a shorted secondary the copperlosses will cause the windings to overheat , the insulation to melt etc.

#### colin55

##### Well-Known Member
Briefly, same number of turns means one-to-one, same out as in.
More turns on the secondary, higher voltage out, and draws more current from the primary.
Less turns on the secondary, lowers voltage out, and draws less current from the primary.
More turns on the secondary, higher voltage out.
Less turns on the secondary, lowers voltage out.

Current is a separate issue.

When it gets too hot, the thermal fuse
blows and disconnects the fan from the power. The fan is not usable
after that but at least there was no fire

The latest "thermal fuses" are resettable. Wait 10 minutes and the fan or heater will work. But you must turn the power off for the fuse to re-set itself. A very clever device.

Suppose you have a transformer with the secondary not connected to a load.
As the AC waveform rises, the voltage allows a current to flow in the primary winding and this produces magnetic flux in the core of the transformer. This flux cuts all the turns of the primary and produces a voltage IN THE OPPOSITE DIRECTION. Thus, at any point in time, the incoming voltage may be 20v, but the back voltage may be 19.5v. Thus the forward voltage will only be 0.5v.
This effect continues right up to 315v, the peak voltage for a 240v AC line. The back voltage will be as high as 310v.
This magnetic flux will also cut the secondary winding, but because this winding is “open circuit” the flux will not be affected.
This is why a transformer that is unloaded (no load on the secondary) will only get slightly warm and take almost no current.
Now, if we load the secondary, the magnetic flux that is constantly cutting the secondary turns, will pass through the turns and produce a voltage (it produced a voltage before but the voltage was not connected to anything). This voltage will appear on the secondary and thorough the load. The resistance of the load will cause a current to flow.
This current will “use up” (reduce) the magnetic flux and thus the back voltage produced by the flux will be less.
This will cause a higher current to flow in the primary winding.

No we have the situation of a “short” such as the secondary leads shorted together.
This is the same as putting a very heavy load (low resistance) on the secondary.
A very high current will flow in the secondary and this will reduce (use up) the magnetic flux considerably and thus a high current will flow in the primary.
It is simply a multiplication of the voltage and current in the primary that produces a higher than normal wattage consumption. This will heat up the primary winding and eventually damage the insulation. If a turn from one layer touches a turn from a lower layer (normally these turns will be 20 - 50v different in voltage) and they “weld together” it will produce a number of turns just like a shorted secondary.
This will cause a large current to flow in the primary and the heating will increase.
Eventually the transformer will “burn out.”

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#### MrAl

##### Well-Known Member
Hello again,

The latest "thermal fuses" are resettable. Wait 10 minutes and the fan or heater will work. But you must turn the power off for the fuse to re-set itself. A very clever device.
Really? All of the fans i have bought in the fairly recent past have
had the standard cheapie little 2 or 2.5 amp thermal fuses installed. I'll
most likely be looking again for another new one in a month or two.
Perhaps i'll look at the models with resettables in them first this time.
I am tired of replacing those little buggers which means taking the covers
off and sometimes even more disassembly.

What make/models do they install the resettable fuses in?

#### crutschow

##### Well-Known Member
Well yea, actually I wanted to know how does the FLUX cause the transformer to get hot and then burns or melts? It seems to me that the interacting between primary and secondary fluxes cause that phenomenon?
There is one magnetic flux coupling between the primary and secondary windings. The flux only provides coupling, it does not directly generate heat, only by it's interaction with the windings and the core. If you had a perfect transformer with perfect magnetic material, no leakage flux, and superconducting wire, then you would have no heat generated.

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#### Hero999

##### Banned
Some appliances have resettable thermal fuses and others have the single use variety.

I've even found a wallwart that had a resetable bi-metal thermal cut out!

I think the government should pass a law saying that thermal fuses shall be resettable where practical, to minimise e-waste.

#### colin55

##### Well-Known Member
The re-settable fuse I am talking about is called a poly fuse (poly switch). It needs the supply to be removed before it will automatically reset.
These fuses are in $14.00 2,200 watt room heaters. I think this is about the most sophisticated of all the fuses. The bimetal trip is on all our 4-outlet power boards (cost$3.30) but it has to be manually reset. How they can make and sell a power board for $3.30 is beyond me. They originally sold for$15.00 when our wages were $55.00 per week. Our wages are now$650.00 per week (for less hours) and the power boards are \$12.00 for a pack of 4. The miracle of electrical, electronics, and the Chinese.

#### MrAl

##### Well-Known Member
Hi again,

Oh ok, by the wording it sounded like you might have known some fans that
have a resettable fuse built in. That sure would be nice, but then im not
sure what the UL rules are for this kind of consumer product (fan) as to if
they allow them to be resettable or demand that the one time use devices
be incorporated instead. This is part of why i was so interested too.

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