Transformer Short Circuit: Test Results and Solutions

[Pagedown]

I have been using a 25VA transformer without any problems, but it happened to trip the elcb/mccb once I plug it in.

I tested the primary windings and the secondary windings. The secondary windings seems to be short circuited with resistance of 0.3 ohm while primary with 70ohm. I tested other transformers and all secondary windings seems to be short circuit, even some primary windings.

Are all transformers like that?

 

[MATLABdude]

Remember that a transformer coil is just a long piece of wire.

 

[Pagedown]

ya, so do you mean the multimeter will show it is shorted?
Thus, why do the trip occur?? if it is shorted, my connected circuit will be shorted too right?

 

[MATLABdude]

It'll appear as a short if your coil has low-enough resistance (my multimeter beeps on continuity test--shows a short / connection--for anything under 50 ohms).

Just because it has low resistance doesn't necessarily mean that it's shorted--a short would mean that you'd have current bypassing the coil instead of going through it (short circuit). Additionally, because it is a coil, it'll have some impedance as well, when you feed it AC.

As to why it's tripping: you need to back up a little. When you say that you've been using the transformer "without any problems", and that it just happened to trip the breaker, do you mean that you're installing a new breaker or that just today, it tripped the breaker?

what is the rating on your breaker? And what are you feeding the transformer?

If you have a regular breaker (an MCCB), it'll trip if it's not big enough to handle the required current (as might be the case when you have a short that doesn't vapourize when you put current through it). If you have a ground-fault breaker (an ELCB) and have some leakage from either the neutral or hot to ground, then it'll also trip (in addition to the regular MCCB stuff).

 

[Pagedown]

The breaker has not been replace, in the lab, I have been using this transformer without any problems. But just today, no matter what i change, it trips the breaker. and i detect the short circuit until the transformer. No problem with circuit.

 

[MATLABdude]

Did you try plugging in another transformer of the same make? If so (and it still trips), have you tried plugging it into another circuit serviced by a different breaker?

 

[Pagedown]

I solved it, i think the transformer secondary wasnt connected properly to the circuit, thus it remained open.After connecting properly to the circuitry/load, it functioned properly.

But why would an open secondary transformer trip the elcb/mccb?

 

[Zryn]

But why would an open secondary transformer trip the elcb/mccb?

It won't. When I was working on circuits in the labs I would plug a transformer in and just have it sit there humming (terminals covered) as an audible warning for live electricity.

Is the transformer an auto-transformer possibly (one coil with internally joined reference wires and multiple voltage outputs), and did your circuit have any ground reference or is everything floating?

You only really have two options: you're pulling too much current (MCCB trip) or you've got current going where it shouldn't (ELCB trip).

Fuses (or even fuse wire) are pretty cheap if you really can't figure out what the problem is. Spread them liberally around your circuit and if any blow you have an over current problem, if none blow you probably have a leakage problem. Just try not to electrocute anyone!

 

[Pagedown]

It is a normal power transformer 240V to 12V, 25VA. The hot and neutral is connected to a rectifier.

The circuit has only one ground reference. That is connected to the rectifier leg.

 

[sophiecentaur]

I solved it, i think the transformer secondary wasnt connected properly to the circuit, thus it remained open.After connecting properly to the circuitry/load, it functioned properly.

But why would an open secondary transformer trip the elcb/mccb?

If the secondary of a properly designed transformer is left open circuit then there should be no (very little) primary current. If your "wasn't connected properly" involved the secondary being shorted then you could expect a lot of current to flow. Sounds like your bridge rectifier was connected by putting a diode right across the secondary ? Did the diode survive? They can be tough little devils.

 

[Pagedown]

It is a silicon full wave bridge rectifier. With an open circuit, isn't the primary acts like a short circuit between hot and neutral as there is only a small resistance and no magnetizing force to oppose it(open secondary)?

Now the problem was trailed to the sense resistor, 0.1 ohm at the neutral wire.

It is connected to an opamp for gaining purposes. Does it need another transformer before going to the gain stage? is there a short in the ground in the sense that the opamp ground and the neutral?

 

[sophiecentaur]

Look at the basics of transformer theory. Go to wikipedia, at least, before you make such a serious error. Do you seriously think that no secondary current means high primary current? The self-inductance of any decent primary prevents any more than a trace of primary current when unloaded. The input to the primary 'sees' the resistance in the secondary circuit scaled UP in by a factor of the turns ratio squared - i.e. a lot!

Btw, what 'sense resistor'? That could not affect the operation of the transformer - unless it were connected across one of the windings.

 

[Zryn]

Now the problem was trailed to the sense resistor, 0.1 ohm at the neutral wire.

Are you making an AC to DC power source with PID feedback control out of curiosity? Try powering the Op-Amp's with a floating power source (use an isolating transformer between the mains and the normal power source) and remember that oscilloscopes are internally grounded too if you make measurements and they come out funny or cause problems.

If you somehow manage to put a 0.1R resistor in parallel with the circuit then you would expect an over current fault, but it should be in series on the neutral (to create a voltage proportional to the return current value to eventually modify the input duty cycle to move the output power closer to the users desired power, right?).

Perhaps if you posted the schematic you are trying to make, someone could give you some tips if there's any problems. At this point its hard to see whether the problem lies in your understanding of theory or a potential wiring error.

 

[Pagedown]

This is my schematic, the function of this circuit is to translate voltages and currents into DC levels to be read by a MCU.

It has a built in simple +5V power supply to supply components, and the voltage & current sensings circuitry. The opamp is powered by the built-in +5V and -5V converted and supplied by a charge pump DC-DC converter TC7660 (not drawn in schematic).

Once the oscilloscope is connected across the output of the opamp (for current sensing), the breaker trips.