[SOLVED] False Open or shorted Primary transformer in circuit tests

[danny davis]

Measuring a transformer In-circuit , AC step down, AC 115 vac to 30 VAC

If there is an open or shorted component on the secondary winding of a transformer, can this measure O.L over load on the Primary winding?

If there is an open or shorted component on the secondary winding of a transformer, can this measure below 10 ohms on the primary winding?

If one or two of the isolated secondary winding coils on the transformer are open or short, can this cause the primary winding to measure a different ohm reading? it will it be the same ohm measurement? does the resistance go up or down on the primary winding?

One primary winding that has 3 isolated secondary coils

If the Primary winding is shorted or open, can this cause the other secondary coils to measure different ohm readings? does the resistance of the secondary coils go up or down?

 

[Tunelabguy]

Measuring a transformer In-circuit , AC step down, AC 115 vac to 30 VAC

If there is an open or shorted component on the secondary winding of a transformer, can this measure O.L over load on the Primary winding?

If by "overload" you mean that the primary draws excessive current, then yes.

If there is an open or shorted component on the secondary winding of a transformer, can this measure below 10 ohms on the primary winding?

An open component would likely cause less of a load. A short would cause more of a load. But you cannot measure anything about the secondary by using an ohmmeter on the primary. An ohmmeter will only tell you the resistance of the wire in the primary, and it could very well be 10 Ohms. But it would be the same resistance regardless of what is connected or shorted in the secondary circuit.

If one or two of the isolated secondary winding coils on the transformer are open or short, can this cause the primary winding to measure a different ohm reading? it will it be the same ohm measurement? does the resistance go up or down on the primary winding?

As I said, the ohm reading on the primary is not affected by anything that goes on in any secondary winding.

If the Primary winding is shorted or open, can this cause the other secondary coils to measure different ohm readings? does the resistance of the secondary coils go up or down?

Likewise, ohm readings of the secondary windings are not affected by anything that goes on in the primary. The only thing an ohmmeter reading on a transformer winding will tell you is the resistance of the wire in that winding. That is all. It does not tell you anything more.

 

[Audioguru]

First you must understand what happens to voltage and current (Ohm's Law) when a component is open or is shorted but it seems that you do not know.
Do you know what happens to voltage and current when a transformer is overloaded?
If the secondary of a transformer is shorted then what tester would you use to measure an overload on its primary?

Does the DC resistance of the wire of one winding affect the DC resistance of another winding?

What is "impedance"?

 

[danny davis]

If by "overload" you mean that the primary draws excessive current, then yes.

If if a component was shorted or open on the secondary Circuit , the primary winding will draw more current? and overload the primary? causing it to short or open?

The only thing an ohmmeter reading on a transformer winding will tell you is the resistance of the wire in that winding. That is all. It does not tell you anything more.

What else is there to know? the ohm meters can measure if the transformer is SHORTED or OPEN,

Is there any other kind of short or open a transformer is that is not a resistance short or open?

Can you measure a transformer IN-CIRCUIT without any problems of interaction of the other secondary circuit?

Or do you really have to disconnect all the transformer wires to really test it for shorts and opens ? or can I do this in circuit?

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Do you know what happens to voltage and current when a transformer is overloaded?

The voltage and current raises up , because the resistance inside the transformer goes down , why i'm not sure

If the secondary of a transformer is shorted then what tester would you use to measure an overload on its primary?

An AC power meter or AC volts meter

Does the DC resistance of the wire of one winding affect the DC resistance of another winding?

It shouldn't , because the coil are isolated

What is "impedance"?

AC resistance or frequency impedance

 

[Audioguru]

Question from Audioguru: "Do you know what happens to voltage and current when a transformer is overloaded?"
The voltage and current raises up, because the resistance inside the transformer goes down

No, the voltage does not rise, it drops.
Correct, the current rises.
No, the resistance in the transformer does not change. The LOAD resistance is too low.

Question from Audioguru: "If the secondary of a transformer is shorted then what tester would you use to measure an overload on its primary?"
An AC power meter or AC volts meter

Correct, an AC power meter.
Correct, an AC volt meter will simply show that the voltage is too low.
An overloaded transformer will get hot.

Question from Audioguru: "Does the DC resistance of the wire of one winding affect the DC resistance of another winding?"
It shouldn't, because the coils are isolated

Correct for another reason. The AC impedance of one winding is coupled to other windings, not DC resistance.

Question from Audioguru: "What is "impedance"?"
AC resistance

Correct.

You knew a few of the answers so you should know more about simple transformers.

 

[danny davis]

the voltage does not rise, it drops.
Correct, the current rises.

Where does this extra current come from? it makes the primary coil work harder into making the extra current?

No, the resistance in the transformer does not change. The LOAD resistance is too low.

Load resistance is the secondary circuit? the circuit board with components on the secondary become Low resistance as a Load , why? because an open or short changes the Load resistance

Cuts the load resistance in half or more when there is a short or open on the secondary circuit which causes the Primary coil to get overloaded, so the current does up? and the voltage goes down

I would think the primary coil would get overload with there is an EXTRA load , when the secondary circuit has Extra load than normal

A short or open cuts the secondary circuit in half or more right? or does this Double the resistance in the secondary circuit?

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C

orrect for another reason. The AC impedance of one winding is coupled to other windings, not DC resistance.

So I can check if the primary coil and all the isolated secondary coils are shorted or open? or good? while IN CIRCUIT?

or an a component that is open or shorted cause a FAKE or FALSE meter reading that a electronic tech would this the SECONDARY coil is open? or shorted? even tho the secondary coil is good

 

[Audioguru]

A transformer has a power rating determined by the size of its core and the thickness (resistance) of its windings. Let us say that your 120V to 30V transformer is rated at 90W. Then its maximum secondary current is 90W/30V= 3.0A. The load resistance is 30V/3A= 10 ohms.

The maximum primary current is 90W/120V= 0.75A.

Something happens to the load to cause its resistance to be too low, maybe only 5 ohms. Then the current is doubled to 6A and the power is also doubled to 180W.
Why do you think the transformer is overloaded with a current too high when the secondary is open? Open means disconnected then there is no load so there is no current so there is no power so there is no overload.

Yes, the primary gets overloaded when there is extra load from the secondary.

I do not know if your transformer tester can measure the transformer in a circuit.
You would need to test a good transformer to see if the original transformer tests the same or not.

A short is the OPPOSITE to an open but you do not understand that. A short draws a very high current but an open draws NO current. Resistance probably has nothing to do with something (maybe a transistor) in the secondary circuit drawing too much current.

I do not know if your transformer tester can be used in circuit and I do not know if you have a good transformer to compare the original transformer with.

You will probably NEVER see a shorted or open power transformer so why are you talking about it?

 

[danny davis]

I do not know if your transformer tester can measure the transformer in a circuit.
You would need to test a good transformer to see if the original transformer tests the same or not.

I mean measure the ohm resistance of each secondary coil and primary coil, does the DC resistance of the coil change? because the Secondary coils go direct to the circuit or channels of circuits to supply AC or DC power

So if a component is shorted or open , that means it changes it's resistance or load , which causes the transformers coil IN CIRCUIT resistance to change?

 

[Tunelabguy]

If if a component was shorted or open on the secondary Circuit , the primary winding will draw more current? and overload the primary? causing it to short or open?

Stop talking about shorts and opens as if they are similar. They are opposites. Speaking only of shorts, if a component connected to the secondary of the transformer shorts out, it will draw more current, causing the secondary to draw more current, which causes the primary to draw more current, which makes the primary get hot. What it does after that depends on how hot it gets. If it gets hot enough to melt the wire, that could cause the primary winding to become open. More likely it will just blow a fuse.

What else is there to know? the ohm meters can measure if the transformer is SHORTED or OPEN,

I see. I thought you were asking if an ohmmeter on the primary could detect a shorted component on the secondary. But you were actually thinking that the short on the secondary would cause the primary to burn out, and maybe go open circuit. Yes, that is possible. But it is not guaranteed. The primary might not get damaged at all. But if the primary does get damaged, then an ohmmeter can tell you that,but only after you turn off the power and disconnect anything else that the primary is connected to. And you said you wanted an in-circuit test. This is not an in-circuit test.

Can you measure a transformer IN-CIRCUIT without any problems of interaction of the other secondary circuit?

Usually, no.

 

[danny davis]

if a component connected to the secondary of the transformer shorts out, it will draw more current, causing the secondary to draw more current, which causes the primary to draw more current, which makes the primary get hot.

Why does the transformer draws more current when the resistance is very low?

What's is the ohm law formulas that has a overload formula applied to ohms law?

It's not V/R = I because it's not overloading when i'ts V/R= I , i need a formula that applies the overloading to V/R=I

Can you measure a transformer IN-CIRCUIT without any problems of interaction of the other secondary circuit?
Usually, no.

Why can't you measure a transformer in circuit?

 

[Audioguru]

Why does the transformer draws more current when the resistance is very low?
What's is the ohm law formulas that has a overload formula applied to ohms law?
It's not V/R = I because it's not overloading when i'ts V/R= I , i need a formula that applies the overloading to V/R=I

Why can't you see that V/R = I then if R is reduced then I is increased?? Don't you know simple arithmetic??

As you show, overload is usually caused by too much current (I in your formula). If the normal output of the transformer is 30V/3A which is 10 ohms then if the load reduces to 5 ohms the current is doubled to 6A then the 3A transformer us overloaded and gets hot.

Why can't you measure a transformer in circuit?

Because you do not know the current it is supposed to normally draw. It might never draw its maximum rated current and you might not even know its maximum rated current. Out of a circuit you should measure no or very low primary current and a secondary voltage a little higher than normal.

 

[danny davis]

If the normal output of the transformer is 30V/3A which is 10 ohms then if the load reduces to 5 ohms the current is doubled to 6A then the 3A transformer us overloaded and gets hot.

Yes I understand , it's the power rating on the transformer that makes the difference, on how much current it can handle

Out of a circuit you should measure no or very low primary current and a secondary voltage a little higher than normal.

I just measure the ohm resistance of the primary coil and isolated secondary coils with my DVM meter

What tests do you do in circuit or out of circuit when testing a Transformer?

 

[betwixt]

Danny, you have already told us you can measure the secondary while it is disconnected. So test your transformer by disconnecting the secondary and turning the AC power on to the primary. Measure the AC VOLTAGE at the secondary, if it is correct the transformer is good, if it is too low the transformer is faulty. End of test.

Brian.

 

[Audioguru]

I just measure the ohm resistance of the primary coil and isolated secondary coils with my DVM meter

Why? Resistance is a DC measurement but a transformer works only from AC. Measuring the DC resistance of the coils with a DVM will show if the transformer is burnt out (then it would measure open) but looking at it and smelling it will also tell you that.

What tests do you do in circuit or out of circuit when testing a Transformer?

I think you are wasting your time measuring and talking about a transformer. It is simply some wire around a core. AC goes in and AC comes out.

 

[danny davis]

So test your transformer by disconnecting the secondary and turning the AC power on to the primary. Measure the AC VOLTAGE at the secondary, if it is correct the transformer is good

If a secondary isolated coil was shorted , can this cause the primary to overload? or change it's DC resistance?

Any reason why I can't test the Secondary coils in Circuit? if there is a component that is shorted or open, the secondary coils DC resistance would change?

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Resistance is a DC measurement but a transformer works only from AC. Measuring the DC resistance of the coils with a DVM will show if the transformer is burnt out

What other test or measurements would u do?

How else can a transformer fail?

What are these AC transformer fails you guys are talking about? I guess I only know about how to test a transformer in DC resistance fails

 

[betwixt]

Danny - Imagine my transformer has 1000 turns on it's primary and the resistance from one end of the winding to the other is 10 Ohms.
What would the resistance be if turn 727 and turn 728 were shorted to each other?
Would your DVM with say 1% accuracy be able to confirm whether the short was there or not?

The same transformer has 100 turns on it's secondary side, it's thick copper wire with a resistance of 0.01 Ohms from end to end.
What would be the resistance if turns 5 and 6 shorted to each other?
Would your DVM with it's 1% accuracy be able to confirm if the short was there or not?

Take a transformer (any power transformer will do), connect it's primary to the AC supply with your DVM is series, set to the AC AMPS range, measure the current through the primary.
Now short out the secondary and see what happens to the primary current.
From that, work out if something different would happen if the short was inside the transformer instead of outside it.

NOTHING can make the resistance of the wire change except for it's normal temperature coefficient. It is perfectly normal for the resistance to change slightly with temperature but that would not indicate a fault or be reason to create a fault.

Brian.

 

[Audioguru]

If a secondary isolated coil was shorted , can this cause the primary to overload? or change it's DC resistance?

Why don't you know that a short causes high current which is an overload??
Why on earth do you think a transformer winding will become shorted?? Because it was made in a 3rd-world country with a name you never heard of?
Because it fell off a truck?

Any reason why I can't test the Secondary coils in Circuit? if there is a component that is shorted or open, the secondary coils DC resistance would change?

OF COURSE NOT! A coil is simply a piece of wire. Its resistance is not changed by a shorted or open component in its load.
HELLO! You forgot that a transformer works with AC, not DC. The IMPEDANCE (AC resistance) of its load is changed.

What other test or measurements would u do?
How else can a transformer fail?

You are wasting time by testing transformers.

What are these AC transformer fails you guys are talking about? I guess I only know about how to test a transformer in DC resistance fails

Your English is awful. Maybe you are talking about "failures" not "fails".
A quality transformer rarely fails or rarely has a failure.

 

[danny davis]

What would the resistance be if turn 727 and turn 728 were shorted to each other?
Would your DVM with say 1% accuracy be able to confirm whether the short was there or not?

DC resistance, it might change 1 ohm or less , so how can you test this type of short than?

The same transformer has 100 turns on it's secondary side, it's thick copper wire with a resistance of 0.01 Ohms from end to end.
What would be the resistance if turns 5 and 6 shorted to each other?
Would your DVM with it's 1% accuracy be able to confirm if the short was there or not?

No, would be in the milliohms , would need a milliohm meter

How would u test this type of short?

You forgot that a transformer works with AC, not DC. The IMPEDANCE (AC resistance) of its load is changed.

So how do u test,check and verify the AC resistance of the primary and secondary coils?

If a component on the secondary circuit is shorted or open it changed the transformers secondary coil AC resistance? not DC resistance?

 

[Audioguru]

A winding on a transformer is simply a length of wire. The wire has the same DC resistance if the load of the transformer becomes shorted or becomes open.
If the load on the secondary becomes shorted or open then the primary "sees" a different impedance and the DC resistance of both coils remain the same as before.

If a transformer has an AC input but has no output then you can measure the resistance of the windings if you want.