Transformer phase

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I have a toroidal transformer with a single primary and secondary.

How do I find out which leads are in phase and which are out?

I thought about hooking up my dvm (old fluke 87) to the secondary and "tapping" a 1.2 v battery (AA, C, etc.) To the primary and see if that would say if the secondary leads were connected to the dvm in phase or out. However being a dvm I don't know if it's reaction time will be fast enough.

Any other ideas?

TIA,

Troy
 
Connect the primary to a low voltage AC source. Then connect the secondary in series with it. Then measure the secondary's voltage, and then the voltage between the outer terminals. Both windings are in phase if the voltage between the outer terminals is the sum of both the primary and secondary voltages.
Best regards!
 
Hi Troy.
You can actually use a DVM to check transformer phasing but you have to be very careful as you will work on the primary voltage!

You connect you 115Vac to the primary winding as usual (the primary terminals we call 1 and 2). Then, the unusual part: You connect one end of the secondary winding (we call that terminal 3) to one terminal of the primary winding (we may use terminal 2). If you put your DVM in 200V AC range and measure from terminal 1 of the primary to the second terminal of the secondary winding (we call that terminal 4), you will either measure more than 115V (then, the primary and secondary windings are in phase) or less than 115V (then, the primary and secondary windings are out of phase).

Is that explanation clear?
 
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Thank you all for the replies.

This is not a mains transformer (logical conclusion) but rather a signal transformer. Current to a speaker converted to a voltage for signal manipulation.

So no 110v testing as the primary is to low a load and would fuse.

I will try 12vac with a resistor in series to see if I can get that to work. I need to break down and get a new oscope.
 

PRR

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Joined 2003
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> The ear doesn't care

The specific application is not clear.

Troy, you have not given any voltage, ratio, or impedance numbers. It is hard to talk in general, and the practical hacks vary.

I agree with no 120V even if the tranny were good for it. I assume you will get like a 6VAC heater transformer. A 10 Ohm resistor helps limit the accidental smoke.

I will start with a 1:1 ratio case.

Put the two windings in series any way (you can document!). Drive one winding. You either get 12V or zero V. Check the other way also. (A dead tranny or jumper will be zero both ways.)

With a significant ratio the difference in-phase or out-phase will be smaller; but for many practical ratios the difference is clear.

If you are using a 1:1000 current transformer, the difference may be unclear. (Your power line is not 0.1% stable minute to minute.) And the 1-turn side is too heavy a load for a heater transformer, while the sense coil will not induce decent voltage in the 1-turn.

In fact this is no different than wiring a NFB tube amp. No matter how carefully you account inversions and dots, on smoke-test it howls. The traditional method is to leave the OT leads long, and swap them as needed.
 

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Had a few minutes to play yesterday and I can confirm the 1.2vdc batter method was a waste of that time.

I will rummage through the power supply bins for an AC wall wart and try the addition / subtraction method.

Thank you for the advice, more to come later this week.
 
> The ear doesn't care

The specific application is not clear.

Troy, you have not given any voltage, ratio, or impedance numbers. It is hard to talk in general, and the practical hacks vary.

I agree with no 120V even if the tranny were good for it. I assume you will get like a 6VAC heater transformer. A 10 Ohm resistor helps limit the accidental smoke.

I will start with a 1:1 ratio case.

Put the two windings in series any way (you can document!). Drive one winding. You either get 12V or zero V. Check the other way also. (A dead tranny or jumper will be zero both ways.)

With a significant ratio the difference in-phase or out-phase will be smaller; but for many practical ratios the difference is clear.

If you are using a 1:1000 current transformer, the difference may be unclear. (Your power line is not 0.1% stable minute to minute.) And the 1-turn side is too heavy a load for a heater transformer, while the sense coil will not induce decent voltage in the 1-turn.

In fact this is no different than wiring a NFB tube amp. No matter how carefully you account inversions and dots, on smoke-test it howls. The traditional method is to leave the OT leads long, and swap them as needed.

I had some time Tuesday to try as you suggested. No joy. Apparently the 1:x ratio was too high for a meaningful voltage measurement.

I will be buying a new o-scope shortly and will try looking at the phases that way.
 
Can you measure inductance? Many DMMs have an inductance range.

Connect the windings in series. Measure inductance. Reverse one winding. Measure again. The highest inductance corresponds to the windings being in phase.

PS as this is not a power transformer the thread is in the wrong place. Maybe 'parts' rather than 'power supplies'?
 
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Thank you all for the replies.

This is not a mains transformer (logical conclusion) but rather a signal transformer. Current to a speaker converted to a voltage for signal manipulation.
We are missing some *important* information here.
*How* are you going to use your signal transformer?

* IF you will rectify secondary voltage and use that DC for something , phase is irrelevant.

* but IF you are using that Audio voltage (which is AC) directly, as in some kind of NFB loop , or processing it some way or the other (filtering, etc.) but still staying AC, then phase is most important.

So which one it is?
Please provide details on how you plan to use such secondary voltage.
 
It is part of a nfb loop so proper phase is required. The primary is a short stout wire so it has very low resistance and inductance in a 1:x (high x) ratio.

I think the only way to test it out of circuit will be to put a signal on the secondary and check phase on the primary.

However in typing this I realized I can make a simplified audio circuit and test it in a similar fashion as it will be used once I get the new Oscope.

Once again you need the proper tools to do good work.
 
What can you measure? Obviously, with no test equipment you can't measure anything. With some test equipment you can measure a lot of things, with extra thinking sometimes making up for lack of equipment. If you have an AF sig gen and can measure AC voltage then you can check phasing, by measuring the resonant frequency of the transformer with a capacitor in parallel.
 

PRR

Member
Joined 2003
Paid Member
Do you realize that a Current Transformer's secondary MUST be "shorted"?

If left open, an ideal transformer would induce a high impedance in the primary, which you do not want. Also a high primary current with open secondary makes HIGH secondary voltage. Power CTs can be lethal if the load falls off.

Finger-size CTs are a little different than pole-top metering CTs. Their cores saturate and the secondary voltage will not go over maybe a Volt. (Will vary with frequency.) Obviously once saturated they don't give a useful reading.

I still do not understand the Need To Know. NFB loops are often connected wrong the first time. The system howls at maximum output. Simply reverse the leads. (If this is a loudspeaker, probably best to first-try with a dummy load so you don't annoy the neighbors.)
 
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