Determining Turns Ratio of Transformer

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RSRecords

Well-known member
Joined
Jun 8, 2009
Messages
318
Can you determine the ratio of transformer by measuring the nominal impedance of the Primary vs secondary?
For example:
10 ohms across primary
42 ohms across secondary

Is there a formula or is it a 1:4 transformer?
I recall it not being that simple but I couldn't find the thread discussing it. There's no way to determine reflected impedance without destroying it correct?

Thanks

 
Yes. The impedance ratio is just the square of the turns ratio (or in other words the turns ratio is the square root of the impedance ratio). For example, if turns is 1:4 then impedance is 1:16.

But 10 ohms and 42 ohms is pretty low. Is this a transistor transformer? Those sound like DCR measurements which is not useful. You need AC impedance.

Rather than trying to measure impedance, just measure the voltage ratio. The voltage ratio equals the turns ratio. Then square that to get impedance.

Update: I had the square / square root relation reversed. Fixed.
 
squarewave said:
Yes. The impedance ratio is just the square of the turns ratio (or in other words the turns ratio is the square root of the impedance ratio). For example, if turns is 1:4 then impedance is 1:16.

But 10 ohms and 42 ohms is pretty low. Is this a transistor transformer? Those sound like DCR measurements which is not useful. You need AC impedance.

Rather than trying to measure impedance, just measure the voltage ratio. The voltage ratio equals the turns ratio. Then square that to get impedance.

Update: I had the square / square root relation reversed. Fixed.

Ok thanks! It's an mci jh110c output transformer.

So i applied a 1k sine wave at 1Vac RMS to the primary. Secondary reads 1.3vac.
1:1.3

so impedance ratio 1:1.69.

so if the output is supposedly 600 ohms. would that be
1/1.69=x/600
.59=x/600
354=x
354:600

Does that look right?

 
RSRecords said:
Ok thanks! It's an mci jh110c output transformer.

So i applied a 1k sine wave at 1Vac RMS to the primary. Secondary reads 1.3vac.
1:1.3

so impedance ratio 1:1.69.

so if the output is supposedly 600 ohms. would that be
1/1.69=x/600
.59=x/600
354=x
354:600

Does that look right?
Yes. The primary impedance is just 600/1.169 =  355.

Cheers

Ian
 
There are a few things to be clear here. You have 10Ω and 42Ω. I assume it is rather DCR of the Pri and Sec. Doesn't tell you much--often those wound with different wire diameters...

Then you measured 1:1.3 voltage ratio--already much better. Once you know inductance of one of the Pri then it gives you much better idea what's going on. Say, you assume -3dB at 30Hz and measure Pri inductance as 1H. Then from the equation:
L = X / ( 2 × π × f )    Where X is impedance and f is frequency (another way of thinking about it is the low end response of the transformer is its inductance for given low end response)
you find that
X= 1Hx (2 × π × 30Hz )=188.4Ω (so in this case you would have a 200Ω transformer, which is pretty close)
Your Sec impedance is 188.4Ωx1.69=318.4Ω

Of course, your Pri inductance will be different. Once you are able to measure it you will know exactly what your transformer is.

Best, M

 
RSRecords said:
There's no way to determine reflected impedance without destroying it correct?
Absolutely not! Evaluating the reflected impedance is quite simple; once you knopw the voltage ratio, thus the impedance ratio, you apply that ratio to the load impedance. EG if you load your 1:1.69 xfmr to a 600 ohm load, the reflected impedance at the primary is 600/1.69 or about 355 ohms. That is useful to assess the drive capability of the driver; it must be capable of driving 350 ohms. This does not take into account losses or magnetizing current, but usually it does not make a big difference. A good designer (old-fashioned) will design the driver stage for being capable of driving 200 ohms.

Now if you want to assess the reflected impedance at the output, the quick n' dirty formula doesn't work, because of the losses and DC resistance of windings, that are much higher than the driver stage's output impedance. I'm talking about solid-state here, vacuum tubes are different.
 
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