# How to measure transformer impedance?

Discussion in 'General Electronics Chat' started by james101, Apr 16, 2010.

1. ### james101 Thread Starter New Member

Jan 12, 2010
17
0
Hi,

I have an audio transformer (1:1 ratio) connected to the output of a Low Power Audio Amplifier (MC34119) as shown in the attached picture belwo. How do I measure the output impedance at the transformer secondary?

The datasheet for the MC34119 Audio Amplifier is here
http://www.freescale.com/files/analog/doc/data_sheet/MC34119.pdf

Thank you very much in advance!
James

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2. ### retched AAC Fanatic!

Dec 5, 2009
5,201
315
You could use a DMM in resistance mode while the transformer is NOT powered or connected to anything and measure between the secondary leads. (Even though with a 1:1 it would be the same on both sides.)

3. ### james101 Thread Starter New Member

Jan 12, 2010
17
0
That's the DC resistance. I would like to find the AC impedance at 1Khz.

Is the transformer's secondary AC impedance simply the output impedance of the audio amplifier?

4. ### Bychon Member

Mar 12, 2010
469
41
An audio transformer is like a lens. Id doesn't have impedance of its own. It reflects the impedance of the load back to the driver. If you put a 4 ohm speaker on a 1:1 transformer, the driver will believe it is talking directly to a 4 ohm load (at audio frequencies).

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5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,394
355
To measure the output impedance at the secondary of the transformer, as driven by the amplifier, do this:

Apply a 1 kHz signal to the input of the amplifier, so that the output is not too large, but easily measurable, perhaps 1/10 volt. Carefully measure the output voltage with no load connected. Next connect a variable load resistance to the output, and adjust its resistance until the output voltage is reduced by half. Measure the value of the load resistance. That value will be the output impedance.

Or you can use a fixed resistor. Measure the open circuit output voltage with sufficient drive to give you a small but easily measurable output voltage; call this voltage Vo. Now connect a resistor of suitable value; start with 10Ω. Measure the output voltage with the load resistor, RL, connected; call this voltage VL.

The output impedance is given by Zo = RL*(Vo/VL - 1).

If the output voltage doesn't change much when you connect the load resistor, use lower value resistors until it does.

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6. ### james101 Thread Starter New Member

Jan 12, 2010
17
0
Thanks guys! I can measure the impedance now.