How to measure impedance of transformer

M

MNQ

Guest
Hi All,

Can anyone advise me on how I can measure the impedance of a transformer.
The datasheet states it should be 600 ohms. I have taken a voltmeter set to
ohms and it only shows a few ohms. This is what I think it should be
without anything connected. I imagine I need to put an ac voltage across it
.. what do I do then?

The transformers part number is GR128417. This is a Gardners audio
transformer made many years ago.

Any help would be appreciated.

Naveed
 
Naveed,

The input impedance depends on the output load impedance of the xformer and also
depends on frequency. Assuming it is properly loaded, it is easy to find input
impedance experimentally. Connect a sine wave signal generator of about 1000 Hz.
and a series resistor of about 600 ohms to the input. Measure the AC voltage
across the input of the xformer. If it is 600 ohms you will get 1/2 half the
input voltage at that point. In other words the series resistor and the input
acts as a voltage divider. Good luck..

George

MNQ wrote:

Hi All,

Can anyone advise me on how I can measure the impedance of a transformer.
The datasheet states it should be 600 ohms. I have taken a voltmeter set to
ohms and it only shows a few ohms. This is what I think it should be
without anything connected. I imagine I need to put an ac voltage across it
. what do I do then?

The transformers part number is GR128417. This is a Gardners audio
transformer made many years ago.

Any help would be appreciated.

Naveed
Click to expand...
 
I read in sci.electronics.design that MNQ <corlioni1976REMOVE@yahoo.co.u
k> wrote (in <c3s5md$hff$1@rdel.co.uk>) about 'How to measure impedance
of transformer', on Wed, 24 Mar 2004:

Can anyone advise me on how I can measure the impedance of a
transformer. The datasheet states it should be 600 ohms. I have taken a
voltmeter set to ohms and it only shows a few ohms. This is what I
think it should be without anything connected. I imagine I need to put
an ac voltage across it . what do I do then?

That '600 ohms' is the iterative impedance of the circuit in which the
Click to expand...
transformer is intended to be used. In other words, it's intended to
work from a 600 ohm source impedance into a 600 ohm load.

You can't measure the 'impedance of a transformer' in the sense you
mean. There is no such thing. There IS such a thing in another sense
entirely, related to what it does at the extremes of its useful
frequency range, but that's a long story for another day.

Your transformer isn't confined to use in 600 ohm circuits. It will work
with a range of source and load impedances but its frequency response
and its losses ('insertion loss') depend on those impedances: they would
presumably have their best values with a 600 ohm source and a 600 ohm
load. But you could use it with a 60 ohm source and a 10 000 ohm load if
you wanted. You might lose 2 dB of signal level compared to a direct
connection, and there might be a peak in the frequency response at, say,
8 kHz, but you might be willing to accept that.

In order to help further, you need to explain what you want to use the
transformer for.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
MNQ wrote:
Hi All,

Can anyone advise me on how I can measure the impedance of a transformer.
The datasheet states it should be 600 ohms. I have taken a voltmeter set to
ohms and it only shows a few ohms. This is what I think it should be
without anything connected. I imagine I need to put an ac voltage across it
. what do I do then?

The transformers part number is GR128417. This is a Gardners audio
transformer made many years ago.

Any help would be appreciated.
Click to expand...
The 600 Ohms are specified at 1kHz.
It now depends on the impedance on the other side. It can be 4 Ohms for
example. Whatever. Have a termination resistor at the other side with
the proper impedance. Then you'll be measuring the 600 Ohms
at 1kHz on your side.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
George

Thanks for your speedy response. Much appreciated.

Naveed


"George Steber" <steber@execpc.com> wrote in message
news:4061A040.F9020EEE@execpc.com...
Naveed,

The input impedance depends on the output load impedance of the xformer
and also
depends on frequency. Assuming it is properly loaded, it is easy to find
input
impedance experimentally. Connect a sine wave signal generator of about
1000 Hz.
and a series resistor of about 600 ohms to the input. Measure the AC
voltage
across the input of the xformer. If it is 600 ohms you will get 1/2 half
the
input voltage at that point. In other words the series resistor and the
input
acts as a voltage divider. Good luck..

George

MNQ wrote:

Hi All,

Can anyone advise me on how I can measure the impedance of a
transformer.
The datasheet states it should be 600 ohms. I have taken a voltmeter
set to
ohms and it only shows a few ohms. This is what I think it should be
without anything connected. I imagine I need to put an ac voltage
across it
. what do I do then?

The transformers part number is GR128417. This is a Gardners audio
transformer made many years ago.

Any help would be appreciated.

Naveed
Click to expand...
 
The impedance seen by a transformer on one side depends on the impedance of
the other side AND the turns ratio of the transformer.
SO it is pointless trying to measure it in this case. With an ohm-meter you
are actually measuring the DC resistance of the copper winding of the
transformer.

If the data sheet says that it is a 600 ohm transformer then this means that
it has been designed to be connected to a circuit that has an impedance of
600 ohms. The 600 ohms actually refers to the circuit and not the
transformer itself.
 
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