LONG post about Impedance of audio transformers.
Hit the Delete Key now, if not interested.
Gang,
I've asked the question to the list on how to characterize small
size audio transformers. Recently I got a bunch of them and wanted to
know their characteristics to give them a good use.
So far, no replies to my message. Taking advantage of the five day
holiday we have here in Spain (Holy week and Easter) I have done some
homework. Here are my advances so anyone can comment, reply,
or even trash'em.
a) Main goal of a transformer is to convert electric current into a
magnetic flux and back into electric current. The flux obtained is
proportional to the number of ampere-turns and, avoiding losses, is
transferred from the primary (Np * Ip) to the secondary (Ns * Is).
b) The relationship among voltages is directly proportional to the
number of turns of primary and secondary, respectively, and the ratio
between turns (equal to the ratio of the voltages) is called the
transformation ratio: Np/Ns or Ep/Es.
c) Combining both equations, we may see why a 110/220 to 12 transformer
draws a little current on the primary but may provide a hefty current on
the secondary Is = (Np/Ns) Ip
d) An ideal transformer reflects the impedance that the secondary sees
following the relationship Zp = (Np/Ns)^2 * Zs
First try was to measure winding resistance. The problem is that they
use different wire gauges for primary and secondary (remember the
amperage ratio?) so pure resistance doesn't help although it serves to
decide what is going to be our primary winding, what the secondary.
An easy way to characterize the transformer, hinted by Juanito, EA2KQ,
is to use an ol' bridge. Two fixed resistors for one leg, a pot and the
transformer on the second leg, an AC voltmeter from left to right, and an
AC generator from top to bottom (you are wise enough to figure it out,
wontcha?)
An important point is to load (to terminate) the secondary of the
transformer with a pure resistor. I used a 8.2 ohm resistor since I
wanted the transformer for downstepping impedances: ? to 8 ohm.
Now the second question. At what frequency should I carry the test?
Well, remembering old generators with a fixed modulation of 1 kHz, I
tried that for a starter.
Oh, boy! what a Pandora box! Thanks God I had a transformer I was sure
of its characteristics.
Sample one: Know Z 200:8 ohms. Resistance 8:1 ohm.
Bridge result: 210:8 So far, so good.
Sample two: Unknown. Resistance 130:8. Bridge result: 400:8 ohm.
Sample three: Unknown. Resistance 60:105. Bridge result: 125:8 ohm.
Uh! Houston, we've got a problem!
Then I decided to try another way: if they are transformer, let's
them to play their trick. I took the AC generator and injected on the
secondary a sinusoidal wave, measuring the voltage at the primary with
an oscilloscope.
The voltage ratio on sample one was 5.1:1. Using equation on d),
it means that an impedance of 8 ohms on the secondary gets converted on
8 * 5.1^2 = 208 ohm. Not bad!
The other samples didn't worked as well as this one. Sample two had a
3.8:1 ratio, which gives as a result an impedance ratio of 115:8, which
didn't corresponds with the 400:8 obtained before.
Sample three had a lower voltage on the primary than on the secondary:
3.2 Vp / 5.2 Vs. which makes things ever worser.
Finally, I found a method suggested by "The Filter Doctor" Ed Wetherhold,
W3NQN. Using the output of a rig (which has a defined output impedance
of tipically 8 ohm), coupled to the secondary of the transformer, he
advises to, while measuring the voltage at the primary winding with an
AC voltmeter and NO load, increase the audio tone up to a level of 1 Vpp,
level which has to remain constant along the test. This voltage is V1.
Then, without changing anything, terminate the primary of the transformer
with a pure resistor "R" between 1000 and 1500 ohm (for a 200:8
transformer) and measure the resulting voltage, which is going to be V2.
Use then the magic formula Zp = R (V1-V2) / V2 and you'll get the
impedance of the primary for this setup (considering the impedance of the
source).
I haven't tried that since I am not sure any of my rigs will have such
"defined" output impedance. But I will try soon.
Let me know if you have any thoughts.
jon, ea2sn
(C) copyright Jon Iza 1997
-- Jon Iza ea2sn since 1978 qrp'er at large! <mailto:ea2sn@jet.es> ** One's needs are proportional to the square of his/her incompetence ** URE - ARRL - DARC - VERON - ARI - WIA - NZART - AMSAT-UK - TAPR - IDRA EAqrp 26 = Gqrp 1216 = ARCI 5153 = qrp-L 313 = NorCal 701 = ARS 202
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]