Experiments with transformers.
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On this page measurements are described on:
Audio transformer 952.431
12 V mains transformer
Ignition coil
Audio transformer TR10.16
Audio transformer P037S
Measurements on audio transformer 952.431
Audio transformer model 952.431 has 2 output's, one with a impedance of 8
Ohms and the other with 16 Ohms impedance.
If we load the output with this (normal) value, the input impedance (measured at
the connection 0.125 Watt) will have a value of about 80 k.Ohm.
But if we increase the loadresistor on the output, the input impedance of the
transformer will also increase.
In the following measurements, I loaded the 8 and 16 Ohm outputs with several resistor values, and measured the input impedance, the frequency range and efficiency of the transformer
Transformer = 952.431
Measurement number 
Load resistor (Ohm) 
Impedance of output (Ohm) 
Increase of load resistor 
Input impedance (k.Ohm) 
Frequency range 3dB (Hz.) 
Efficiency 
1  16  16  1x  81.4  7718000  0.79 
2  16  8  2x  151.8  14013300  0.82 
3  32  16  2x  149.5  14313000  0.83 
4  32  8  4x  255  2507700  0.74 
5  64  16  4x  251  2507700  0.74 
6  64  8  8x  400  3705200  0.57 
Conclusions:
With increasing load resistor, the input impedance will also increase, and
frequency range will reduce.
The efficiency has it's highest value, when the load resistor is twice it's
normal value.
A frequency range of 2507700 Hz. (measurement 4 and 5) is well useable for
reception of medium wave stations, because they don't transmit audiofrequencies
above 4500 Hz.
If we don't mind some reduction of bass responce, we can load the output of this
transformer with 4 times it's normal value.
This gives a large increase in input impedance, wich gives a better impedance
match in our crystal receiver.
The efficiency is only reduced a little bit compared to the normal load
resistor.
Measurements on a mains voltage transformer.
Also normal mains voltage transformers can be used to transform audio
signals.
To become a high input impedance, the transformer must have a low power rating.
The primary voltage of the transformer must be as high as possible, so a 230
Volt transformer is better then a 110 volt transformer.
The here tested transformer has the following specifications:
Input voltage: 220 Volt **
Output voltage: 2x 12 Volt
Power: 1.2 Watt
Maximum output current: 2x 50 mA
** Nowadays the mains voltage in the Netherlands is 230 Volt, this transformer
is from the time it was still 220 Volt.
The two 12 volt windings are parallel connected, and form the output of the
transformer (low impedance).
The 220 Volt winding is the input of the transformer, with a high impedance.
Transformer: 220Volt in  2x12V out 1.2 Watt
Measurement number 
Load resistor (Ohm) 
Input impedance (k.Ohm) 
Frequence range 3 dB (Hz.) 
Efficiency 
7  120  25.7  753600  0.51 
8  240  38.2  1305700  0.68 
9  470  59.8  24011300  0.65 
10  1000  91.4  45020000  0.54 
11  2200  127.8  74027000  0.36 
With this transformer, we see at higher impedances, the frequency range will
go up, both low and high 3dB point.
Compared to audio transformer 952.431, this transformer has lower input
impedance, wors low frequency responce and a lower efficiency.
With bigger mains transformers (with higher power) the efficiency will be higher, but these have a lower input impedance.
A ignition coil used as a audio transformer.
A ignition coil of a car can also be used as a audio transformer.
A coil has two windings, one low voltage winding (with low impedance), and one
high voltage winding (with high impedance).
In a ignition coil the ground connections of the two windings are connected to each
other.
Test setup of the ignition coil. The type number of the used coil is
LA12.

In this test I used the high voltage winding as input winding.
The output (low voltage winding) is loaded with different resistor values and
the following properties are measured:
 input impedance at 1 kHz
 frequency range (3 dB)
 efficiency at 1 kHz
Load resistor  Input impedance at 1 kHz  Frequency range (3dB)  Efficiency at 1 kHz 
47 Ohm  148 k.Ohm  450 Hz....19 kHz  0.64 
68 Ohm  178 k.Ohm  600 Hz....19 kHz  0.58 
100 Ohm  212 k.Ohm  600 Hz....18,6 kHz  0.53 
220 Ohm  275 k.Ohm  1 kHz....13 kHz  0.31 
Both input impedance as efficiency reach there highest value in the middle of
the frequency range, and this is a higher frequency then 1 kHz.
But because frequencies around 1 kHz are most important for crystal receivers, I
measured at 1 kHz.
The low frequency response of this ignition coil is limited, but however it is
useful for crystal receivers.
We can e.g. load the output with a modern 2x 32 Ohm headphone (=64 Ohm), the
input impedance will then be about 170 k.Ohm.
Because in the ignition coil the two windings are connected, it's not possible to
series connect several coil's to get a even higher impedance.
Measurements on audio transformer TR10.16
In this measurement, the input impedance, frequency range and efficiency is measured of audio transformer TR10.16Audio transformer TR10.16 Brand: Visaton 
Load resistor (Ω) 
Load connected to output  Input impedance (kΩ) 
Frequency range 3 dB (Hz) 
Efficiency 
16  0 and 16  16.1  90 Hz ... 50 kHz  0.86 
16  0 and 8  28.7  200 Hz ...37 kHz  0.75 
16  0 and 4  43.8  320 Hz ... 25 kHz  0.73 
16  4 and 16  48.6  340 Hz ... 24.8 kHz  0.65 
16  8 and 16  74.7  690 Hz ... 13 kHz  0.32 
16  4 and 8  89.6  850 Hz ... 11 kHz  0.20 
Measurements on audio transformer P037S
In this measurement, the input impedance, frequency range and efficiency is
measured of audio transformer P037S.
This transformer has output connections of 0, 8 and 15
Ω.
A load resistor of 16 Ω is connected on several ways to the output.
Audio transformer P 037 S Brand: Eagle 
Load resistor (Ω) 
Load connected to output  Input impedance (kΩ) 
Frequency range 3 dB (Hz) 
Efficiency 
16  0 en 15  16.2  100 Hz ... 50 kHz  0.72 
16  0 en 8  28.7  220 Hz ... 43 kHz  0.67 
16  8 en 16  56.6  650 Hz ... 20 kHz  0.28 