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

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 250-7700 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

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. I don't know from what model car it comes.

 

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.

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Measurements on audio transformer TR10.16

In this measurement, the input impedance, frequency range and efficiency is measured of audio transformer TR10.16
This transformer has output connections of 0, 4, 8 and 16 Ω.
A load resistor of 16 Ω is connected on several ways to the output.
 

Audio 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

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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

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