Measuring Transformer Ratio

Transformer (Voltage vs Turn vs Impedance) ratio

Formula:

Vp/Vs = Np/Ns = (Zp/Zs)^1/2

Vp= Primary voltage
Vs = Secondary voltage
Np = Primary winding turn
Ns = Secondary winding turn
Zp = Primary impedance
Zs = Secondary impedance

Showing here is a very good formula which pointed the relationship between voltage, turn and impedance of a transformer.

Identify an Unknown transformer

Vp/Vs = Np/Ns

1. Measure the resistances of each windings with an ohm meter. Higher reading represent higher turns.

2. Connect a voltage supply(6vac, 12vac) to the high turn side.

3. Then measure the voltage on both sides.

4. If the measured voltage is Vp = 6v, Vs1 = 6v. Vs2 = 0.158

5. The ratio are Vs1 = 6v / 6v = 1 and Vs2 = 0.158v / 6v = 0.026.
If we supply 240v, then Vs1 = 240 x 1 = 240v. Vs2 = 240 x 0.026 = 6.24v

6. Finally this transformer is known by 240: 240 : 6.3v. It can be use to identify OPT too.

Identify Transformer OPT Impedance

Vp/Vs = (Zp/Zs)^1/2

1. Measure the resistances of each windings with an ohm meter. Higher reading represent higher turn.

2. Apply 6v to the lowest reading pair and measure the primary winding(should be the highest ohm reading).

3. I have 150v on the primary, Let put it into the formula, assume we need to plug it into a 8 ohm speaker load.

150v/6v = (Zp/8)^1/2

Zp = 625 x 8 = 5000

From here, we know that if you have a 150v : 6v transformer, you may ‘use’ it as a OPT with impedance of 5000 :8.