Reusing a salvaged known yellow transformer from a USB charger and using it in H bridge configuration

Question

I have salvaged a transformer from a defective USB charger (witch was not mine).

the print on it refers to an actual USB charger from the same brand. additionally 12 V in big also printed on it. No datasheet can be obtained on it. these like a coin small yellow transformers, most of us have pretty sure have seen.

My "Home Improvement Shop multi meter" fluctuated around 0.9 Ω or so, not so 1 Ω. symmetrical center tapped on 1 side. 2 coils on the other side, with looks like the primary side. There is difference in thickness of the wires on the sides, not sure if that indicates the high-power side.

I want to power it with 12 V using a H bridge 4 MOSFET circuit.

I don't know the original configuration of the transformer, I kind of threw away the board, but a common 15 V transformer I have around showed some 29 Ω on the primary.

The resistance is pretty much to low for my multi meter, I'm wondering if this transformer will allow me to run it in a H-bridge. It's like you need to solder an extra resistor to see if there is any difference.

That's the board I want to make or would I simply cause a short circuit?

Answer 1

As long as it's not defective (e.g. shorted wire inside) it can be used for different applications but you need to know all the parameters of the transformer.

• Measuring winding resistances will not tell you anything other than the DC resistance which can be used (partially) for calculating copper losses.
• You do need to measure the inductances (with an LCR meter or with a signal generator-scope pair) as well as the turn ratios. You can calculate the turns ratios from the inductances (\$N_1/N_2=\sqrt{L_1/L_2}\$) but better to use a signal generator and a scope (make sure the signal generator should be able to generate the switching frequency of your application).
• Measure the wire diameters as they will give you an idea about the maximum allowed current to flow through.
• If you are sure that it used to belong to a working wall charger then the primary-secondary insulation should be sufficient for your application. Otherwise you may want to test the insulation as well because the secondary voltage in your application is quite large relative to the primary voltage.

When driving a transformer with pulses you need to ensure proper/guaranteed reset otherwise the transformer's core will saturate after a couple of pulses and the circuit will stop working. Also the switching frequency should be selected to ensure acceptable core loss to prevent overheat (NOTE: Core properties start to change as the temperature goes beyond 100~120°C).