Electrical – How much will the output voltage of a transformer drop as I add loads


I have a transformer that says 6V 600mA 3.6VA, AC current (but the voltage measured is actually 8.65Vac).
I will add a 4 diode rectifier WITHOUT capacitor (I say without in order to simplify the calculus). I want to connect LEDs (200mA per LED). So, I should be able to connect 3 LEDs.

I want to know how to calculate the series resistor for each LED. But I don't know the REAL output voltage of the transformer. Will the voltage drop after I add each LED (this is what I think it will happen) or only after I reach the maximum power consumption?

Best Answer

With a transformer that small, the load regulation will be very poor, maybe 20% (that is, loaded voltage = 80% of unloaded voltage), or could even be worse. The voltage will drop steadily as you add more load.

Your best bet is to measure it under load. You can do this safely by measuring the voltage off load, then compute the dropper resistors for that voltage. Attach the LEDs. The transformer voltage will drop under load, so having the wrong values will be safe. Take new measurements under load, rinse and repeat.