One way is to attach various resistors and measure AC volts. That gives you an idea when the secondary output voltage starts to drop, thus letting you know how much power you can get from your rewired transformer.
A resistive type load is recommended because it behaves in a predictable manner. You may use heating elements, or plain incandescent bulbs..
You are not understanding how current ratings work.
A transformer produces voltage, not current. Current is what you draw from it.
The voltage will drop and the temperature will rise as more current is drawn. The current rating is decided by how much voltage drop is acceptable and how hot it can be allowed to run.
Brian.
Firstly, take note of what FvM wrote in post #3, it will work but that kind of transformer is not optimal.
... ... ... .if you apply a too low ohm load you will overload the Tx, this is what fuses and polyfuses & circuit breakers are for - protection if you short the output or apply too low ohms ...
Provided you have enough capability to provide the maximum current you need, that is true. Amps represent the load, all you have to do is make sure the source is capable of producing them safely.amps will take care of themselves
To compound your problem, the RMS value isn't the same as the peak value, it is somewhat below it. That means when you measured 48V RMS, the peaks were quite a lot higher, in fact 1.414 times the RMS voltage. 48V RMS = (1.414 * 48) = 67.8V. In your power supply there are capacitors which charge up to that peak voltage so you have far more than is safe. You really want the power supply to see no more than 40V which means the RMS voltage must be (40/1.414)=28.3V or lower.
if filtered it should rise to almost 70V.
So I take a resistive load like a heating element or light bulb and measure it's resistance using my multimeter. I hook that load up to my transformer and measure the voltage drop on both sides of the load. Then I use the formula v/r to find I. Will that work?
It isn't correct. The RMS reading of the AC from the transformer will be fine but the DC range will not give an accurate result. It is accurate for steady DC voltages, such as from a battery or regulated PSU but after the bridge rectifier you don't have a steady voltage (unless you add capacitance), you have a voltage constantly changing between zero and peak and back again 120 times a second. The meter can't measure a moving target accurately and will show something approximating to the average of the waveform. To measure it accurately, add a capacitor across the output of the bridge rectifier and divide the DC reading by 1.414 then if you want absolute accuracy, add the voltage dropped across the two conducting diodes in the bridge (about 1.5V)My Digital Multi Meter DMM (Extech 470 with True RMS) has settings for A/C power so when I measure A/C power it calculates the RMS and gives me the correct reading. When I measure D/C power I change the setting for that and so it gives the correct reading.
If you mean this:
It isn't correct. The RMS reading of the AC from the transformer will be fine but the DC range will not give an accurate result. It is accurate for steady DC voltages, such as from a battery or regulated PSU but after the bridge rectifier you don't have a steady voltage (unless you add capacitance), you have a voltage constantly changing between zero and peak and back again 120 times a second. The meter can't measure a moving target accurately and will show something approximating to the average of the waveform. To measure it accurately, add a capacitor across the output of the bridge rectifier and divide the DC reading by 1.414 then if you want absolute accuracy, add the voltage dropped across the two conducting diodes in the bridge (about 1.5V)
Brian.
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