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Testing SMPS Transformer Without Load
I believe many of you (electronic repairer) have tested the output voltage of a linear transformer before. Just set your meter to measure AC voltage and place the probes across the output of the linear transformer. If the linear transformer printed as 9 VAC output then you can expect about 9.5 to 10 Vac because transformer have tolerance too. Now this is different if you want to check the output of SMPS transformer. You can’t just solder out the output pin and power on the equipment and check the output voltage. Since there is no more load, the primary side will sense an overvoltage which could shutdown the power supply or totally blow up the power components. I have came across this situation (blowing up the power side components-power IC, power FET, resistors and etc) before thus in my mind i always says no-no to removing only the smps transformer output pin and power On.
If you would like to test the output of the SMPS then place your probe after the secondary diode output pin and check the DC voltage. It is a common sense that if you get the right output voltage then that’s mean the SMPS transformer is producing good AC voltage thus it is not necessary to check all the output pin of the SMPS transformer.