Troubleshooting Switch Mode Power Supplies using flyback tester
is the power supply section can be check using flyback tester? Switch mode power supplies consist of primary and secondary section. If any component's in either one of the section found defective, it may cause power to blink, no power, power shutdown or low power. The dick smith flyback or line output transformer (lopt) tester is very handy and versatile when comes to finding fault locate in the primary and secondary section. Usually when a technician wants to confirm whether the secondary diode working or not, he have to desolder one leg to measure in order to get a precise reading. Checking diode in circuit normally don't produce accurate result. In this article i will show you how to check the secondary diode without lifting the diode from the circuit.
the flyback tester probes connect to the primay winding of the switch mode power transformer, a good result will usually between 4-8 bars LED lights up. If it show only one or two led bars or the flyback tester led goes off (no light), this indicate problems in primary side, primary winding shorted or a secondary diode shorted. Be sure to discharge the big filter capacitor and unplug the ac before checking! It is easy to determine the primary winding of the switch mode power transformer. Connect one probe to the drain pin of the power field effect transistor (fet) while the other probe to the positive side of the big filter cap. If you follow the drain and the positive side of the filter capacitor, it will eventually lead you to the two pins of the switch mode power transformer. This two pins is the primary winding of the switch mode power transformer.
Assuming the result you get from a working power supply primary winding is 7 bars, if you use a long nose plier to purposely short across one of the secondary diode at the secondary section. You will definitely observed that the 7 bars of the flyback tester led will drop and all the led goes off ! This mean that if in any actual situation when checking the primary winding of any equipment and the led goes off-suspect one of the diode in the secondary have developed a short circuit. If you measure the primary winding and it shows 4- 8 bars (depends on equipment and design) then you have to move on to check other components than the primary winding and secondary diode. This can save you time and time is money!
is the method also can be use to check the primary winding of flyback transformer. Sometimes a small secondary diode shorted at the secondary side of the flyback transformer (G1 or Vcc) circuit can cause the whole power supply to blink, shut down and low power. Thus it is important to make sure that all secondary diodes and primary winding are in good condition before proceed to check other components or circuit. Power supply checking can be frustrated if you do not know how to use the right way to check it. If you do not have a flyback tester be sure to get one and you will definetly like it .