HOW TO TEST :
use a stepdown transformer to input 36V a.c. Into the EMI circuit and test if 36V a.c. comes out of the EMI filter circuit,
including EMI coil and X/Y cap. If yes, they are OK.
![Popcorn :popcorn:](https://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpg.pagespeed.ce.yJ6TmC5fSa.jpg)
Fuse : checking continuity
EMI filter (coil) : no idea
Safety capacitors (X/Y class): check if there are no shorts, checking capacitance using multimeter in-circuit looks good
In real situation, after fuse blowed, bridge rectifier will be destroyed and smoothing cap. capacitance must be checked to see if it is dry. If yes, will make fuse blowed. In half bridge SMPS, however, fuse and bridge rectifier are ok even one of smoothing caps dry out.
![Popcorn :popcorn:](https://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpg.pagespeed.ce.yJ6TmC5fSa.jpg)
Bridge rectifier : check if there are no shorts, diode check using multimeter in-circuit failed, out-of-circuit was ok.
Bulk capacitors : check if there are no shorts, checking capacitance using multimeter, check ESR value using ESR meter both in-circuit and out-of-circuit was ok
Varistor / surge absorber : no idea, reads as open circuit in-circuit using multimeter.
It must be checked, located at and connected to the primary side of the SMPS transformer . It absorbs back emf
about - 800V, when SMPS power transistor is off, due to -Ldi/dt.
Thermistor / inrush current limiter : in-circuit reads about 15 ohms at room temperature, changes resistance when temperature increases / decreases. Must be checked
Resistors : checked if there are no shorts, measured them and checked color codes to see if they correspond. The starting resistor connected from the large cap to the baase of power transistor must be checked.
Diodes : check if there are no shorts, diode check using multimeter in-circuit and out-of-circuit
Capacitors : check if there are no shorts, checking capacitance using multimeter, check ESR value using ESR meter both in-circuit and out-of-circuit.
Those SMPS transformers almost NEVER die. For Beginners, no need to check them. For computer power supply, there are 2 to 3 transformers called voltage output transformer, standby transformer and drive transformer. For opto-coupler, no need to check. Its faulty will not make fuse get blowed.
![Popcorn :popcorn:](https://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpg.pagespeed.ce.yJ6TmC5fSa.jpg)
Power transformer : check if there are no shorts, diode check on base / emitter / collector out-of-circuit
Opto-coupler : can only test the LED using diode test, no base exposed for transistor. removed if out-of-circuit and built a small test circuit
Transformer : no idea how to test it. Always read this is unlikely to fail but you never know.