SAFETY The primary danger to you is from the input side of the supply which is directly connected to the AC line and will have large electrolytic capacitors with V or greater DC when powered often, even if the supply does not work correctly and for some time after being unplugged especially if the power supply is not working correctly but does not blow fuses. WARNING: The filter capacitors used in many switchmode power supplies can store an amount of energy that can kill - always discharge and confirm this before touching anything. There is also risk of instantly destroying expensive parts of the supply and any attached equipment as well like the switchmode power transistor if your probe should slip and short something either directly or by killing the feedback circuit. These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Note that the danger to you is not only in your body providing a conducting path, particularly through your heart.

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You are on page 1of 2 Search inside document Here are some tips for smps faults — Smps blows fuse — if fuse blows in smps , there is short circuit in the ps circuit. What is the method of switching , parts used , and make test points. Main filter is centre point for checking smps. Measure resistance across the two terminals of main filter , capacitor action must come. Instead of capacitor action it shows low resistance across mf than there is shorted part I the supply line , disconnect all parts one by one connected with the supply line , by disconnecting which part removes short , check that part or line for short , replace.

Probable faulty parts may be — shorted — bridge rectifier or diodes making rectifier , mf , any protection part if used , smps out put transistor or switching regulator ic STR ,sm transformer , short on output of smps can also cause fuse blow , transient protection capacitors placed across rectifier or diodes. Smps dead no out put from smps — There might be many faults for this symptom.

If fuse do not blows , and there is no output than follow the following tips — Check voltage on mf. If no voltage on mf , than ac to dc circuit is to be checked. If voltage available at mf than ac to dc part of circuit is ok. If str is used check voltage on collector or drain terminal of str , if voltage is not available than check parts coming in the path of supply from mf to str , check for open line , lr , smps transformer. If voltage on collector terminal is available than measure voltage on base or gate terminal if voltage is not coming than check parts giving supply to base pin of str , if voltage is coming than check for oscillator failure , parts giving feed back for oscillation , or parts used for protection , short in out put of smps , or circuit loading smps.

If transistor circuit is used in output and driver and IC in oscillator circuit and output is not coming , than first check driver transistor base or gate voltage if voltage is not normal than check oscillator circuit by measuring voltage of ic and comparing it with normal voltage to access fault , supply to ic , oscillator faulty , protection on , time constant parts faulty.

If collector or drain voltage is missing than check parts coming in the supply path, lr limit resistance , fuse , eht primary , if voltage on collector voltage is coming than check output transistor and replace it. Always check for protection circuit if smps starts for a fraction of a second.

Donot force circuit to start , search for fault only. Always discharge MF before attempting any repair work. To locate faulty line you may apply 12v on the line from any urg ps used for tape recorders.

Smps out put is low — if smps givel low voltage output than the fault is mostly in the error amplifier, control, softstart circuits. Output loading may also affect the output voltage some time.

Measure voltages and compare them with normal voltage given the circuit diagram, probable parts may be faulty zenar diode in the error amp, faulty control circuit parts,transistor, IC , opto coupler faulty, filter on error voltage line faulty. Smps output is high — if smps output is high first shut down set , check fault either in switch off position or use variac or low voltage transformer. Never keep on in this fault it may damage other parts also.

Check for — error amp circuit, zener diode, optocouoler, filters on error amp line, transistor, IC, resistance. Humming — if there is humming in the ps then picture and raster will shake or produce hum bars in raster or picture.

Check power filters on all power lines, check for leakage in diodes of mr. Than check for weak earth connections , open shielding , signal components close to ac parts or have common ground. Related Interests.


Smps Faults Tips

Some designed have four outputs, some have five and so on. Each of the output is connected to an individual circuit. For instance, a 6. If one of the components or circuit developed a short circuit, the voltage vcc supply line will drawn a lot of current and thus the power will shut down, fluctuate, low output power or even no power at all. You must know how to isolate or repairs the problem otherwise you will be wasting your time troubleshooting good components in the power supply section.


Switch Mode Power Supplies



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