Testing PWM UC3842 With Analogue Meter
If you are a seasoned electronic repairer, you would have come across many times with this PWM UC3842 IC before. This IC normally use in the SMPS power primary section in Monitor (LCD and CRT Monitors), fax machine, TVs and etc. Sometimes whenever power blows we do not know whether the power IC was affected or not thus we just direct replace the IC and retest the equipment. Generally whenever the power FET shorted this power IC would also gave way. This IC have other equivalent part numbers too like KA3842, IP3842, HA17384 and etc.
Is there a way to test this power IC? I’ve read an article about testing the pin 5, 6 and 7 of this power IC with an analogue multimeter many years back. The article mentioned that one have to treat the three pins like the normal bipolar transistor. If you get a shorted or leaky reading then the IC is bad and if you get a good transistor “like” reading, then the power IC is 99% good. So the question is how accurate is this testing method?
In fact i have personally tested the method and found it to be not that accurate thus i had stopped testing it with analog meter. The reason for it was that i do came across quite a numbers of the power IC test result was good but no power after fixing it. A new replacement restored the power problem. Second reason was that the Monitor display have distortion and after the replacement of the power IC it worked good. Again the Power IC was tested good.
Then, how do i test the UC3842 IC? I just simply replace with a new one and retest the equipment. Do you know that the equipment itself is the best way to test the Power IC and in fact all ICs? If it work i consider the problem solved, and if it doesn’t work, i will concentrate on other parts or area. This IC is not expensive and one should keep at least a few for testing and replacemnet purposes.
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Truckman Jones
July 18, 2008 at 9:15 pm
I found this method of testing this pwm, practice shows that this method works ok, but I cannot guarantee also I'm not the author of the desing...
http://www.sosmonitores.com.br/imagens/pwm/Pwm.jpg
If LED is flashing ic is good, the pot regulates the frequency. Otherwise IC is bad.
Hope that it'll help anyone...
admin
July 21, 2008 at 4:00 pm
Thanks for sharing but the link is not working. Anyway thanks again for the good tip.
Jestine
Líbero
October 23, 2008 at 8:03 pm
The complete link is: http://www.sosmonitores.com.br/dicas/testpwm.html
admin
October 23, 2008 at 10:03 pm
Hi Libero,
Thanks for sharing the circuit. It will be very helpful!
Jestine
Jon Pippard
August 4, 2009 at 9:26 am
Very often the IC is OK, but the startup or bootstrap electrolytic has dryed out. Before changing the IC change the bootstrap capacitor, usually 10 -47uF 25V.
admin
August 5, 2009 at 8:08 pm
Hi Jon,
Thanks for sharing and yes many times the e-cap cause lots of problem like low output voltage, no power and shutdown too.
Jestine
RAJENDER SHAH
August 7, 2010 at 1:17 am
sir,
plz. tell me how to test pwm ic with multimeter?
admin
August 8, 2010 at 4:47 am
Hi Rajender,
You can only test the pwm ic using voltmeter and scope. Find the spec of the ic and look for the vcc oin from there power on the set and measure the vcc pin of the ic to see if the supply is good or not and if it is good you can then use scope to check the output of the ic with scope.
Jestine
alan
February 27, 2018 at 7:32 pm
Can someone reupload the test circuit please