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Can this rail fault cause the oscillator to give bad signal ?

BGAmodzX

Member
Hello everybody.

I am troublshooting a device with 2 power supplies a high power bridge and an auxiliary one using the uc3842 ic.

I have changed the ic with a new one and checked the aurrounding circuitry .

I noticed that the ic Vcc supply fluctuates between 12 v dc and 14 v dc
and a bad output signal.

Can the supply rail be the problem ?
 

throbscottle

Well-Known Member
If it's fluctuating it might be a sign of a faulty rectifier or failing smoothing cap.
If possible, I would try to test the pwm circuit with a stable input supply.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
By FAR (easily more than 95%) the biggest cause of problems in SMPSU's are low quality electrolytic capacitors, which go high ESR - sometimes just in a matter of months.

Easiest way to check them is an ESR meter (no point at all doing so with a capacitance meter), or failing that (and if the PSU is still working, as in this case) use a scope to check the ripple on each of the electrolytics.
 

BGAmodzX

Member
By FAR (easily more than 95%) the biggest cause of problems in SMPSU's are low quality electrolytic capacitors, which go high ESR - sometimes just in a matter of months.

Easiest way to check them is an ESR meter (no point at all doing so with a capacitance meter), or failing that (and if the PSU is still working, as in this case) use a scope to check the ripple on each of the electrolytics.
Hi thanks for the hints. btw i have an lcr meter with dissipation factor D for capacitance.
 

BGAmodzX

Member
If it's fluctuating it might be a sign of a faulty rectifier or failing smoothing cap.
If possible, I would try to test the pwm circuit with a stable input supply.
Hi again am working on this board again.

I have changed the electrolytic capacitor feeding the uc3842 , also repaired two 100 kilo ohm power resistors giving supply from main Dc bus , and i still have bad signal on ic output.

i also checked the rt, ct circuit and the optocoupler circuit.

mystery.

This is the auxiliary switcher :

 

MacIntoshCZ

Active Member
what do you mean by bad signal on ic output?
this chip has 2v5 feedback comparator for voltage and 1V comp for current sense. It should oscilate on full duty cycle if its not above this voltages
 
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