16 Sep
2014
16 Sep
'14
11:10 a.m.
What I find is that the UC3842 isn't getting the juice it needs.
Monitoring pin 7 (Vcc) shows that when I switch the supply on, Vcc
ramps up to 14V, then drops down to 0.8V and stays there. When I
switch the power supply off again, after a while Vcc ramps up to 14V
again, then slowly ramps down to 0V. Any ideas as to what would cause
this are welcome.
I have a disk enclosure with a faulty PSU featuring a UC3842. As in your case,
I found Vcc to be low around 1.3V. I didn't notice if it was higher briefly
at switch on or not. Vcc was initially supplied by a relatively large value
startup resistor which was cooked enough that I could not read the markings.
Not being sure what chicken and egg situation might have led to this state, I
tried applying an independent 16V supply to the UC3842. It responded by
drawing 500mA (which the current limiter on the power supply was set to) and
getting quite hot. It continued to do this when I removed it from the circuit
and powered it with nothing else connected to it. Maybe your UC3842 is in
similar distress?
Regards,
Peter Coghlan.