18 Sep
2008
18 Sep
'08
3:13 p.m.
Tony Duell wrote:
That's fine.
Ouch!. According to the TI datasheet I've just looked at, the reference
should be between 4.75V and 5.25V. Either the TL494 is defective or
soemthign else is pulling it up (which may have damaged the TL494
anyway). Or I suppose youre meter could be way out of calibration, but
since the 8V line checks out I think that's less likely
That sounds as thouhg it's not oscillating and that the output transsitor
is never turned on.
Well. if it's as low as that the outputs should certainly be oscillating.
But shouldn't the protection circuit designed to shut the supply down if
the 8V rail drops too low have operated at this point? It compares the 8V
rail after 5 diode drops with the reference voltage, if the former is
lower, then pin 3 is pulled high, shutting the supply down. So check pin
3 again...
I am beginning to suspect that ther TL494 may have failed. It's not a
difficult-to-get chip.
I replaced the TL494 this afternoon (thanks to a local electronics store
that actually has useful stuff!) but it does not appear to have made any
difference. I get the same voltages on the TL494's pins as I did
before. Any other thoughts?
Thanks once again...
Josh
Now check
for the 5V reference voltage from pin 14 of U4 (the TL494).
Hmm, I get 6.75V (measuring from Pin 7 (gnd) to 14.)
Now, I
think that TL494 should be oscillating, at least at startup. See if
you get anything on pin 8. A TL494 datasheet would be a useful thing to have!
7.91V on pin 8. (I have a datasheet at
http://yahozna.dyndns.org/scratch/tl494.pdf). I
scoped it out, and pin
5 is oscillating, a nice sawtooth.
OK, the oscillator is running, but it's not driving the output stage.
What
voltage do you see on pin 3 (Comp) of this chip? That's the output of the
error amplifier IIRC. It appears there's an overcurrent shutdown circuit
connected there, maybe that's shutting it down.
0.06V on pin 3.
-tony