- ... 3.63V is too low for the potential divider U4, R35, R40, Q8 base, so the Q9 collector node is definitely connected to something else - I wouldn t haveMessage 1 of 47 , Aug 2, 2007View SourceAt 6:54 am ((PDT)) Wed Aug 1, 2007, ian mcphedran wrote:
>I have removed Q9 again.3.63V is too low for the potential divider U4, R35, R40, Q8 base,
>The 5V line is 4.32V, U4 is running hot, and the input current at 12.5V is
> Q9 collector pad reads 3.63V, base pad reads 0V as of course does the
so the Q9 collector node is definitely connected to something
else - I wouldn't have expected the /OE inputs of Q3
to represent a significant load, they are CMOS inputs
and hence high-impedance, aren't they?
When Q9 was hot, something was sourcing current to it:
now it's gone, something is sinking a little current from the
collector node but the high current is still flowing from U4.
The only other stray thought I have is that you are testing
in the aluminium box, the box is not isolated from supply,
and there is a short. Unlikely, but...
- ... Perhaps, but output near the +ve rail could be: a) inputs driven to output saturation b) output pulled high externally (i.e. by a pins 7-8 short) c) deviceMessage 47 of 47 , Aug 3, 2007View Source
>U8 86 FAt 2:44 am ((PDT)) Thu Aug 2, 2007, in Digest 1865 Tony Parks wrote:
>U8 pin 7 reads 4.2V
>Sure looks like U8 is a problem.Perhaps, but output near the +ve rail could be:
a) inputs driven to output saturation
b) output pulled high externally (i.e. by a pins 7-8 short)
c) device failure which also emulates (a) or (b).
Both (b) and (c) can explain the U8 heating
but only (c) is that device's fault.
(There's no source for the heating current in case (a).)
Meanwhile, none of these explain why:
d) Q9 ran hot and
e) Q9 replacement ran hot
This doesn't seem to be in a related part of the circuit,
so was Ian just unlucky with two rogue 2N3904?
[BTW, my apologies for saying that Q9 collector pad voltage
3.63V with Q9 removed seemed too low. I recalculated as
3.64V - within the DVM last digit uncertainty.
(Vcc-Q8vbe) * R40/(R35+R40) + Q8vbe =
(4.32-0.65) * 22.1/(4.99+22.1) + 0.65 = 3.64V ]
>Let me know what components you need to restore your boardI continue to admire your outstanding support for this project;
>and I will send them right away at no cost.
"May Great Leader Tony live ten thousand years!"