11 Apr
2012
11 Apr
'12
12:42 p.m.
On Apr 11, 2012, at 3:00 PM, Rob Jarratt wrote:
Thanks for that. I am left wondering about one thing
though. I believe the
c-e resistance of the transistor will have a material effect on the
frequency of the oscillation of the 555, which is controlling the chopper
transistor of a switched mode PSU. I don't know how this resistance might
vary between different transistors and how this might affect the chopper and
the PSU as a whole.
The reason I think it affects the oscillation is as follows: The transistor
switches in a resistor that affects the 555's timing. Before the failure of
the transistor in question the 555 oscillated at about 28KHz. After the
failure it oscillates at about 14KHz. As a test, using just a 9V battery, I
shorted the C-E terminals of the transistor with a piece of wire just to
bring the switched resistor back into the circuit; when I do this the 555
oscillates at 33KHz, so the transistor must be adding resistance that
influences the timing.
The resistor at the emitter is actually part of the bias network, I think.
You get much more accurate biasing on a BJT if you put a resistor at the
emitter end as well as a Thevenin network at the base because the one at
the emitter will react to gain variations and act as a form of negative
feedback.
Note also that the discharge pin on the 555 is an output (active low),
typically tied to the ramp capacitor through a resistor (R22 in this
case, 80.6K). In this case, I think the transistor is acting as a
switchable constant-current source to give the capacitor voltage a
linear ramp (or at least to add current to that coming in by default
through R23).
That, at least, would seem to jibe with the fact that you see an
increased switching frequency when you short it; the cap should be
ramping faster, but the discharge transistor is still going to more
than overcome the current coming through R41.
Bottom line, though, is that BJTs don't have a "resistance" per se
across C-E like FETs do (except as a parasitic effect of their
cursed physical connections). Rather, they act as a constant current
device modulated by temperature and the voltage across the B-E diode
(which is itself modulated by Ibe current and temperature). There is
a finite minimum Vce(sat) voltage drop which you will hit when the
transistor is in saturation, but I don't think you're running that
way. I think either an A55 (which is a 1978 vintage transistor
still produced by OnSemi) or a 2N2907 type (a more modern equivalent
with the right specs where it counts) will do the trick for you
rather nicely.
- Dave