Re: VN3515 substitute please


"ehsjr" <ehsjr@xxxxxxxxxxxxxxxx> wrote in message
news:PhtSf.4701$Ub.33@xxxxxxxxxxx

If the MOSFET were on _and_ the voltage were high, the current would
be
quite large obviously leading to the sudden death of the LED.

But it shouldn't kill the led even if the Mosfet was a dead short.

I mean in my circuit, without the current limiter.

That circuit is supposed to provide constant current independent
of Vcc. (Obviously there are limits - Vcc could be so low that
you can't draw 3-4 mA or so high that parts melt.) Lets say
you have 100 volts on the collector of TR4. When there's 4ma
current through the 180, it produces .72V, which biases TR3 on.
That in turn increases the drop across R5 which was 100K*IbeTR4
to 100K*IbeTR4 + 100K*IceTR3 - which reduces the base current
to TR4. Note that we never use the 100 volt figure. The equation
would be identical regardless of voltage.

How did the circuit designer come to pick the 100K resistor value?

Maybe looking at it another way would help:
TR3 will do its damndest to keep the voltage drop across the 180
equal to its own Vbe. The only way it can do that is by telling
TR4 "you're applying too much current to the 180".

But in any event - you're getting a 1800% increase in voltage
and only a 50% increase in current.

Ok, thanks. That makes sense, so I'm wondering why SwitcherCAD shows
such a variation in the current. I'll have to play around with the
simulator some more.

I guess
it's not a bad thing to include as a failsafe, but I don't see it as
really necessary for my one-off project. I'll have to see if the
opto-isolator I use in place of the LED leads a short life. It
seems
ironic that the circuit author would take the effort to include the
current limiter that won't really be exercised, but ignore the max
Vce
on TR1 and let the resistors and bias save it, but it still takes a
bit
of a voltage hit while waiting for C2 to charge. So far everything
is
working fine without the current limiter.

BTW, I subbed a 2n3904 for the BC547. Don't laugh too hard, but I
used
an IRF830 for the MOSFET since that was all I had on hand. It
probably
shouldn't work since it really needs more gate voltage, but it works
fine so far. Back to the topic, can you recommend a common
(jelly-beanish) low power (TO-92 is good), logic level MOSFET,
N-Channel
of course. Thanks for the input.


The 2N7000 might be the most common.

I have 50 of them as of this afternoon when the UPS guy came. ;-) I
also ordered 10 Zetex ZVN2106A parts (lower Rdson/more current but
otherwise similar) to keep on hand.

.

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