Does that actually flash? With just a 5 volt supply?
John
Does that actually flash? With just a 5 volt supply?
John
Jamie
no it does not, I just through some values at it, I just wanted opinions about zenering the emitter. normally you would use at least 10volts or more..
I always thought avalanching the (BE) wasn't a very good idea.
Jamie
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yes, but this will unlatch and flash for you.
This circuit is very simply. I was just curious as to longevity of the transistor in such a configuration.
Jamie
-- How so? with the base connected to air all you've got is two diodes in series opposition.
It's a transistor negative-resistance effect that Spice apparently doesn't model.
In your sim, you have 23 volts base-emitter, so the Spice model is very wrong.
John
Yeah. I ran into this before when I tried to use the e-b junction as a zener. AFAIK, LTSpice does not model Veb at all.
-- Thanks for that; I wasn't aware of it.
The default LT SPice models for standard parts don't seem to be very good, especially as regards breakdown voltages. A 1N914 has a reverse leakage of 102 nA at 100,000 volts!
John
I learned about this effect years ago when I had to repair something that was oscillating because the base resistor became open on a npn circuit. I thought it was a fluke, defective transistor etc. It was later that I found out it was a known effect. Some where around 1980 or so..
And no, LtSpice wont show it. Why? I don't know.
Jamie
Can anybody explain the voltages and currents here?
Version 4 SHEET 1 880 680 WIRE -224 -32 -288 -32 WIRE -224 16 -224 -32 WIRE -96 64 -160 64 WIRE 64 64 -16 64 WIRE 112 64 64 64 WIRE 224 64 192 64 WIRE 64 112 64 64 WIRE 224 128 224 64 WIRE -224 160 -224 112 WIRE 64 240 64 192 WIRE 224 240 224 192 FLAG 64 240 0 FLAG 224 240 0 FLAG -224 160 0 SYMBOL diode 208 128 R0 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL voltage 64 96 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value -100000 SYMBOL res 208 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL npn -160 16 M0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res 0 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 1K SYMBOL res -272 -48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1K TEXT 280 80 Left 2 !.tran 0 1 0
LTSpice doesn't know how to handle the reverse voltage on the base nor the reverse voltage on the anode of the diode. That is, it doesn't handle reverse breakdown as you said in your last post in this thread.
Like our QC slogan goes at work "We Test to pass not test to fail!"
Nice how there isn't any Vbe R-break down either!
Jamie
-- Trying to bring down a winner, as usual? If you find fault with the diode model, why not fix it by writing your
Will Larkin even try?
No :-) ...Jim Thompson
-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et |
=A0 =A0 2N2222
=A0|-
=A0 120
=3D=3D=3D
inions
s or
n..
I believe you can just edit the standard.dio file to add a Vrev =3D xx theres several other parameters you can add to the different transistor/diode models
-Lasse
Any Spice is dangerous, as your recent post, with 22 volts Vbe, demonstrates. Simulation is no substitute for thinking.
Obviously it does zeners, so all the 1N914 model needs is some reverse breakdown params. It's just interesting that the model supplied doesn't have them.
The 100 kilovolts volts across a b-e junction is even more interesting. I assume that Vbr specs slow down a simulation, so weren't included.
John
But look at the currents versus time!
John
-- One would think that by making a statement like that you'd be qualified as a judge of what constitutes thinking and what doesn't.
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