8 Apr
2012
8 Apr
'12
1:34 p.m.
On 2012 Apr 8, at 1:17 PM, Mouse wrote:
Why not? Except for higher forward drop on the B-E pins (because it's
actually two B-E junctions in series), I don't see any reason you
wouldn't see basically two-diode behaviour from a Darlington pair.
As shown in the datasheet that somebody provided earlier in the
thread, it's not a pure darlington pair - it has built-in pull-down
resistors across the internal BE junctions and a reverse-voltage-
clamp diode across CE.
I have been desoldering the power transistors and
testing them with
a DMM using the diode tester. There are eight in all, four test OK
as two diodes. However, the other four are marked 8702 TIP125,
IIRC, the TIP125
is an NPN darlington, basically 2 transsitors with
one driving the other to get a higher gain )at hte expense of some
other parameters). You won't see the normal 2-diode behaviour with
those. > 1st - b-e 1.95V e-b 0.82V b-c OC c-b 0.66V
c-e 0.58V e-c OC
> 2nd - b-e 1.91V e-b 0.83V b-c OC c-b 0.68V c-e 0.58V e-c OC
> 3rd - b-e 1.90V e-b 0.81V b-c OC c-b 0.67V c-e 0.59V e-c OC
> 4th - b-e 1.75V e-b 0.75V b-c OC c-b 0.56V c-e 0.58V e-c OC
This is..strange. Combined with the above, I'm led to suspect that
what you people are calling a Darlington pair is not what I thought it
was. I thought a Darlington pair was two bipolar transistors of the
same kind (both NPN or both PNP) in a single package, connected
internally like so:
---------------
| |
| -----+---|----c
| | | |
| --- | |
| | c| | |
b----|--|b | | |
| | e| | |
| --- --- |
| | | c| |
| --|b | |
| | e| |
| --- |
| | |
| ---|----e
| |
---------------
As far as I can see this should show open circuit when reverse biased
on the b-e or b-c pins, and shouldn't conduct either way on the e-c
pins with the b open.
Do you people mean something else, or am I missing something, or what?