RE: Transistors [Not OT :^]Hello Tim,

    thank you for suggesting the use of opto-isolators. 4N25 optoisolators
from no-name manufacturers are cheap. It worked very well on my circuit.
Instead of 2 transistors and 4 resistors (that doesn't worked), I'm now
using two opto-isolators and two resistors. I'm even using the same two
resistors for the two (+5V and +13V) LEDs, so the circuit is much simpler
right now.

    Now I have another question (administrative this time), why all the
messages I receive from PICLIST have 6 spaces in the start of the subject
line? It's what causes the "Re:      Re:      Re:      Re:      Re: Subject"
sindrome.

    Best regards,

    Brusque
----- Original Message -----
From: TIM
To: PICLIST@MITVMA.MIT.EDU
Sent: Thursday, December 16, 1999 2:14 PM
Subject: Re: Transistors [Not OT :^]


hello , gentlemen,  why don't you go with a single opto- isolator?..turning
the led on the transistor conducts.....collector to emitter?
cost 2 usa dollars in the worst case.small 6 pin dip
package...4n25..4n27...til111...etc.? both types will work depending how
it's pins are wired in to the circuit...npn vs. pnp.. i have gotten them to
work where conventional thinking says they won't work....
most can handle 100 ma current thru  emitter,collector junction...or you can
even use it to trigger a higher current pass transistor.
-----Original Message-----
From: Michael Rigby-Jones <mrjones@NORTELNETWORKS.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, December 16, 1999 5:47 AM
Subject: Re: Transistors [Not OT :^]


Hello Gremmen, Michael, Des, Tony et all,
Greetings!
>>This picture looks awfully complicated to just switch a 13V supply on/off.
> Looks perfectly fine and sensible to me.
> etc etc etc...
    I agree that the figure seens a bit complicated to just switch 13V
on/off, but anyway, I've tried it and it didn't work.
I suspect you have a wiring fault then.  The circuit looks entirely
operable, and I have used simmilar high side switching schemes many times in
the past.  What voltages are you seeing at TR1 and TR2 collectors with and
without the 5volt control?
The circuit isn't particularly complex, it's just well designed with pull
down resistors on the bases of the transistors, which are often left out.
You could reduce the circuit to 2 transistors and 3 resistors if you are
looking for a minimalist approach, but the problems this can cause are
probably not worth it.
>> Remove R2, R5 and TR2. Replace R1 and R4 by 10K and use the collector of
>> where TR2 used to be to switch the supply on/off. Resistor R3 can be
omitted
>> completely. On the VPP line (which is directly connected to TR1) you
could
>> add a resistor of, let's say 68R, to reduce the current on the VPP line.
    I've tried it and it didn't work either. It does work however to switch
5V programming power.
This scheme can work, but only if you use an open collector to drive the
transistor, such as pin RA4 on a PIC.


Regards
Mike Rigby-Jones