A bit OT - Transistors in parallel
Archiver
Posts: 46,084
I would like to use parts on hand to switch power to a small circuit.
Power would always be applied to the "power switch" portion of the circuit,
which would then switch power on/off to the function portion of the circuit.
I have some NPN transistors that can handle up to 50 mA of collector
current, and at saturation, Vce is 0.4 V.
If I use one transistor, with the base hooked to the "power switch" and an
open collector output (connected to my functional circuit's common), then
when switched off, no current should flow, the power is off. When the
transistor goes into saturation, it should drop 0.4V and I'll have approx 5V
across my circuit, power is on (up to 50 mA), correct?
Now, say I need to supply my functional circuit with up to 150 mA. Could I
place 4 or 5 transistors in parallel (for a safety margin), each base tied
to the "power switch", each emitter tied to ground, and each collector tied
together to the "common" of the functional circuit? They should be able to
supply 200 or 250 mA ideally... However, the slight difference in each
transistor would cause more current to flow through some, and less through
others. Perhaps the safety margin above should compensate this uncertainty?
I've read about capacitors in series in order to increase their voltage
handling capabilities, and using "bleeder resistors" to help maintain the
same voltage drop across each cap. I was thinking this may help in the
application I'm talking about, assuming I'm using resistors large enought
that when all transistors are off, minimal current flows through the
functional circuit, but I'd rather have !OFF! than "well, 1 mA is pretty
much off..." However, if a transistor opens, then the others will have to
make up the difference, possibly causeing every single one to fail, or, if
one shorts, the circuit will always be on, maybe causing other problems when
the supply voltage suddenly jumps up 0.4V!
Does this make sense? I know that simply buying a power transistor (or at
least one that can handle to current of the circuit) would be a better
solution, but I don't have one, and I'm also simply curious...
Steve
Power would always be applied to the "power switch" portion of the circuit,
which would then switch power on/off to the function portion of the circuit.
I have some NPN transistors that can handle up to 50 mA of collector
current, and at saturation, Vce is 0.4 V.
If I use one transistor, with the base hooked to the "power switch" and an
open collector output (connected to my functional circuit's common), then
when switched off, no current should flow, the power is off. When the
transistor goes into saturation, it should drop 0.4V and I'll have approx 5V
across my circuit, power is on (up to 50 mA), correct?
Now, say I need to supply my functional circuit with up to 150 mA. Could I
place 4 or 5 transistors in parallel (for a safety margin), each base tied
to the "power switch", each emitter tied to ground, and each collector tied
together to the "common" of the functional circuit? They should be able to
supply 200 or 250 mA ideally... However, the slight difference in each
transistor would cause more current to flow through some, and less through
others. Perhaps the safety margin above should compensate this uncertainty?
I've read about capacitors in series in order to increase their voltage
handling capabilities, and using "bleeder resistors" to help maintain the
same voltage drop across each cap. I was thinking this may help in the
application I'm talking about, assuming I'm using resistors large enought
that when all transistors are off, minimal current flows through the
functional circuit, but I'd rather have !OFF! than "well, 1 mA is pretty
much off..." However, if a transistor opens, then the others will have to
make up the difference, possibly causeing every single one to fail, or, if
one shorts, the circuit will always be on, maybe causing other problems when
the supply voltage suddenly jumps up 0.4V!
Does this make sense? I know that simply buying a power transistor (or at
least one that can handle to current of the circuit) would be a better
solution, but I don't have one, and I'm also simply curious...
Steve
Comments
transistors are not exactly the same and in this situation, most likely, one
or two would draw more current than the others and then increase in
temperature which will draw more current, etc. Eventually smoke. You can
balance the current draw to some extent by puting a series resistor on each
but why. You're talking of using at least 10 parts to do the work of one.
Just use a proper transistor and do it right in the first place. Check out
this web site for a recommended output transistor arrangement which will
work with the stamps, provide a good safety margine and protect the stamp.
jim
http://www.geocities.com/jimforkin2003/
Original Message
From: Steve Ziuchkovski [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=KFtIQ9-rD1d3kt06TcHq3XOOYCegqKCVt4bo01F-o0gMpJjwSSKHCS3OzKZMhru-TuWKZDjpnDNjoH0]zman97211@y...[/url
Sent: Tuesday, June 10, 2003 4:49 AM
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] A bit OT - Transistors in parallel
I would like to use parts on hand to switch power to a small circuit.
Power would always be applied to the "power switch" portion of the circuit,
which would then switch power on/off to the function portion of the circuit.
I have some NPN transistors that can handle up to 50 mA of collector
current, and at saturation, Vce is 0.4 V.
If I use one transistor, with the base hooked to the "power switch" and an
open collector output (connected to my functional circuit's common), then
when switched off, no current should flow, the power is off. When the
transistor goes into saturation, it should drop 0.4V and I'll have approx 5V
across my circuit, power is on (up to 50 mA), correct?
Now, say I need to supply my functional circuit with up to 150 mA. Could I
place 4 or 5 transistors in parallel (for a safety margin), each base tied
to the "power switch", each emitter tied to ground, and each collector tied
together to the "common" of the functional circuit? They should be able to
supply 200 or 250 mA ideally... However, the slight difference in each
transistor would cause more current to flow through some, and less through
others. Perhaps the safety margin above should compensate this uncertainty?
I've read about capacitors in series in order to increase their voltage
handling capabilities, and using "bleeder resistors" to help maintain the
same voltage drop across each cap. I was thinking this may help in the
application I'm talking about, assuming I'm using resistors large enought
that when all transistors are off, minimal current flows through the
functional circuit, but I'd rather have !OFF! than "well, 1 mA is pretty
much off..." However, if a transistor opens, then the others will have to
make up the difference, possibly causeing every single one to fail, or, if
one shorts, the circuit will always be on, maybe causing other problems when
the supply voltage suddenly jumps up 0.4V!
Does this make sense? I know that simply buying a power transistor (or at
least one that can handle to current of the circuit) would be a better
solution, but I don't have one, and I'm also simply curious...
Steve
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/