Is there a device that can...

[pbabin]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the Vce would be less than 50v.

Ideally, it would be a 4 legged device that would be in a case similar in size to a TO-92 device.

I know that an opto-isolator could do it but they're too expensive in most cases.

Thanks in advance
PB


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[Winfield Hill]

pbabin wrote...

I often have to use an NPN transistor (emitter on ground) to turn
on a PNP transistor (emitter on +B) to efficiently switch positive
voltage. Is there a device that can replace the two transistors
and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the
Vce would be less than 50v.

Yes there are many, using MOSFET high-side switch architecture,
with onboard current- and thermal-limiting circuitry.

Ideally, it would be a 4 legged device that would be in a case
similar in size to a TO-92 device.

Oops, not in a TO-92. Small SMT is available though. Typically
thay have 5+ pins, the 5th being used for a status info output.

 

[John Popelish]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the Vce would be less than 50v.

Ideally, it would be a 4 legged device that would be in a case similar in size to a TO-92 device.

I know that an opto-isolator could do it but they're too expensive in most cases.

There are devices that accept a low side control signal but switch the
high side. They also often include over current detection. Here is a
5 pin example.
http://www.vishay.com/docs/72219/72219.pdf

 

[John Fields]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

 

[Mebart]

I know that an opto-isolator could do it but they're too expensive in most cases.

I'm not sure i understand your logic..........

An opto is too expensive??

Two low power transistors should cost 20 to 40 cents each, then you
have to buy associated resistors and design the PCB, which is larger
and costs more with each additional component. It takes more labor to
build the multiple transistor assembly too.

An opto should be under 2 dollars, has no additional components and is
available in SMT or dip packages.

Is the opto really more expensive than the multiple component solution
when you consider the total cost?

M

PS Both Sharp and HP have some interesting opto's, although they are a
bit pricey and you probably don't need anything more than a basic
unit.

 

[Spehro Pefhany]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the Vce would be less than 50v.

Ideally, it would be a 4 legged device that would be in a case similar in size to a TO-92 device.

I know that an opto-isolator could do it but they're too expensive in most cases.

Thanks in advance
PB

Maybe you could find a dual complimentary BJT with built-in resistors
that would reduce your source driver to a single 6-lead SMT part,
similar in size to a TO-92. You could certainly use two such "digital
transistors" (complimentary) and they should be available in
through-hole if that's an issue.

They also could have the base resistors that you have appear to be
omitting, which could improve the high-temperature leakage performance
of your current setup.

There are also bipolar IC source drivers from Allegro, Toshiba and
others that might work for you if you need a bunch of drivers
together. They tend to have more voltage drop than an ideal bipolar
solution because of the difficulty of making good PNPs in the typical
bipolar process, so your "efficiently" comes into question.


Best regards,
Spehro Pefhany

 

[OBones]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the Vce would be less than 50v.

Ideally, it would be a 4 legged device that would be in a case similar in size to a TO-92 device.

I know that an opto-isolator could do it but they're too expensive in most cases.

Thanks in advance
PB

why do you need two resistors ?
The cumulative gain means that the base current is ridiculously small in
both of the transistors. At worst, you only need one resistor, not two.

 

[Jim Thompson]

why do you need two resistors ?
The cumulative gain means that the base current is ridiculously small in
both of the transistors. At worst, you only need one resistor, not two.

Spoken like an amateur. Ever hear of leakage? How can you ensure
turnoff? Inductive load? On and on and... we designers fret the
details ;-)

...Jim Thompson

 

[OBones]

Spoken like an amateur. Ever hear of leakage? How can you ensure
turnoff? Inductive load? On and on and... we designers fret the
details ;-)

Yes, but still, for simple applications, the extra resistor is not
required. It all depends on the load, but most of the circuits I've
built don't require it.

 

[James Meyer]

Spoken like an amateur. Ever hear of leakage? How can you ensure
turnoff? Inductive load? On and on and... we designers fret the
details ;-)

...Jim Thompson

The way I usually build that circuit is with a resistor in series with
the NPN base to limit current when it's driven from a voltage source and a
resistor between the NPN collector and the PNP base again to limit current
there. Prudence dictates a base-emitter resistor at each transistor to mitigate
effects of collector-base leakage.

That makes two transistors and four resistors for a discrete design.
Designers who have the luxury of working directly in silicon could probably
design transistors with characteristics that would allow the elimination of all
the resistors.

Jim "The other one" Meyer

 

[James Meyer]

Yes, but still, for simple applications, the extra resistor is not
required. It all depends on the load, but most of the circuits I've
built don't require it.

Could you list a few of the products containing the circuits you
designed? I'd like to add them to my list of "Things to avoid".

Jim

 

[Fred Bloggs]

The way I usually build that circuit is with a resistor in series with
the NPN base to limit current when it's driven from a voltage source and a
resistor between the NPN collector and the PNP base again to limit current
there. Prudence dictates a base-emitter resistor at each transistor to mitigate
effects of collector-base leakage.

That makes two transistors and four resistors for a discrete design.
Designers who have the luxury of working directly in silicon could probably
design transistors with characteristics that would allow the elimination of all
the resistors.

You could use a cheap MOSFET like the 2N7000 to eliminate two resistors
by replacing the NPN circuit- leaving the PNP base current limiting and
its base pulldown resistors.

 

[John Larkin]

The way I usually build that circuit is with a resistor in series with
the NPN base to limit current when it's driven from a voltage source and a
resistor between the NPN collector and the PNP base again to limit current
there. Prudence dictates a base-emitter resistor at each transistor to mitigate
effects of collector-base leakage.

If you put a resistor in the NPN emitter, it becomes a
constant-current sink. So if you don't care about PNP leakage current
(and in most cases it's not an issue) that reduces to two transistors
and one resistor.



V+
|
|
e
+--------b
| c
| |
| +-------out
c
ttl in-----------------b
e
|
|
r
|
|
gnd

with obvious gotchas.


John

 

[Winfield Hill]

James Meyer wrote...

Jim Thompson wroth:

The way I usually build that circuit is with a resistor in series
with the NPN base to limit current when it's driven from a voltage
source and a resistor between the NPN collector and the PNP base
again to limit current there. Prudence dictates a base-emitter
resistor at each transistor to mitigate effects of collector-base
leakage.

That makes two transistors and four resistors for a discrete design.
Designers who have the luxury of working directly in silicon could
probably design transistors with characteristics that would allow
the elimination of all the resistors.

You've described the classic level-shifting pass-element switch,

.. hv in ---+--- E C ------- out
.. | B pnp switched
.. R2 |
.. | |
.. '------+
.. |
.. R4
.. |
.. C
.. logic -- R3 -+- B
.. control | E npn
.. R1 |
.. | gnd
.. gnd

The scheme I prefer, current switching, saves two resistors and
has a few other advantages to boot. There is a requirement that
the switched voltage be higher than the logic-control voltage.

.. hv in ---+-- E C ------- out
.. | B Q2 switched
.. R2 | pnp
.. | |
.. '-----+
.. |
.. C
.. logic ----- B Q1
.. control E npn
.. |
.. R1
.. |
.. gnd

The current provided by R1 is the desired base-drive current, plus
the base discharge current Vbe/R2. The base-drive current level is
provided independently of any changes in the switched power voltage.
The circuit is suitable for switching high voltages, for example
if Q1 is a mpsA42, up to 300V can be switched. If Q2 is a MOSFET,
chose R2/R1 to provide about 12V of gate voltage for the ON state.

Note, no Q1 base-emitter resistor is needed for the OFF condition,
because the logic-control line is strongly driven to ground, which
bypasses any leakage currents, and also rapidly turns off Q1.

 

[OBones]

Could you list a few of the products containing the circuits you
designed? I'd like to add them to my list of "Things to avoid".

That's personal projects, nothing production.

 

[Pooh Bear]

I often have to use an NPN transistor (emitter on ground) to turn on a PNP transistor (emitter on +B) to efficiently switch positive voltage. Is there a device that can replace the two transistors and two resistors required to switch the +B supply?

The required collector current would be less than 500ma and the Vce would be less than 50v.

Ideally, it would be a 4 legged device that would be in a case similar in size to a TO-92 device.

I know that an opto-isolator could do it but they're too expensive in most cases.

I often have to do this too.

Economically the discrete solution, like the one you're using, is the best.

Graham

 

[Pooh Bear]

why do you need two resistors ?
The cumulative gain means that the base current is ridiculously small in
both of the transistors. At worst, you only need one resistor, not two.

One word. *Leakage* !

Graham

 

[Pooh Bear]

That's personal projects, nothing production.

Try making a few thousand or tens of thousands and you'll see why a b-e resistor is a
very good idea.

Graham

 

[John Larkin]

One word. *Leakage* !

One more word: *nanoamps* !

John

 

[OBones]

OBones wrote:




Try making a few thousand or tens of thousands and you'll see why a b-e resistor is a
very good idea.

Then by all means, please point me to some sort of tutorial or best
practice guide, because the ones I've got never mentionned that kind of
setup in practical terms.