Is there a device that can...

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

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.

--
 Thanks,
    - Win
Reply to
Winfield Hill
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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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Reply to
pbabin

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?

less than 50v.

size to a TO-92 device.

cases.

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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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?

--
Yes, a relay.
Reply to
John Fields

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?

less than 50v.

to a TO-92 device.

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.
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Reply to
John Popelish

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.

Reply to
Mebart

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?

less than 50v.

to a TO-92 device.

cases.

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

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

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

Reply to
John Larkin

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.

--
 Thanks,
    - Win
Reply to
Winfield Hill

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?

be less than 50v.

size to a TO-92 device.

cases.

One more word: *nanoamps* !

John

Reply to
John Larkin

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?

less than 50v.

to a TO-92 device.

cases.

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.

Reply to
OBones

On Sun, 05 Jun 2005 08:22:37 -0700, 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.

Jim "The other one" Meyer

Reply to
James Meyer

On Sun, 05 Jun 2005 18:09:35 +0200, OBones wroth:

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

Reply to
James Meyer

mitigate

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.

Reply to
Fred Bloggs

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?

less than 50v.

size to a TO-92 device.

cases.

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.

Reply to
OBones

That's personal projects, nothing production.

Reply to
OBones

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?

less than 50v.

to a TO-92 device.

cases.

I often have to do this too.

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

Graham

Reply to
Pooh Bear

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?

less than 50v.

size to a TO-92 device.

cases.

One word. *Leakage* !

Graham

Reply to
Pooh Bear

wroth:

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

Graham

Reply to
Pooh Bear

wroth:

resistor is a

Just look at leakage current on a data sheet ( Icbo IIRC ) and find the tempco if you can !

Consider hfe range. Basically all the tolerances and worst case situations that might occur simultaneously.

A b-e resistor is cheap insurance.

Graham

Reply to
Pooh Bear

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