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[DPRG] Please Clarify when to use npn or pnp transistors (fwd )

Subject: [DPRG] Please Clarify when to use npn or pnp transistors (fwd )
From: R. Bickle rbickle at swbell.net
Date: Wed Jul 11 16:28:32 CDT 2001


Here's a simple explanation of NPN vs. PNP transistors.

First of all, in all of the applications I have used BJT transistor in, I
rarely use any PNP types. Of course, I am generally using them as switches
to control relay coils, light bulbs, LED's and so on for instrumentation

As you probably have read, NPN transistors need to see current flowing from
base to emitter in order to conduct. Depending on the current applied to the
base, and the characteristics of the device, the transistor will conduct in
the linear region (i.e. constant current region) or in saturation.
(Saturation is normally what I'm looking for in switching applications.)
Keep in mind that an NPN transistor needs to have the base voltage at one
diode drop (0.6V) higher than the emitter. For this reason, NPN transistors
are usually connected to "sink" to ground rather than "source". For example,
If I want to switch on a light bulb at 12 volts, I would connect one end of
the bulb to +12 and the other end to the collector of an NPN transistor. The
emitter of the NPN will be connected to 12V GND, and the base to the
switching source. Most general purpose TO-92 transistors have a gain of
about 100 or so. (this is the beta or hfe, check your datasheet of the
part). This means that if my light bulb requires 100mA to operate, then the
base current to the transistor needs to be 100mA / 100 = 1 mA. Keep in mind
that a BJT is a current controlled device. It does not care what the base
voltage is, so long as it is greater than the emitter voltage plus 0.6. This
also means that I can switch a BJT with 5VDC, and control a light bulb
running on 12VDC, as long as the two grounds are connected. Make sure to put
a series resistor on the transistor base in order to limit the base current.
For my example above, you would need (12V - 0.6V) / 1mA = 1.14K or about 1K

PNP transistors operate in an opposite manner from NPN's. The base of a PNP
transistor needs to be lower than the emitter - 0.6V in order to conduct. If
I were to use a PNP transistor for my light bulb example, I would connect
the emitter to +12VDC, and the collector to the light bulb. The other end of
the bulb to ground. Now, in order to switch the bulb on, you would have to
take the base below (+12V - 0.6V = 11.4V) to turn on the bulb. Thus PNP
transistors are generally used for "sourcing" current to a device while NPN
transistors "sink" current to a device. The two types of transistors are
useful in applications like H bridges where you need a complimentary drive
circuit. (Although for most applications, I would probably use MOSFET's for
an H bridge instead of BJT's, they have lower on resistance.)

As you mentioned, one very confusing way of teaching semiconductor theory is
the "electron and hole" method. With regard to transistors, this is in my
opinion more confusing than helpful. Basically, there are two schools of
thought on electricity flow. The conventional current flow goes from
positive to negative, while electron current flow goes from negative to
positive. In reality, it's the little electrons doing the moving, so,
electricity really flows from negative to positive. In order to teach
semiconductors, many textbooks tell you that when electrons flow one
direction, holes flow in the other direction. (You can think of a PNP's
emitter "emitting" electrons, and an NPN's emitter "emitting" holes.) The
hole idea really is very helpful when you get into semiconductor device
physics, it simplifies the equations greatly, but for BJT's I prefer to just
think about the electrons.

Anyway, this has become more long winded than I intended. Hope this answered
your question. Here's a really nice link I found on transistor theory and
many other things.


Good luck,
Rick J. Bickle

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of Dan Miner
Sent: Wednesday, July 11, 2001 3:10 PM
To: 'rljordan at airmail.net'; rich_demartile at prodigy.net
Cc: Dallas Personal Robotics Group
Subject: RE: [DPRG] Please Clarify when to use npn or pnp transistors
(fwd )

I think you have PNP and NPN backwards in your description.

> -----Original Message-----
> From: Robert L. Jordan [mailto:rljordan at airmail.net]
> Sent: Wednesday, July 11, 2001 2:33 PM
> To: rich_demartile at prodigy.net
> Cc: Dallas Personal Robotics Group
> Subject: Re: [DPRG] Please Clarify when to use npn or pnp transistors
> (fwd)
> Rich,
> I have an opinion but not much knowledge.
> I use NPN to switch Positive ("source DC")
> The collector is connected to +DC
> I use PNP to switch Negative (Ground)("Sink power")
> The collector is connected to -DC (gorund, common)
> In an HBridge you can use "P Channel Mosefts" to switch Positive
> and "N Channel Mosfets" to switch ground. The P channel requires a
> voltage of about 7 volts above the source voltage to turn it on. (I
> think) You can use N channel Mosfets on the positive side too, but
> I'm not sure the pros and cons.
> I hope to hear better answers to your question from others!
> Bob Jordan
> Eric Yundt <eric at facetcorp.com> said:
> > DPRG,
> >
> > Anyone up for helping Rich?  Jim is no longer on the DPRG list
> > and doesn't receive email sent to the "dprg.org" address.
> >
> > Rich DeMartile wrote:
> > > From: "Rich DeMartile" <rich_demartile at prodigy.net>
> > > To: <jbrown at dprg.org>
> > > Subject: Please Clarify when to use npn or pnp transistors
> > > Date: Wed, 11 Jul 2001 01:16:30 -0700
> > >
> > > Hello Jim,
> > >     I'm new to this stuff.  I've studied solid state electronics
> for the
> > > last couple months, built a solar tracking device that works for
> my
> > > first project, and have a basic understanding of transistor
> usage.  What
> > > I'd like to get cleared up is...when do you use pnp versus npn
> > > transistors, mosfets etc.  I find lots of information regarding
> hole
> > > flow versus electron flow, and I notice that sometimes one is
> used over
> > > the other, but never why.  Looking at your page on H-bridge
> theory I
> > > finally got my first clue to why pnp is used for ...(sourcing
> power?).
> > > Could you please spend the time to explain to a layman what's the
> time
> > > to use one type over another, and maybe some real life examples.
> I'm
> > > starting to think that where the load is in relationship to the
> > > transistor in a circuit dictates what type you would use.
> > >     My lack of understanding in this area is preventing me from
> moving
> > > forward into more exotic control schemes.  I've been limited to
> using
> > > relays so far and they work just fine but...
> > >     Anyway, I can tell you know about these things by how well
> you write
> > > and explain things.  You can check out my solar tracker at the
> web site
> > > I just started if you like.
> > >
> > > http://pages.prodigy.net/rich_demartile/
> > >
> > > Thanks in advance,
> > > Rich DeMartile
> > > rich_demartile at prodigy.net
> > >
> >
> > --
> > Eric
> > _______________________________________________
> > DPRGlist mailing list
> > DPRGlist at dprg.org
> > http://nimon.ncc.com/mailman/listinfo/dprglist
> >
> --
> Robert L. Jordan (Bob)
> www.robotfun.com
> _______________________________________________
> DPRGlist mailing list
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