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'[EE]: BE resistors'
2001\06\26@212839 by Tony Nixon

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Hi all,

Is there any rule of thumb of specifying base to emmiter resistors which
are commonly used to make sure a transistor is off when no base drive is
present. Perhaps while a PIC is resetting and the TRIS bit is
temporarily set back as an input.

I have seen values ranging from 1K to 1M and was wondering why the large
difference.

Many thanks.

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Tony

mICros
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2001\06\26@220505 by David VanHorn

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At 11:28 AM 6/27/01 +1000, Tony Nixon wrote:
>Hi all,
>
>Is there any rule of thumb of specifying base to emmiter resistors which
>are commonly used to make sure a transistor is off when no base drive is
>present. Perhaps while a PIC is resetting and the TRIS bit is
>temporarily set back as an input.
>
>I have seen values ranging from 1K to 1M and was wondering why the large
>difference.

1M is pretty large.
You need to make sure that under all conditions (heat, leakage, gain etc)
that the Q is off.
I'd use something closer to 1k, because it also improves EMI hardness.
That base-emitter junction is a diode, and will rectify RF into base
current, given a chance.
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2001\06\26@221725 by dpharr53

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Tony:

This resistor should be of high enough value as to not add significantly to
the driving current required from the PIC to turn on the transistor.  On the
other hand, if it is very large, the leakage current (transistor base to
collector) could be enough to generate a .6 to .7V voltage drop thereby
forward-biasing the transistor and turning it on when you don't intend to.
For a typical small signal transistor (e.g. 2N3904), the Base to Collector
leakage current is extremely small, which is why you see such a large
variance in this resistor value - in other words, almost anything will work.

However, personally, I would never use a value as small as 1K or as large as
1M.  For small signal transistors, something in the range of 10K to 100K
should work fine.

Dennis

{Original Message removed}

2001\06\26@221755 by Dave Dilatush

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Tony Nixon <.....Tony.NixonKILLspamspam@spam@ENG.MONASH.EDU.AU> wrote...

>Is there any rule of thumb of specifying base to emmiter resistors which
>are commonly used to make sure a transistor is off when no base drive is
>present. Perhaps while a PIC is resetting and the TRIS bit is
>temporarily set back as an input.

The rule of thumb I use, in the case of transistors driven from 5 volt
CMOS logic outputs, is to make the B-E "bleed" resistor about 3 to
five times the value of the base drive resistor.  When your transistor
is supposed to be off, this will provide plenty of sinking for leakage
currents; yet when you're trying to drive the transistor on, the bleed
resistor isn't so small that it robs you of much base drive.

>I have seen values ranging from 1K to 1M and was wondering why the large
>difference.

I think since, in most cases, just about any resistance value will do
the trick at moderate temperatures, the variations you've seen reflect
individual preference more than anything else.

Dave

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2001\06\26@222757 by Tony Nixon

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David VanHorn wrote:
>
> At 11:28 AM 6/27/01 +1000, Tony Nixon wrote:
> >Hi all,
> >
> >Is there any rule of thumb of specifying base to emmiter resistors which
> >are commonly used to make sure a transistor is off when no base drive is
> >present. Perhaps while a PIC is resetting and the TRIS bit is
> >temporarily set back as an input.
> >
> >I have seen values ranging from 1K to 1M and was wondering why the large
> >difference.
>
> 1M is pretty large.
> You need to make sure that under all conditions (heat, leakage, gain etc)
> that the Q is off.
> I'd use something closer to 1k, because it also improves EMI hardness.
> That base-emitter junction is a diode, and will rectify RF into base
> current, given a chance.

I usually use ~1K, and I got the 1M value from a Zetex typical
application note for the FMMT717 "Super SOT" transistor which I am
trying out in a PIC project. The 1M is good for me because I need
absolute minimal power drain from a tiny battery and also a low Vce
drop, as it is being used as a high side switch.

The circuit works, but I wondered if there was a text error, although
looking more closely, all FMMT717 resistors's on the sheet are the same.

EMI was something I didn't consider.



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Tony

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2001\06\26@223624 by Dan Michaels

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Tony.N wrote:
>Hi all,
>
>Is there any rule of thumb of specifying base to emmiter resistors which
>are commonly used to make sure a transistor is off when no base drive is
>present. Perhaps while a PIC is resetting and the TRIS bit is
>temporarily set back as an input.
>
>I have seen values ranging from 1K to 1M and was wondering why the large
>difference.
>

Hi Tony, if you are talking about the base pullup resistor in
a hi-side wired PNP like 2N3906, I typically use 10K - seems to
work ok in my 3 different situations:

1 - PNP used for programmer Vpp, where Rbase = 1K, and drive
   is via an NPN inverter.
2 - PNP used for RS-232 driver, where Rbase = 10K.
3 - PNP used for multiplexed RS-232 driver, where base is driven
   by 2 uC's, each with its own Rbase = 10K [ie, wired-AND config].

As others have said, too large and the pullup is not effective,
too small and it robs some of the base drive.

best regards,
- dan michaels
http://www.oricomtech.com
===========================

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2001\06\26@230433 by Brent Brown

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> Hi all,
>
> Is there any rule of thumb of specifying base to emmiter resistors which
> are commonly used to make sure a transistor is off when no base drive is
> present. Perhaps while a PIC is resetting and the TRIS bit is
> temporarily set back as an input.
>
> I have seen values ranging from 1K to 1M and was wondering why the large
> difference.
>
> Many thanks.
>
> --
> Best regards
>
> Tony

Hi Tony,

10:1 may be a good rule of thumb for this. The good thing about a
rule of thumb is that they are usually anonymous so you can't
blame anyone when they don't work.

But usually I don't use BE resistors at all. Waste of time. Like you
said, if your PIC output pin is driving a transistor then the only
possible time you may need one is during reset when the I/O pin is
temporarily floating. In normal operation the PIC output pin is push-
pull and the series resistor will provide both turn on and turn off
functions.

I wouldn't expect a BJT turn on all by itself, unless it was in a very
noisy environment with long leads etc. But I guess if the transistor
was controling something like a nuclear weapon firing system I
might relent and add another resistor for safety...

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  .....brent.brownKILLspamspam.....clear.net.nz

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2001\06\27@024918 by Vasile Surducan

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On Wed, 27 Jun 2001, Brent Brown wrote:

> >
> > I have seen values ranging from 1K to 1M and was wondering why the large
> > difference.
> >

   Because of h21e, or "beta" factor. All values mentioned above could
work ok. Depends what is the collector load range ( for npn ) and if the
transistor is a supergain ( darlington ) one or a comutation one ( which
have low h21e ). High amplifier factor could mean high resistor value.
So, there is no rule to compute that resistor value except Ohm law and
transistor's equations. ( Measure which is the minimum current to switch
transistor on with real load and assure at least 1 to 10 x that current
for blocking . BTW there are a lot of simple methods to keep transistor
off without that resistor, one is to float the emitter potential at more
or equal than vbe voltage,  using a diode in series with emitter. )
 Vasile

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2001\06\27@040736 by Alan B. Pearce

face picon face
>Is there any rule of thumb of specifying base to emmiter resistors which
>are commonly used to make sure a transistor is off when no base drive is
>present. Perhaps while a PIC is resetting and the TRIS bit is
>temporarily set back as an input.

Place wet finger in the air, see which way the wind blows, then think of a
number and double it, subtract the wind direction....


Seriously, it often depends on what other resistors you may be using. For
this sort of purpose I would tend towards 4k7 to 47k, probably depending on
having a SIP resistor pack of a value somewhere in that range already in use
with a spare resistor. 1k seems a bit low unless there is a requirement to
sink some current from the drive circuit for other reasons, and 1M seems a
bit high as then base emitter currents could start to turn the transistor
on, especially under high temperature conditions, unless it is a FET.

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2001\06\27@090703 by Olin Lathrop

face picon face
> Is there any rule of thumb of specifying base to emmiter resistors which
> are commonly used to make sure a transistor is off when no base drive is
> present. Perhaps while a PIC is resetting and the TRIS bit is
> temporarily set back as an input.
>
> I have seen values ranging from 1K to 1M and was wondering why the large
> difference.

I don't know of any quick answer.  This depends on how fast you need the
transistor to turn off, and what the base drive circuit does to turn it off.
To turn a bipolar transistor off, you only need to make sure its base
current is 0.  However, the minority carriers in the base do not instantly
go away when the base current stops.  This remaining "charge" in the base
region will still allow collector current until it is used up.  If you are
swithching something slow like a relay or a fan, for example, then the turn
off time isn't much of an issue.  If you need to go from saturation to off
as fast as possible, then you want to actively drive the base to about the
emitter voltage.  This will actually suck some of the existing charge out of
the base.  A B-E resistor can help in this case when the external circuit
doesn't actively drive the base to the emitter voltage.

A B-E resistor is also useful in cases where the base drive circuit goes to
high impedence to shut the transistor off.  In addition to speeding the
shutoff, the resistor will keep the base from floating and picking up stray
signals, and will overcome some leakage from the "high impedence" source.

The right value of B-E resistor depends on several factors.  Smaller is
better for turn off speed, but of course requires more current from the base
drive circuit to turn on the transistor.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\06\27@132940 by Robert A. LaBudde

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At 11:28 AM 6/27/01 +1000, Tony wrote:
>Is there any rule of thumb of specifying base to emmiter resistors which
>are commonly used to make sure a transistor is off when no base drive is
>present. Perhaps while a PIC is resetting and the TRIS bit is
>temporarily set back as an input.
>
>I have seen values ranging from 1K to 1M and was wondering why the large
>difference.

The base resistor R(B) and the base-emitter resistor R(BE) form a voltage
divider with the PIC 5V output at one end and ground at the other (assuming
an NPN transistor with emitter to ground).

The base resistor R(B)
The voltage at the base will be 5 x R(BE)/(R(B)+R(BE)), so too large a
value for R(BE) will prevent the transistor from
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ralspamspam_OUTlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
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"Vere scire est per causas scire"
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2001\06\27@133322 by Robert A. LaBudde

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Disregard my fragmentary recent post on this subject.

I mistook the problem, so the answer I was starting was off-base. I then
hit "Send" instead of "Delete".
================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: @spam@ralKILLspamspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
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2001\06\27@140057 by Dan Michaels

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Robert A. LaBudde wrote:

>
>The base resistor R(B) and the base-emitter resistor R(BE) form a voltage
>divider with the PIC 5V output at one end and ground at the other (assuming
>an NPN transistor with emitter to ground).
>
>The base resistor R(B)
>The voltage at the base will be 5 x R(BE)/(R(B)+R(BE)), so too large a
>value for R(BE) will prevent the transistor from


Actually, this wasn't too bad a start. However, in this case,
it might be better to think of the transistor base and Rbe as
forming a current divider on the current supplied through Rb.
If Rbe is extremely large, then most of Ib flows into the
base. If Rbe is very small, then it draws most of Ib to
ground and away from the base. So Rbe must be chosen at some
inbetween value, given how you want the ckt to operate.

Another reason for having an Rbe in this particular ckt,
is to provide a specific path to gnd for collector-base
"reverse leakage" current.

Specifically it is possible, with light collector loads and
high temperatures and cheap transistors and too high an Rb
value or upper end of Rb floating, that the C-B reverse
leakage current can actually turn on the load [by flowing
down into the emitter as the only place left to go].

I once saw this in a poorly designed relay control console
- as the temparature in the lab came up as the day went on,
the relays started to play clicky-clacky tunes. The device
under test was not amused - nor were the air force colonels
in the room at the time. Smokin'.

best regards,
- dan michaels
http://www.oricomtech.com
========================

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2001\06\28@150551 by Peter L. Peres

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Tony, a good rule of thumb is Rbe = Rb * 10. This assumes rail to rail
input drive.

Peter

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'[EE]: BE resistors'
2001\07\08@195244 by Tony Nixon
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"Peter L. Peres" wrote:
>
> Tony, a good rule of thumb is Rbe = Rb * 10. This assumes rail to rail
> input drive.
>
> Peter


Hi all,

Just a short not to say thanks for the responses. This part of the
project appears to work fine.

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Tony

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2001\07\08@204807 by Dan Michaels

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Tony.N wrote:
.......>
>Just a short not to say thanks for the responses. This part of the
>project appears to work fine.
>


Tony, we all appreciate your not saying thanks.

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2001\07\08@210711 by Bob Barr

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Dan Michaels wrote:

>
>Tony.N wrote:
>.......>
> >Just a short not to say thanks for the responses. This part of the
> >project appears to work fine.
> >
>
>
>Tony, we all appreciate your not saying thanks.
>

Dan, you really ought to consider putting a smilie (sp?) on the end of
comments like that one. :=)

On first reading your response, I thought you were being extremely (and
perhaps unnecessarily) sarcastic. Then I looked up and reread Tony's message
a little more carefully. Good one. :=)

Regards, Bob

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2001\07\08@220025 by Alexandre Domingos F. Souza

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>> >Just a short not to say thanks for the responses. This part of the
>> >project appears to work fine.
>>Tony, we all appreciate your not saying thanks.
>Dan, you really ought to consider putting a smilie (sp?) on the end of
>comments like that one. :=)
>On first reading your response, I thought you were being extremely (and
>perhaps unnecessarily) sarcastic. Then I looked up and reread Tony's message
>a little more carefully. Good one. :=)

       Of course, it seems that he wished to say "just a short NOTE to say thanks" heheheh ;oD

       What a missing letter can do...


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