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'[EE]: How to switch 8 volts with a PIC'
2000\08\28@225401 by John Hansen

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Here's a dumb question (which if I'd done the right thing in college and
studied EE I could probably answer... but I didn't). ...

Aside from using a relay, is there a simple way to use a I/O pin from a PIC
(running at 5 volts) to switch a higher voltage (say, 8 volts).  The
current is negligible.   I'm assuming a bipolar transistor switch is not
going to work here because I'm attempting to switch a voltage that is
higher than that which is applied to the base.

Thanks for any assistance!

John Hansen

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2000\08\28@230811 by Dan Michaels

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John Hansen wrote:
>Here's a dumb question (which if I'd done the right thing in college and
>studied EE I could probably answer... but I didn't). ...
>
>Aside from using a relay, is there a simple way to use a I/O pin from a PIC
>(running at 5 volts) to switch a higher voltage (say, 8 volts).  The
>current is negligible.   I'm assuming a bipolar transistor switch is not
>going to work here because I'm attempting to switch a voltage that is
>higher than that which is applied to the base.
>

NPN inverter with collector pulled up to whatever voltage you like,
base driven by PIC thru 10-50K resistor.

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2000\08\28@230829 by Sean H. Breheny

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Hi John,

What did you take in college? Perhaps it helped you in ways EE couldn't!

The answer to this question depends on what the load is. If the load
doesn't need to be ground referenced (i.e., a light bulb or other 2
terminal device doesn't need to be), then you could use a bipolar
transistor between the load and ground, instead of between the load and 8 v
supply.

Even if it does need to be ground referenced, there are many tricks one can
pull with bipolars (i.e., use two of them). You could also use FETs, in
which case you could try various circuits.

In short, please give more info.

Sean

At 09:57 PM 8/28/00 -0400, you wrote:
{Quote hidden}

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2000\08\28@231202 by Gennette, Bruce

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You're assuming wrong - bipolar transistors are voltage amplifiers - if it
has a gain of 100 and you put 5 volts to the base in theory it would let
500V pass (but most 'normal' transistors can only handle moderate voltages -
around 40-60).

Look up 'level shifter' or CMOS to TTL interfacing.
All you have to do is keep the 2 circuits +V supplies separate, have just
one 0V join and the transistor between them.

Bye.

       {Original Message removed}

2000\08\28@233112 by Lance Allen

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On 29 Aug 2000, at 14:10, Gennette, Bruce wrote:

> You're assuming wrong - bipolar transistors are voltage amplifiers - if it
> has a gain of 100 and you put 5 volts to the base in theory it would let
> 500V pass (but most 'normal' transistors can only handle moderate voltages -
> around 40-60).
>

I hate to be difficult.. but a bipolar transistor is a current amplifying device.
The gain is the collector current over the base current.
A FET is a voltage controlled amplifier (V to I amplifier in effect).

Valves are inefficient FETs with a pilot light (that should bring em out
of the wood work).
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

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2000\08\28@234838 by Lawrence Glaister

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Ya gotta like valves.... I like the fet with a pilot light remark.... they
even support
several visual overload indicators - big red spot on plate, blue flash of an
arc over and an Erie blue glow when the have gas :=}
=======================================================
Lawrence Glaister VE7IT             email: lgspamspam_OUTjfm.bc.ca
1462 Madrona Drive                  http://jfm.bc.ca
Nanoose Bay BC Canada
V9P 9C9
=======================================================
{Original Message removed}

2000\08\29@004243 by Plunkett, Dennis

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Now in this case how does one get a Vbe? If we can only drive to 5V then the
maximum supply voltage that can be turned in is 5V + Vbe = 5.7V



Suggestion
if the current is low then:- connect something like a MMBT2907 (PNP) as a
high side switch, with a pull up resistor on the base to the rail that you
are switching, then use a MMBT2222 (NPN) to pull the base of the PNP to
ground (Via a resistor). This will let you drive the NPN with +.7V or better
to turn on (Current is required as suggested). Then the PNP will switch up
to 75V (Watch the current and dissapation). Simple and it does not turn on
when the power is on either and yes it also passes the relevant GM standards
(And Fords too)
Bit this all assumes low current, how much do you need to switch?

Dennis





> {Original Message removed}

2000\08\29@080519 by Olin Lathrop

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> You're assuming wrong - bipolar transistors are voltage amplifiers - if it
> has a gain of 100 and you put 5 volts to the base in theory it would let
> 500V pass (but most 'normal' transistors can only handle moderate
voltages -
> around 40-60).

Bipolar transistors are current amplifiers.  The collector-emitter current
allowed by the device is roughly the base-emitter current times a "gain"
factor.  Of course there's a lot more to the behaviour of a bipolar
transistor, but this is the main characteristic motivating their use in most
applications.

If you're new to this sort of thing, you can get some useful work done by
thinking of the model above, with the addition that a forward biased base
looks like a diode with the accompanying voltage drop (figure about .6 to .7
volts for silicon).


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, @spam@olinKILLspamspamcognivis.com, http://www.cognivis.com

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2000\08\29@080525 by Olin Lathrop

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> I hate to be difficult.. but a bipolar transistor is a current amplifying
device.
> The gain is the collector current over the base current.
> A FET is a voltage controlled amplifier (V to I amplifier in effect).

There are different types of FETs.  Many FETs can be thought of as a voltage
controlled resistance.  Of course, just like bipolars, there is a lot more
to the device characteristics than the simple gain part.

> Valves are inefficient FETs with a pilot light (that should bring em out
> of the wood work).

That depends on whether you are referring to triodes or pentodes.  Triodes
are closer to voltage controller resistances, whereas pentodes are more
voltage controlled current sinks  -- when the pilot light is on ;-)


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, RemoveMEolinTakeThisOuTspamcognivis.com, http://www.cognivis.com

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2000\08\29@154331 by Oliver Broad

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Provided the voltages are tightly defined you could put a 3v zener in series
with the transistor base resistor. Provided you fit a B-E resistor to shunt
away the zener leakage current it ought to work.

You said you wanted it simple. Also I understand that the RA4 open drain
transistor can switch voltages above VDD, though this maybe causes weird
things to happen to older PICs?

Oliver

{Original Message removed}

2000\08\29@223405 by Mark Willis

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I rather liked one guy's BRIGHTLY glowing violet purple tube project in
high school.  Rather pretty, the thoughts that came to me when I saw it
in High School were,
 (a)  That's pretty!
 (b)  Back away safely but quickly!
and
 (c)  This IS pretty, but isn't GOING to be pretty, soon!

Someone else got to the lab-wide off switch fairly quickly <G>  He was
overdriving the tube "just a little", it was quite a sight!  I'm
surprised the envelope didn't shatter when the plate melted...

 Mark

Lawrence Glaister wrote:
> Ya gotta like valves.... I like the fet with a pilot light remark.... they
> even support
> several visual overload indicators - big red spot on plate, blue flash of an
> arc over and an Erie blue glow when the have gas :=}
> =======================================================
> Lawrence Glaister VE7IT             email: TakeThisOuTlgEraseMEspamspam_OUTjfm.bc.ca
> 1462 Madrona Drive                  http://jfm.bc.ca
> Nanoose Bay BC Canada
> V9P 9C9
> =======================================================
> {Original Message removed}

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