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PICList Thread
'[EE]: Driving a solenoid'
2000\08\29@143316 by Lynx {Glenn Jones}

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Hello all,
I need to drive a solenoid. Currently, i have it connected to a NPN
transistor as a low side driver. This is all well and good, but the
transistor requires aprroxamately 100ma of base current which is of course
beyond the capability of the pic pin im using to control the system. Does
anyone have any good ideas of how to drive this 100ma base current? i
tried using a resistor to pull the base high with 100ma and a smaller NPN
transistor to drive it to ground, but this seems like its wasting a lot of
current. Thanks in advance, and best regards. Glenn

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2000\08\29@145533 by David Kott

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> Hello all,
> I need to drive a solenoid. Currently, i have it connected to a NPN
> transistor as a low side driver. This is all well and good, but the
> transistor requires aprroxamately 100ma of base current which is of course
> beyond the capability of the pic pin im using to control the system. Does
> anyone have any good ideas of how to drive this 100ma base current? i
> tried using a resistor to pull the base high with 100ma and a smaller NPN
> transistor to drive it to ground, but this seems like its wasting a lot of
> current. Thanks in advance, and best regards. Glenn
>

Wasting a lot of current is the truth.  You may find that a FET will work
considerably better as a low-side driver for this type of load than a BJT
transistor.  Good power FET's don't consume much current at all, due to
their extremely low on resistance.  Plus, they are voltage controlled
controlled, as opposed to current controlled.  This means that you have no
steady state current flowing on your control side.

Put a 3k(ish) resistor in series between the Gate of the power HEXFET, and
your PIC's output pin.  Inductive loads generate and unpleasant kickback
voltage that tends to capacitively couple through the Gate/Drain to your
driving circuit.  If you drive a PIC's output pin too far below your Vss
reference, you'll cause it to latchup.

http://www.irf.com


-d

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2000\08\29@145538 by Peter Schultz

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Hi,
Options:
Zetex superbeta transistor.
Darlington circuit (Vcesat around 1V, if it is OK)
PNP on high side to drive the NPN
MOSFet (The elegant solution )
Peter

{Original Message removed}

2000\08\29@152420 by David Kott

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> > Hello all,
> > I need to drive a solenoid. Currently, i have it connected to a NPN
> > transistor as a low side driver. This is all well and good, but the
> > transistor requires aprroxamately 100ma of base current which is of
course
> > beyond the capability of the pic pin im using to control the system.
Does
> > anyone have any good ideas of how to drive this 100ma base current? i
> > tried using a resistor to pull the base high with 100ma and a smaller
NPN
> > transistor to drive it to ground, but this seems like its wasting a lot
of
> > current. Thanks in advance, and best regards. Glenn
> >
>
> Wasting a lot of current is the truth.  You may find that a FET will work
> considerably better as a low-side driver for this type of load than a BJT
> transistor.  Good power FET's don't consume much current at all, due to

Did I *really* say "...consume current..."?  Blech.  I meant to say
something to the effect that the device doesn't consume power by converting
it to heat.

{Quote hidden}

-d

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2000\08\29@160502 by Olin Lathrop

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> I need to drive a solenoid. Currently, i have it connected to a NPN
> transistor as a low side driver. This is all well and good, but the
> transistor requires aprroxamately 100ma of base current which is of course
> beyond the capability of the pic pin im using to control the system. Does
> anyone have any good ideas of how to drive this 100ma base current? i
> tried using a resistor to pull the base high with 100ma and a smaller NPN
> transistor to drive it to ground, but this seems like its wasting a lot of
> current.

Use a darlington configuration.  That means you leave the power transistor
with emitter to ground and collector to the coil low side (don't forget the
flyback diode unless you like fried transistors).  A darlington is really
two cascaded bipolar transistors of the same type (NPN, PNP).  Since the PIC
can't source the 100mA the base of the first transistor needs, you insert
another transistor as an emitter follower.  This can be a smaller transistor
because it only needs to supply the 100mA base current to the first
transistor.  Tie the emitter of the second transistor to the base of the
first.  The collector of the second in a pure darlington would be tied to
the collector of the first, but in this case it would be better to tie it to
+5 via a suitable resistor that allows 100mA to flow over the roughly 4.2V
drop.  Tie the base of the second transistor to the PIC output via a
resistor.  This resistor needs to allow the PIC to source 1-2 mA over about
a 3.6V drop.

Did I succeed in totally confusing you 8-)


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

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2000\08\29@174651 by Bob Ammerman

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Darlington pair!

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

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