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PICList Thread
'Driving Motors - Help! -Reply'
1996\07\12@130811 by Mark Jurras

From your description you talk about using a 3V
battery. In your calculations for #3 you use 4.8
volts for the PIC. Do you have two supplies. If so
are the grounds common? If not tell me what you do
have 'cause the calculation are different.

Don't forget the 1.4V base emitter voltage drop
for a darlington.
Resistor    = (PIC Output Voltage-Vbe) / BEI
             = (4.8 -1.4)Volts / 0.12mA
             = 28.3K

Working the equation backwards with the 40K
resistor you calculate and a Hfe of 5K you still
should get 0.425 amps. Maybe not enough to start
the motors but enough to run them. What is the
starting current required?

- -Mark
>>> myke predko <spam_OUTmykeTakeThisOuTspamPASSPORT.CA> 12 July 1996
11:31 am >>>
Hi Folks,

- -snip- -

I originally tried to copy the circuit presented
in the "Runabout Robot" in
Electronics Now, but I've reached a stumbling
block.  The motor driver is an
"H" transistor Driver (a PNP 2N4403 from 3 Volts
(battery) to a motor
terminal to an NPN 2N4401 dropping the circuit to
Ground).  The Transistor
bases are connected to a PIC driver via a 750 Ohm

- -snip- -

3.  What PIC Pin to Transistor Base Resistor Value
should I use?  Assuming
an Hfe of 5K for the Darlington NPN Array and
current requirements of 600 mA
(for some guardband):

 C-E Current = Hfe x B-E Current ("BEI")
 600mA       = 5K (for example) x BEI
 BEI         = 600mA/5K
             = 0.12 mA

 Resistor    = PIC Output Voltage / BEI
             = 4.8 Volts / 0.12mA
             = 40K

So, from these calculations, a 40K resistor should
give me approximately 600
mA through a Darlington Pair NPN Transistor driven
by a PIC Pin.  Is this

Do you ever feel like an XT Clone caught in the
Pentium Pro Zone?

1996\07\12@132918 by myke predko

For Power, I'm using two 1.5 volt dry cells for driving the motors and a
Linear Technology LT1111 which provides 5V to the PIC and I/R Receivers.
The Grounds are common.

Thanx for pointing out my error in the Darlington Calculation.  I don't know
what the start up current is -  I haven't been able to get a good
measurement of it.

{Quote hidden}

Do you ever feel like an XT Clone caught in the Pentium Pro Zone?

1996\07\12@134001 by Mark Jurras

I need to do this in one of my apps. Can you point
me in the right direction for Application
assistance and manufacturers. Who's parts have you

- -Mark

>>> Mark K Sullivan <msullivanspamKILLspamVAX.NIOBRARA.COM>
12 July 1996  12:16 pm >>>
- -snip- -
You should look into low-threshold MOSFETs, too.
MOSFETs don't need gate
current (except when switching) and won't need any
gate resistors in your

- Mark Sullivan -

1996\07\12@161437 by Don McKenzie

Mark Jurras wrote:
> I need to do this in one of my apps. Can you point
> me in the right direction for Application
> assistance and manufacturers. Who's parts have you
> used?
> - -Mark
> try:
They have some driver IC's and/or boards that make it simple.

Don McKenzie
DonTronics Tullamarine, Australia

Picosaurus(tm) 40 pin PICBasic with 8 channels of A-D, and real Uart.
PIC Basic Compiler. Programmers from 15 USD.  Pic-Axe(tm) A New Tool.

1996\07\12@174602 by Ben L Wirz
       Dons right, we have just recently designed a Motor Driver board
that accepts TTL inputs and will control a wide range of currents
and voltages depending on which of the three configurations you use.
Bare double-sided printed circuit boards are available for $17.  This
board really makes it easy to get your motors up and running quick.  If
you would like more information, visit my WEB page at: or email me and I will send it via
snail mail.  I also have the LMD18200 and L293D driver IC's available if
you want to homebrew your own.

H-Bridge Driver Board Specifications

Driver IC Configuration         L293D   L293    LMD18200

Number of Channels              2       2       2        per Board
Avg Drive Current, I avg        1.2     2       3        Amps per Channel
Peak Drive Current, I max       2.4     4       6        Amps per Channel
Min Drive Voltage, Vc/Vs        4.5     4.5     12       Volts
Max Drive Voltage, Vc/Vs        36      36      55       Volts

Dynamic Braking                 No      No      Yes
Over Current Protection         No      No      Yes
Over Temp Protection            Yes     Yes     Yes
Over Temp Flag                  No      No      Yes
Current Feedback Output         No      No      Yes

Price for Complete Kit *        $27.00  N/A     $49.50
Price per Driver Chip           $4.00   N/A     $16.00
Price for Unpopulated PCB       $17.00  $17.00  $17.00

* Complete Kit Includes Driver IC's, PCB Board, and Connectors


Ben Wirz                For Great Deals on Nitinol Wire, H-Bridge IC's,
Wirz Electronics        Polaroid Sonar Units, PIC 16C84's, and more      Hobbyist Robotic & Electronic Supplies, visit:

On Sat, 13 Jul 1996, Don McKenzie wrote:

{Quote hidden}

1996\07\13@130625 by Mark K Sullivan

I have used IRF7201, IRF7202 for logic level FETs.  I don't know if they have a
P channel.

- Mark Sullivan -

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