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'power transistor'
1999\11\10@140018 by camerlin

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hello,

I am working on a project that requires a PIC16F84 to turn on a power
transistor via porta which then will turn on a relay.  The relay will
turn on a 5Vdc motor that draws 1.3A.  My problem is the power
transistor.  The PIC can source 50mA from portA. I need a transistor
that has a Beta of around 30 to do this (I think) (1.3/50mA=26) Does
anyone have any suggestions?  Also will the relay still need to have a
coil current less than 50mA to be turned on?
I'm having a few problems designing this simple power supply for this
project.  If anyone is willing to give me a hand let me know and I'll
send you the schematic of what I have done so far.  I need to have this
project done within the next couple of weeks so I would greatly
appreciate any help.  Thank you!!
Regards,
Chris

1999\11\10@145049 by The Old Crow

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On Wed, 10 Nov 1999, Chris Camerlin wrote:

> I am working on a project that requires a PIC16F84 to turn on a power
> transistor via porta which then will turn on a relay.  The relay will
> turn on a 5Vdc motor that draws 1.3A.  My problem is the power
> transistor.  The PIC can source 50mA from portA. I need a transistor
> that has a Beta of around 30 to do this (I think) (1.3/50mA=26) Does
> anyone have any suggestions?  Also will the relay still need to have a
> coil current less than 50mA to be turned on?

 Use a 'pilot' transistor between the port pin and the power transistor.
Such as a 2N4125 driving a 2N3055.  Port pin low would then turn on the
transitors and the relay (and snub diode) connected from power trnasistor
to +V.

/**/

1999\11\10@152037 by l.allen

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Chris wrote...
>
> I am working on a project that requires a PIC16F84 to turn on a power
> transistor via porta which then will turn on a relay.  The relay will
> turn on a 5Vdc motor that draws 1.3A.  My problem is the power
> transistor.  The PIC can source 50mA from portA. I need a transistor
> that has a Beta of around 30 to do this (I think) (1.3/50mA=26) Does
> anyone have any suggestions?  Also will the relay still need to have a
> coil current less than 50mA to be turned on?

Im a little confused?
You have a PIC that needs to turn on motor that draws 1.3 amps at 5
volts... correct?
You propose to use a relay to overcome the current amplification
needed ... correct?
You are worried about driving the relay from the PIC as there is
probably insufficient drive available from the PIC to energize the
relay coil.. correct?
You therefore want to use a transistor to drive the relay.. correct?

If these assumptions are correct then you dont need a relay at all, I
would recommend an N channel LOGIC LEVEL mosfet such as RFD14N05L or
even up to BUZ101L ( good for 29 amps).
They are cheaper than a relay and dont need another drive transistor.

The Mosfet connects the gate directly to the PIC, the source to
0 volts and the drain to the motor -ve, motor +ve goes to +5volts.
You will need a reverse biased diode across the motor for back EMF
suppression, such as 1N4004
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

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

_____________________________

1999\11\10@160242 by eplus1

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Skip the transistor. Directly connect the PIC to the relay. If you used a
relay (which isn't such a bad thing) it would need to have a coil current
quite a bit lower than 50mA to reliably be activated by the PIC. Radio shack
part number 275-232 should do nicely at 20mA and will handle 1A at 125VAC or
about 17A at 5VDC. Its also tiny. Add a back EMF protection diode to extend
the life of the relay contacts and another to protect the PIC from the relay
coil collapse.

Better yet, get the "The Motor Handbook" by Bob Boucher from
http://www.astroflight.com

You should also check out
http://204.210.50.240/techref/default.asp?url=/io/motors.htm

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{Original Message removed}

1999\11\10@160454 by Mark Willis

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Lance Allen wrote:
{Quote hidden}

Not a bad idea to put a small resistor on the PIC's pin going to the
FET, say 100R or so, instead of directly;  This reduces parasitic
oscillations (from FET capacitance), and safes things if that lead's
long (lead inductance, and inductive pickup problems) - put it AT the
FET for best results.  Perhaps a 10k or so resistor to ground at the
PIC, also! (you DO want the motor to not spin up when the PIC's reset,
and remember that PIC I/O pins default to floating, I assume?  <G>)

Logic Gated FET's there, which is what you want <G>

Maybe a small cap across the motor terminals, and one from each to
ground, if you have RFI problems, etc., also.

 Mark  "Love them HexFet's!"  <G>

--
I do small package shipping for small businesses, world-wide.

1999\11\10@233525 by Harold Hallikainen

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       How about using an FET to drive the motor directly?

Harold


On Wed, 10 Nov 1999 13:57:25 -0800 Chris Camerlin <.....camerlinKILLspamspam@spam@OVIS.NET>
writes:
{Quote hidden}

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1999\11\11@005049 by harryf

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Hi Chris,

I ever had experience. Maybe it has a simmilarity. My microcontroller
(MC68705R3) drives stepper motor 3 Vdc 3 A. What I did is using Darlington
transistor BD139  ( If I am not wrong) from MC to motor. To give/drive
bigger current, I add CMOS between MC and Darlington which has open drain
output. It can give big current source to Darlington.

For Power supply, I use regulator IC with transistor booster.

For relay (to supply MC and motor), it need special relay which can handle
big current in its 'switch'. If not suitable, the relay will be
oscillation (on-off-on-off................)

May be it can be modify......................

Regards,
Harry

Harold Hallikainen wrote:

{Quote hidden}

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