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PICList Thread
'How to make high side switch'
1999\11\12@010502 by Jon Petty

picon face
Hi

I am trying to find the best way to make a high side switch to control an
auxillary power source for my pic in an automotive application. When ignition
switch voltage is present (13.8V or so) I want the pic to control a switch
that will direct 12V battery power to the  5V regulator for perifferals. When
ignition switch is off,  high side switch will cutoff bat supply to 5V
regulator for perifferals.

I would like a logic level signal from pic to control 12V source to regulator.

My plan was to use a pnp mosfet. It said it was a logic level hexfet, but it
did not work. I tried an npn logic level mosfet and it didn't work either. It
appeared the gate logic was dependent on the batter voltage. I got a  pnp
transistor to work when gate voltage was above or below the batt voltage, but
it was the inverse of what I was looking for. I know I am missing something
simple.

It seems so simple I have done it in the past using 5v relays, but I thought
there must be a simplier SS solution .

Could you provide some suggestions on how I can make a logic level high side
switch cheap?

Thanks

Jon

1999\11\12@045539 by Thomas C. Sefranek

face picon face
Jon Petty wrote:

> Hi
>
> I am trying to find the best way to make a high side switch to control an
> auxillary power source for my pic in an automotive application. When ignition
> switch voltage is present (13.8V or so) I want the pic to control a switch
> that will direct 12V battery power to the  5V regulator for perifferals. When
> ignition switch is off,  high side switch will cutoff bat supply to 5V
> regulator for perifferals.
>
> I would like a logic level signal from pic to control 12V source to regulator.
>
> My plan was to use a pnp mosfet.

You mean a P-channel?

> It said it was a logic level hexfet, but it did not work.

They work for me!  I'll bet you didn't use correct gate drive.
Source to battery, gate must go from battery level (Off) to a TTL level or great
er
less than battery. (On)
The PIC can't source battery level voltage without a buffer. (FET)  Or open drai
n
high voltage output.

{Quote hidden}

--
Thomas C. Sefranek  WA1RHP
ARRL Instructor, Technical Specialist, VE Contact.
http://www.harvardrepeater.org
http://hamradio.cmcorp.com/inventory/Inventory.html

1999\11\12@062624 by Michael Rigby-Jones

flavicon
face
part 0 6569 bytes
<P><FONT SIZE=2 FACE="Arial">I am trying to find the best way to make a high side switch to control an</FONT>
<BR><FONT SIZE=2 FACE="Arial">auxillary power source for my pic in an automotive application. When ignition</FONT>
<BR><FONT SIZE=2 FACE="Arial">switch voltage is present (13.8V or so) I want the pic to control a switch</FONT>
<BR><FONT SIZE=2 FACE="Arial">that will direct 12V battery power to the&nbsp; 5V regulator for perifferals. When</FONT>
<BR><FONT SIZE=2 FACE="Arial">ignition switch is off,&nbsp; high side switch will cutoff bat supply to 5V</FONT>
<BR><FONT SIZE=2 FACE="Arial">regulator for perifferals.</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Umm, can't you just connect your 5v reg to the 12v ignition supply?</FONT>
</P>
<BR>
<UL>
<P><FONT SIZE=2 FACE="Arial">I would like a logic level signal from pic to control 12V source to regulator.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">My plan was to use a pnp mosfet. It said it was a logic level hexfet, but it</FONT>
<BR><FONT SIZE=2 FACE="Arial">did not work. I tried an npn logic level mosfet and it didn't work either. It</FONT>
<BR><FONT SIZE=2 FACE="Arial">appeared the gate logic was dependent on the batter voltage. I got a&nbsp; pnp</FONT>
<BR><FONT SIZE=2 FACE="Arial">transistor to work when gate voltage was above or below the batt voltage, but</FONT>
<BR><FONT SIZE=2 FACE="Arial">it was the inverse of what I was looking for. I know I am missing something</FONT>
<BR><FONT SIZE=2 FACE="Arial">simple.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">It seems so simple I have done it in the past using 5v relays, but I thought</FONT>
<BR><FONT SIZE=2 FACE="Arial">there must be a simplier SS solution .</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Could you provide some suggestions on how I can make a logic level high side</FONT>
<BR><FONT SIZE=2 FACE="Arial">switch cheap?</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Thanks</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Jon</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">You could use a PNP bipolar transistor.&nbsp; You will need a resistor from base to emitter to keep it switched off, and then pull the base to ground (through a current limiting resistor) to switch it on.&nbsp; You will probably need to do this with an NPN transistor rather than using the PIC's ports directly.&nbsp; This gives the general idea.&nbsp; The resistor values are of the top of my head, the 2k will be the most important depending the the beta of the pnp transistor and the amount of current you want to draw, basically it needs to be driven into saturation.</FONT></P>
<BR>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">12v</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">____&nbsp;&nbsp;&nbsp; _______ Switched 12v&nbsp;&nbsp;&nbsp;&nbsp; </FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;| e\&gt; /c</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">+-+&nbsp; ---&nbsp; (pnp)</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">| |&nbsp;&nbsp; |b</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">| |10k|</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">+-+&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;|____|</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; +-+</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; | |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; | |2k</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; +-+</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; c|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 4k7</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">(npn)&nbsp; \|_____+---+____From PIC pin</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &lt;/|b |&nbsp; +---+</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp; e|&nbsp;&nbsp; +-+</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp; | |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp; | |10k</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp; +-+</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |&nbsp;&nbsp;&nbsp; |</FONT>
<BR><FONT COLOR="#0000FF" SIZE=2 FACE="Courier">--------------GND</FONT>
</P>

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1999\11\12@225651 by Jason Muhammad

picon face
Just connect the +5v regulator to the Ignition wire. That's what I did
to power my VFD in my car...

{Quote hidden}

--
Jason
========================================
Website: http://www.execpc.com/~milsumai
E-Mail:  spam_OUTmilsumaiTakeThisOuTspamexecpc.com
ICQ # :  12978762
========================================
         .:::.
          ,,,
        _(- -)_
       /  ( )  \
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        |_/ \_|
         | | |
      -TRY PRAYER-

1999\11\14@144922 by l.allen

picon face
>
> My plan was to use a pnp mosfet. It said it was a logic level hexfet, but it
> did not work. I tried an npn logic level mosfet and it didn't work either.

Logic Level means that the switching (fully on channel) voltage on
the gate is around 5 volts... often 3 volts as well ( as opposed to
the 11 volts needed on a typical mosfet). This is easy to understand
with an N channel.... 0 volts =off, 5 volts =on  at any appropriate
voltage on the drain. BUT....... a P channel logic level mosfet is
off at VCC and on at VCC - 5volts.... e.g. at 12 volts vcc 12 volts
on the gate will turn it off, 7 volts or less will turn it on!!!!!
I usually use a low power logic level N channel mosfet from the pic,
the drain of the N channel has a resistor.. say 100K from 12 volts
(in this example) to the mosfet drain, this provides a 0 to 12 volts
swing at is drain that I can connect onto the gate of an ordinary P
channel power mosfet.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

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

_____________________________

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