Dear Jim,

Is it possible to use one Shottky diode (anode connected to collector,
and cathode to your circuit)? I think it have voltage drop not more
then 0.35V.
By the way - to eliminate Uce saturation voltage, you can change
collector and emitter in your switch.

Best regards,
 Alexander Vilson
 mailto:wilson@telesys.ru

Tuesday, April 30, 2002, 10:20:16 AM, you wrote:

JR> Help sought on this issue.

JR> Working on a 18F252 based programmer/ICD
JR> Need to switch 5V on/off from a PIC.
JR> Off means Hi-Z
JR> Need 50mA min though switch.
JR> In the Hi-Z state the switch circuit must be able to isolate the PIC from
JR> 13V that may appear on the output of the switch from other sources.
JR> Use as few components as reasonable (3-4 range if possible.)

JR> My idea is to use a PNP transistor wired as a Common Emitter (that is
JR> Emitter to 5V.) Between this and the pic will be a base buffer resistor
JR> of around 4.7k nominal. Pull the base 0.6-0.7 lower than 5V to switch 5V
JR> to the collector.

JR> All fine so far, this idea has been standard for years.

JR> Now here's the rub. Put 13V on the collector and it will forward bias
JR> the base-collector junction and the 13V will appear at the base lead (less
JR> junction drop.)

JR> My proposed solution is to put a 3.3V zener between the buffer resistor
JR> (PIC side) and the 5V supply. The I/O pin on the PIC will be RA4 as this
JR> is an OC type and will not mind the 3.3V clipping at all.

JR> See http://users.pipeline.com.au/newfound/proposed.gif

JR> I know I could just use a different I/O line and use the internal diode to
JR> clip the excess voltage but this does not sit easy with me. Would rather
JR> use an external diode and keep the voltage on the pic I/O to less than its
JR> supply, not 0.65V higher.

JR> Nice in theory so far.

JR> Now the questions that fill my head.

JR> Firstly is it considered bad design to cause a forward bias condition in
JR> the PNP transistor like this does?

JR> If so why? (Pardon me if I'm in complete ignorance, I have no formal EE
JR> qualifications.)

JR> Could this destroy the transistor eventually?

JR> Is there a better way? An NPN emitter follower would work as the 13V reverse
JR> biases the base-emitter junction but I do not want the 0.65 volt drop.
JR> Could compensate with a 5.6V zener on the base and wire the collector to
JR> 13V but then would need a buffering stage between the base and the PIC.
JR> Nice circuit (used to switch VPP on the parallax/techtools picwriter
JR> and the WARP-17,BTW) but would rather keep component count down.

JR> See http://users.pipeline.com.au/newfound/voltsw_w17.gif

JR> I do not have knowledge of FETs etc and maybe there is a ready made solution
JR> by using a more exotic type transistor?

JR> Anyone with comments?

JR> Regards,

JR> Jim Robertson
JR> NEWFOUND ELECTRONICS





JR> NEWFOUND ELECTRONICS
JR> mailto:newfound@pipeline.com.au
JR> http://www.newfoundelectronics.com
JR> MPLAB compatible PIC programmers.

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