Needs hlp to understand small part in ICD2 clone of Pics

[abdalla]

Hi..

I Need hlp to understand small part in ICD2 clone of Pics



what RC2 do in this circuit?? .... as i can`t understand its job ??..

here is the schematic (www.icd2clone.com)


thx

 

[danciurea]

abdalla,



The line named RC2 pulls the "target MCLR" to ground.
T3 acts like a switch which opens when it's emitter is pulled down to ground. You can see in the schematic that the base is connected to VCC therefore when the emitter is pulled to ground by the RC2 line the B-E junction is forward biased allowing the transistor's C-E junction to conduct. In this circuit configuration that transistor allows the RC2 line to switch voltages higher than VCC.

On the other hand I think that D7 is useless here. I removed both D7 and D6 in my version of ICD2 and works ok.

I hope I was clear enough.....

 

[abdalla]

thx so much

but i`m wondering if we removed this T3 completely

i think the line will be pulled down because ADC voltage divider--> A0

and so if Neither 12.5v nor 5v is applied on line the line will be grounded..!!!

 

[danciurea]

you cannot remove T3 completely. If you remove it, you will apply the programming voltage of around 13v directly to line RC2. As a result the internal clamping diode will be forward biased and it will inject the 13v into the 5v supply.

Completely unwanted I might say....


On the other hand the voltage divider which senses the VPP voltage is completeley useless for pulling the line to ground. Why? Because it has high resistance compared to a pull up resistor used to keep the MCLR pin up on the target MCU.

But if you still want to remove some components you can happily remove the D6 and D7 which are redundant in my opinion.

 

[btbass]

I do not agree that D6 is redundant.
If you have 5volts on T2 emitter and 13volts on its collector, you will have 8volts reverse across the pnp transistor, the junction typically acts like a zener at about 3 volts reverse.
The same argument goes for T3 and D7.

 

[danciurea]

You might be right about D6 and T2, but D7 is always forward biased and I cannot see any advantage from using it there.


Anyway, on my ICD2 I removed them both and until now it works with no problem.

When I removed D6 I wasn't sure If it's correct what I'm doing or not, so, I tried it without diodes for few days, I saw that it works and that was it.