On 7/25/05, Bob Blick <bblick@sonic.net> wrote:
> Here's something interesting I discovered while doing some testing on a
> 16F876. I had RB7 output a high(to test the P-channel mosfet) and RB6
> output a low(to test the N-channel mosfet), and put a series of three
> different loads between them, and after measuring the voltages, calculated
> the equivalent on-resistances.
> 
> The N-channel mosfet has lower resistance than the P-channel, of course.
> But they also behave differently. At higher loads the N-channel resistance
> stays pretty much the same. But the P-channel resistance increases
> noticeably with load.

 Not only with the load but also with the supply voltage. I've made
some tests on internal pull-up resistor (basically some P-channel
mosfet) values as a function of supply voltage ( between 2V and 6V ).
It's a disaster.

best regards,
Vasile 


> 
> Anyway, having been one of those in-between kids, too young to have made
> my own transistors and too old to have seen mosfets in school, I hadn't
> expected this differnce in behavior :)
> 
> I'd like to test this again but with smaller loads, to reduce the effect
> of drain voltage and see if the P-channel reverts to straight resistance
> at light loads, but time doesn't permit, as this was part of a different
> test. Another test would be to see what happens at Vsupply=3.
> 
> I put a chart of this up:
> 
> http://bobblick.com/PIC16F876_load_test.html
> 
> Perhaps this will be of interest to some of you.
> 
> Cheerful regards,
> 
> Bob
> 
> 
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