internal resistance

Archiver

Hi Stampers,

does anybody know the value of the stamp's internal resistance per pin?
Does it make sense to use pull-downs with a similar resistance?

Thanks for help,

Uli



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Archiver

>Hi Stampers,
>
>does anybody know the value of the stamp's internal resistance per pin?
>Does it make sense to use pull-downs with a similar resistance?
>
>Thanks for help,
>
>Uli

Hi Uli,

The internal resistance of a Stamp pin is about 25 ohms sourcing and
10 ohms sinking. The short circuit current is about 80 milliamps
sourcing and 140 milliamps sinking--warning!--these currents are out
of the safe range for continuous operation, so always use a current
limiting resistor.

Those figures are for the BS2p and BS2pe. I don't have the figures
for the other Stamps in front of me.

No, in answer question, it makes no sense at all to use pull-downs
with a similar resistance (same as the internal resistance). Pullups
in the range of 1000 ohms to millions of ohms make sense, depending
on the situation.

The internal resistance is nonlinear. What that means is that it is
like a simple resistor at light loads, and turns into a current
source at heavy loads.

You can measure the internal resistance yourself. Set a Stamp pin
HIGH (or LOW). Hook up a voltmeter between Vdd (Vss) and the stamp
pin. It should read zero. Now connect a 1000 ohm resistor between
the pin and Vss (Vdd). Now there will be a voltage reading, about
0.1 volt. Now you can calculate the internal resistance:
Rinternal = 1000 * Vmeter / (Vdd - Vmeter)
For example:
Rinternal = 1000 * 0.123 / (4.98 - 0.123)
= 24.7 ohms

If you were to use a pulldown resistor of 25 ohms, the voltage at the
pin would be pulled down to about Vdd/2=2.5 volts.

Input and output resistance are a fundamental engineering measurements.

-- Tracy