Calculating Current

[stratego]

I am embarrassed by my low level of understanding but at least I am trying.

I recently purchased the "What's a Microcontroller guide" and BASIC Stamp HomeWork Board Set. I was hoping that someone could help me understand how to calculate the current going into the microcontroller on diagram 3-6 (book version 2.2 and 3.0 have the same diagram.)

Thank you

[Mike Green]

You may have some trouble measuring the current since it's on the order of a microAmp. Normally current is measured by running the current through a resistor that's small in relationship to the other resistances in the circuit, then measuring the voltage across the resistor. There's already a resistor in the circuit that you can use for this (the 220 Ohm one), but the voltage you'd have to measure is low (maybe 0.2V) and hard to measure reliably without special equipment or some kind of amplifier close to the circuit.

You could replace the 220 Ohm resistor with a much higher value resistor and that would give you a higher voltage, but then the voltage you'd measure would depend partly on the amount of current drawn by the voltmeter. That's likely to be lower than 1uA, but still would affect the measured voltage.