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EVERYTHING FOR ELECTRONICS
Bias on Production Run
Regarding Ryan Clarke’s January
2014 article on building a PIC-based
remote temperature sensor, I
checked the calculation of the base
bias resistor for Q1. Clarke
apparently used the minimum battery
voltage ( 2.7V), the saturation voltage
of the transistor (0,75V), the Beta
value of 10 from the sidebar, and
the measured minimum Ic value of
3.87 mA for the RF module.
The resulting 5,039 ohms will
indeed keep the 2N3904 in
saturation for all battery voltages ...
for that particular RF module.
However, if you were designing
this circuit for production, you'd have
to plan for some RF modules to draw
the specified minimum Ic current
( 11 mA). That would yield a bias
resistor value of 1,772 ohms, or
1.6K ohms as the closest E24
resistance value. This would
admittedly reduce the battery life a
little, but that's better than having
some circuits fail due to variations in
the RF module Ic current.
Absolutely, and my first "go
around" with the design was built
that way. However, this is not a
production design, but one tailored
to what was on hand. So, I got more
specific on the resistor as you see.
Thank you for your feedback. I
April 2014 7