John Bonnett wrote:
> How / where would you install a decoupling capacitor ? Across the
> terminals so it can act like a mini-battery to keep the altimeter
> powered up
> for a few milliseconds in case the switch contacts bounce open under the
> launch impulse ?
> If so, for a 9VDC battery-powered altimeter like my RRC2, how big of a
> capacitor would you recommend, and how would you bleed off the charge
> when the switch is moved to the 'off' position ?
How big should the capacitor be? Good question. Difficult answer.
The goal is for the capacitor to provide the required current for some period of
time. So one of the variables is just how long that time is. The other two
variables are how much current is being drawn and how much voltage drop can be
The equation to compute the capacitor size is simple:
C = I * t/ V
C = capacitance in Farads (aka coulombs/volt
I = current in amperes (aka coulombs/second)
t = time in seconds
V = allowed voltage drop
Now we need to know what values to use.
Alas, most altimeters that are designed for 9V batteries use older three
terminal regulator that require fairly significant voltage in excess of the
regulated output in order to function correctly. The typical LM7805 has a
dropout voltage of 2V. Which means that the input cannot go below about 7V
before the regulator cannot maintain the output voltage. Some modern low dropout
regulators have dropout voltages of 0.1 volts or so.
So assuming that the battery is at 9V, a 2V drop is tolerable.
The other variable is how much current must be provided. Assuming for the moment
that this is 10mA, we can do some calculations.
If we want to hold things together for 1ms...
C = 0.010A * 0.001s/ 2V = 5uF
No "bleed" resistor is required as the electronics will drain the charge.
David W. Schultz
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