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DCC Impulses Column - 6
before, this diode will drop 0.7 volts, so that reduces the volt-
age across the resistor.
Let’s calculate the voltage across the resistor. The DC supply
provides 14.3 volts. The battery charges to 9.6 volts and the
diode will drop 0.7 volts. So, the voltage across the resistor
when the battery is fully charged, but under load, will be:
14.3 volts – 0.7 volts – 9.6 volts = 4 volts
Now that we know the voltage and current, we can calculate
the resistance and power:
R2 = E / I = 4 / 0.023 = 173 ohms
P = E x I = 4 x 0.023 = 0.1 watts
A standard resistor close to this is 180 ohms rated at least ⅛ watt
(¼ watt is very common). The slightly larger resistor will reduce
the on-going charge current, which is safer for the battery.
8
8: Complete circuit diagram.
A very common diode for this circuit is the 1N4001, rated at
1 amp and 50 volts. The 1N400x series are all rated at 1 amp,
with the last number increasing with higher voltage ratings.
Thus, a higher rating, such as 1N4007 will work just fine.
Bringing it all together
Okay, we now have each module designed. We need a switch
to disconnect the battery from the camera, to keep the camera
from discharging the battery when no track voltage is present.
Bringing this all together, we get the complete circuit diagram.
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Until next month, I wish you green boards.
MRH-May 2013
