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DCC Impulses Column - 6
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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.
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MRH-May 2013