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Ohm’s law is R = E / I (Resistance in ohms = Volts / Amps). Let’s
plug in what we know: 5.7 volts and 0.12 amps.
R1 = E / I = 5.7 / 0.12 = 47.5 ohms
You can’t go to the store and buy a 47.5-ohm resistor. Resistors
come in standard values and 47 ohms is the closest value. It
will result in a slightly larger voltage for the camera, but well
within normal design parameters.
Resistors are also rated in watts (how much power they will
dissipate). Let’s see how much power (in watts) will be being
put into the resistor.
Another formula is P = E x I (Power in watts = Volts x Amps). If
we plug our numbers in, we get:
P = E x I = 5.7 x 0.12 = 0.684 watts
Okay, if we put 0.684watts into the resistor, it must be rated to handle
at least that amount. A standard value larger than 0.684 is 1watt.
Warning: this is quite a bit of power and the resistor will get
warm – not hot. Make sure that it is located so that it won’t
touch plastic items: a shell or plastic tape.
6: Camera power supply circuit diagram.
DCC Impulses Column - 4
6
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MRH-May 2013