Two guys have asked for the math, so I will post it to the whole list.
This is for the value of a "dropping" resistor to be used in series with an
LED. The purpose of the dropping rsistor is to keep the current in the LED
below the value that will fry it. LED's do not obey Ohm's law. In the
simplest theoretical model, any voltage above the "turn on" voltage of the LED
will cause an infinite current to flow, obviously burning out the LED. This
simple minded model isn't quite correct, but the truth isn't that far off. An
LED used without a dropping resistor is very likely to fry.
You need two values for the LED and one for the battery.
For the LED:
Forward Current, If (commonly in range 10 - 20 mA)
Forward Voltage, Vf (commonly in range 1.6 - 2 volts)
For safety, use typical values rather than maximum, especially on the
current.
For the battery:
Open circuit voltage, Vo (for a winch battery, probably 13.8 or 6.9 volts)
Ohms law says I = E/R or R = E/I where R = resistance (ohms), E = voltage
(volts), I = current (amps)
The only tricky part is what voltage to use. In this case, you subtract the
LED forward voltage from the battery voltage, so the equation is:
R = (Vo - Vf)/If
An example (from memory):
If = 20 mA
Vf = 1.6 V
Vo = 13.8 V
R = (13.8 - 1.6)/0.020 = 610 Ohms
Power across this resistor will be P = EI = (13.8 - 1.6) * 0.020 = 0.244
watts, so a 1/4 watt resistor will be just adequate. 1/2 watt might be safer.
If you use a higher resistance, the power will drop, so 1/4 watt would be
sufficient. You can get the equivalent of a 610 Ohm 1/2 watt resistor by using
two 1220 Ohm 1/4 watt resistors in parallel.
This is a MINIMUM value for the dropping resistor. Any less Ohms and you risk
burning out the LED. More Ohms will dim the LED, but will keep it safe.
Someone mentioned that the inductance of the motor might create voltage spikes
when the solenoid opens. To be safe, it might be best to use a dropping
resistor perhaps double or triple the minimum value. The LED will be dimmer,
but will be somewhat safer against voltage spikes. Whoever suggested 10K Ohm
resistors was probably thinking with this in mind, also 10 K Ohms is probably
enough dropping resistance to keep most readily available LEDs safe, so you
don't have to do the arithmetic.
To get higher resistance, you can use resistors in series. When they are in
series, the resistances add.
Mark Holm
[EMAIL PROTECTED]
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and
unsubscribe messages must be sent in text only format with MIME turned off.
