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Calculating the resistor values is done as follows.
The equation is the canonical VR = I * Rm. VR, in this case, is the rated
voltage of the motor. You must measure the resistance of the motor with
a ohmmeter- just place the two leads of the meter on each of the leads
of the motor. This value is R. I = VR / Rm (solving for I from the equation
above) With this form, calculate I, the current. This is the amount of
current the motor likes to operate at. We will get back to this value
in a moment.
Now, the Cricket's motor ports put out 9 Volts. By using
the below equation with the 9 volts of the Cricket and the current (I)
that was solved above, we calculate the value of resistor we need to solder
in line with the motor. The equation is: 9 V = I * (Rm + RN), where Rm
is the resistance of the motor measure already, RN is the value of the
resistor needed (in Ohms), and I is the current that was solved for above.
Calculate RN. This resistor should be soldered in series with the motor.
But first we need to calculate the power rating of the resistor we use.
P = I * I * RN. Solve for P (power). This value is in watts. Resistors most
commonly come in 1/8, ¼ and 1 watt values. If the value calculated falls
in between these values, choose the higher rating. Resistors can be purchased
at Radio Shack or DigiKey in most common values.
When you are ready to
test out your motor, watch for three things: 1. The motor chip on the
Cricket is hot (the motor chip is located right next to the motor LED).
This means that the motor is drawing too much current- you need a bigger
resistor (more Ohms!) 2. The resistor is getting hot. This means that
there is too much current flowing through the resistor; choose a bigger
resistor (in Ohms or in Watts- more Ohms will mean the motor will spin
slower, more Watts will not effect it). 3. Funny smell coming from the
motor- too much current through the motor; bigger resistor (in Ohms!).
An Example: I have a pager motor that is rated at 3 Volts. I measure
its resistance with a ohmmeter at 60 Ohms. The equation again is VR =
I * Rm, so I solve: 3 V = I * 60 Ohms I = .05 Amps I then plug that value
into the equation 9 V = I * (Rm + RN). In our case it is: 9 V = .05 A
* (60 Ohms + RN) Solving for RN, we have RN = 120 Ohms. Now lets calculate
the power we need: P = I * I * RN In our case, it is: P = (.05 Amps)(.05 Amps) * 120
Ohms. Solving, P = .3 Watts. This value is bigger than the ¼ Watt resistor
that is common. We will have to find a 120 Ohm, 1 Watt resistor to use
this motor.
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