Let's Make Robots!


Hi everyone!

I am just getting myself familiar with the basics of robot making, and I'm struggling a bit with resistors!

How am I supposed to know the minimum resistance in OHMs of a resistor for something like a servo so that it doesn't burn out, without knowing the max voltage of the servo?

Thanks in advance! Any good reading material on this subject would also be appreciated!


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Use 330 Ohm resistor on signal line.

What do you mean by signal line?

And how do you assert 330 Ohm?

There is power (usually red), ground (usually black), and signal that is usually either white or yellow.

He came up with 330 ohms because that is what the PICAXE boards use on their output lines that people connect servos to.

The signal line is the one that gets connected to an actual uC (microcontroller) pin and is the one that you send PWM signals out to the servo to make it move.

You may want a resistor between the microcontroller pin and the servo signal input to limit the current that can flow out of the microcontroller pin.

( As already mentioned an RC servo has 3 wires Signal, Power and Ground. )

Normally, the current drawn by the servo through the signal wire is very very small ( <0.1 milliamperes ) but in case the servo burns out or the pin is accidentally connected to ground the current from the microcontroller pin may exceed the capabilities of the chip and it will be destroyed ( for Atmel microcontrollers the maximum current is about 40 milliamperes ). To prevent this from happening a current limiting resistor can be used. Ohm's law says that the current = voltage / resistance. Since we know the maximum voltage of the microcontroller pin (usually 5 volts, sometimes 3.3) and we know the maximum current we can calculate the minimum resistance necessary to limit the current R = voltage / current => R = 5 V / 0.04 A = 125 Ohm. If we leave a safety margin we might use a 200 Ohm or 330 ohm resistor, whichever is more convenient.

We could even use a 1 kOhm resistor, or larger, but if the resistor value is to large the voltage "seen" by the servo controller may be too small. The voltage "seen" by the servo is proportional to the ratio between the limiting resistor and the impedance of the controller input like this: sensed voltage = supply voltage * sense impedance / (sense impedance + limiting resistance), so the greater the limiting resistance, the smaller the sensed voltage. The main point is we don't usually want to limit signal current to less than 1 mA.

For the power and ground wires you always want to REDUCE the resistance to avoid losses, so you would never want to put a resistor on those lines.

Well actually you may want to put a current sensing resistor on the power line, but you want that resistor to be as small as possible, usually much less than 1 ohm.

Ok, great, that clears things up!

So I'm never going to need to use a resistor on the power/ground cables for components (with the exception of sensor resistors)? How does that work for a 3v battery and a 1.5v servo?