I posted this in Motors because thats what it was at first, but it’s turned into more Electrical and I haven’t gotten a response there yet, so here we go.
I want to hook up a servo to the Edubot kit. It says how to do that inpage 7 of the eduManual (pwm outputs section). The only problem is the +battery pin shoots out 7.2 volts (or whatever the battery is currently at). The servo’s we got in the kit are rated from 4.8v to 6v, so the controller’s voltage needs to be dropped with a resistor. Thing is, I’ve never done these calculations, so I want to make sure they’re right.
Ohm’s law solved for resistance is R = E / I, or Resistance = Voltage / Amperage. I’m guessing E is the voltage supplied. As for amerage, the servo specs list two opperating currents: 7.7mA @ 6v for “Idle Current” and 180mA @ 6v for “Running Current”. One gets a resistance of 935 ohms, and the other gets a resistance of 40 ohms. So the question is which do I use? I’m guessing I use the higher amerage (40 ohms), because thats the limit and the motor can take less current if it needs to, but like I said, I’ve never done this, so all of this is a complete guess.
So my question is to hook the servo’s up to the 7.2 volt EduBot pwm ports, is the resistor I need to use 40ohm?
I’m pretty new at the whole custom circuitry thing, so I’ll let someone else answer your resistor question since my answer won’t be very reliable.
But I wouldn’t be too worried about directly plugging that servo in. From my experience they can usually handle up to around 9 volts w/o any trouble.
I’m not exactly positive, but here it goes. The ratio of the voltage drops is equal to the ratio of the 2 loads (resistor and servo). You want a maximum voltage of 6 volts on the servo. So the voltage drop on the resistor is 1.2 V. The maximum resistance (935 ohms) of the servo is the minimum current (7.7mA) Keeping the ratios the same, the circuit would need 187 ohms of resistance when it is not moving. However, the circuit would only need 8 ohms when it’s moving… My suggestion is to purchase a 6V regulator from digikey.com It’ll cost around $1
We’ve been running servos like those in the kit straight off the pwm output with no trouble. I wouldn’t worry about it. Unless one of our electrical gurus like Al says otherwise.
It is VERY common to overvolt RC servos to get more performance out of them in RC Cars/Trucks. I have a feeling the life of them will be reduced though by doing so. Good thing they are very cheap.
hmm… one of my electrical mentors explained it to me like this the other day.
At 6 volts, the voltage drop we want is 1.2 volts. 1.2v / .18 amps gets a resistance of 6.6ohms. But, if we rearrange the equation for Voltage, then E = R * I. Therefore, at idle current, the voltage drop becomes 6.6 * .007 = .046 volts. So basically, using just a simple resistor is fine when you’re running it, but when it’s idle, you’re still essentially shooting out the whole 7.2 volts.
Now all of you are saying that it’s fine to overload the servos. Thats okay, but our mentor-guy came up with a neat little circuit that keeps the voltage between 3 and 5volts. So if anyone ever wants to run two servo’s out of the edubot and feel like teaching a bit about electricity, here ya go.
I never knew that such an issue existed so, I simply plugged the servo into the EDU Bot and it worked just fine. However, you are planning on running it for extended periods of time you might run into trouble so stick with your resistor idea.
All,
I like the approach given in the post above from SuperDanman when you know that the battery voltage is high. Although the parts count is high and it takes space to build, it is a very reliable way to insure the correct voltage is supplied to the servo. However, when the battery voltage drops below about 7 volts the regulator starts to drop out of regulation and it essentially turns into a resistor.
A simple diode (1N4001) in series with the battery lead will drop the voltage by 0.6 volts, cathode (banded end) toward the servo. Add two diodes in series for a 1.2 volt drop. The diodes are easy to install in the PWM cable with a little solder and a piece of heatshrink.
However, FIRST has been using servos for years connected directly to the output ports of the controller since those PWM outputs are at internal 5 volt reference instead of battery voltage. We have used servos in the past to initiate the action of small doors for instance. (Our Magic Tongue release of last year for one.) We have also used servo to initiate the change from brake to coast on speed controllers, where the servo actuator contacts a micro-switch which in turn is wired to the brake/coast jumper on the speed controller.
