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Author Topic: Current limiting circuitry for servos  (Read 965 times)
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Hi guys,
Apologies if this is a dumb question - I've searched the forum for answers, but none seem to quite give me an affirmative yes or no.

I'm going to be driving 6 servos - (Hitec HS 645mg High Torque servos to be exact)
Half the time only 2 servos will be active, but at other times 4 or even occasionally all 6 will be running for a short period)

I'm powering the system from a regulated power supply, set to 6V, with maximum 2.5A output.

I'm a little confused by weather I need a Motor Shield or not. Can I string all the servos off the same power supply, without a current limiter protecting each servo? - Just set the PWM line when I want the servo to actuate?
Presumably the supply will give whatever current is drawn, and no more, so when only 2 servos are running, I don't risk burning them out?
....or do I have my head in coo-coo land?!

Thanks

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nr Bundaberg, Australia
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The PSU will supply exactly what's needed and no more, so you should be right to just connect them all as long a 6 don't draw more current than the PSU can supply.

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Rob
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Rob Gray aka the GRAYnomad www.robgray.com

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fabulous - Thanks Rob - that's exactly what I wanted to hear!
I'll probably need to supply them with dual sides of the power supply to feed enough current to the little blighters when they're under load...but if that's all I must do its not so bad...!

Thanks again
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Your power supply should have a + voltage and a Common or ground. Ground from your servo power supply should be connected to Arduino Ground. Servos don't need current limiting resitors.

If you don't supply the servos from both sides of the supply they won't work...
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Yep! common ground - check! I've been frustrated  by that one in the past!
Thanks again!
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Hi,

Regarding
Code:
I'm going to be driving 6 servos - (Hitec HS 645mg High Torque servos to be exact)
Half the time only 2 servos will be active, but at other times 4 or even occasionally all 6 will be running for a short period)

If you want the servos to hold their positions when inactive, you will still need to pulse them from the Arduino. Fortunately the Servo library makes this very simple and will continue to pulse your servos in the background with whatever position you last set.

In your application you might need a larger current power supply, I could not find the current specs for you chosen servos, but if all 6 of the are operating against a load they could draw 6+ amps.

For an example of using the servo library to control upto 12 Servos see my blog - scroll down to the second and third post. From the moment you call the attach function, the servo library will begin pulsing the servo to set and maintain a centered position, any position you set after this will be maintained in a similar fashion without your code having to do any work. This will ensure that your servos maintain their positions even under load in your project.

rcarduino.blogspot.com

Duane B
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Thanks Duane,
That's really helpful!
fergal;
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You really need to test a servo and see just how much current is drawn by a servo when it is at the desired position and not holding any load. The actual current will probably be very low, as the internal h-bridge will be getting little or no pulses to try to move the motor. If the h-bridge is getting pulses to move the motor, then the servo is not in its commanded position.
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Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   smiley-cool

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