Needing help with a high amp system.

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Amstel
 
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Needing help with a high amp system.

Post by Amstel »

Hi Peeps,

I'm currently in the process of building a spider bot (4 servos per leg, 16 total, using Arduino) and I'm having some power issues. If I understand the specs of the servos correctly and from what I've researched, each servo (MG995 off brand) needs 2 amps when moving but can drop down to 1 amp if holding position. Theoretically I'd need 24 amps for the bot to standup and move one leg.

I want to clarify that I'm self-taught, so I'm learning as I go which leads to my problem. Am I understanding the relationship of servos to amps correctly and if so, how do I achieve a 24-amp system? I haven't seen a setup that is similar, nor do I know what to buy to create a system that would meet that demand. Also, if I do have a high amp system like that, I'm going to assume it would fry the Arduino.

Any help is appreciated!

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adafruit_support_bill
 
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Re: Needing help with a high amp system.

Post by adafruit_support_bill »

I haven't worked with the MG995s, but based on the torque specifications, 2A peak sounds about right. The non-moving current draw will depend somewhat on your mechanical design and how much static load the servo needs to resist to maintain position. No-load current is probably closer to 0.5A.

There are LiPo and NiMh battery packs that can deliver very high currents. Check with sites specializing in battery packs for RC racing or drones. The limitation of course is run-time. A 7.4v 5200mAh pack will only last about 13 minutes at a 24 A discharge rate.

In many cases it is possible to significantly reduce servo load (and associated current draw) with a counter-sprung design. I've used surgical tubing as countersprings for both servo arms and hexapod legs.

Properly wired, the power would be delivered directly to the servos, it will not be flowing through your Arduino, so you don't need to worry about frying the Arduino.

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Amstel
 
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Re: Needing help with a high amp system.

Post by Amstel »

Hi Bill,

Thank you for the reply and explanation. I'll definitely look into the RC batteries and a counter-sprung design. I hadn't thought of using that method to reduce servo load.

My next issues/ questions are using the Adafruit 16-Channel 12-bit PWM/Servo Driver - I2C interface - PCA9685 in conjunction with a high amp circuit. I had added a capacitor to the board to help with the power dips. My concern is if I am running 24 amps through the board, is there a possibility of it burning or melting the board? Essentially, I fear if I put that number of amps through the board, it will just melt.

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adafruit_support_bill
 
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Re: Needing help with a high amp system.

Post by adafruit_support_bill »

With that many high-torque servos active at one time, you are likely to be overloading the power and ground traces on the board. There are a few ways to deal with that:

1) Make a wiring harness that splits off power and ground to a direct battery connection - bypassing the power traces on the board.
2) Use some Rev Robotics Servo Power Modules with some servo jumper cables.
3) Beef up the power and ground traces on the board by soldering some bare 14 AWG wire in parallel with the traces on the bottom of the board.

In all of the above cases, you would also need to make a ground connection from the battery to the board for your signal reference.

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Amstel
 
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Re: Needing help with a high amp system.

Post by Amstel »

Pardon the super late reply Bill. But thank you so much. This has given me a lot to research and look into.

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