Hi everyone,
I have problem understanding current. Therefore, I am going to ask few questions and I hope you can understand my weakness and point me to the right direction.
How to calculate needed current (amp) for my project ?
I understand that the more current (amp), the better. But why some motor have a max current rating? What would happen if i go over the max?
Online resources are welcomed too.
Thank you
A motor uses current. How much depends on how much work it’s doing, but there is a max amount it can ever use. If a motor can use 5 amps, and it’s connected to a battery that can deliver 10,000 amps, the motor will only use 5 amps.
So you mean that extra amps than the rated won’t harm the system?
Okya, let’s say I’m building a humanoid with 20 dgrees of freedom. If the max current for each servo is 500mA, does it mean that I need to provid at least 500mA x 20 for the servo to be safe?
Thanks
That’s exactly want I mean.
The power delivery system (batteries, wires, connectors, etc) all need to be able to supply the servos with the current (power) they need. If all 20 servos have a 500mA max rating then you would need to supply 10 amps for the bot as the worst case scenario. That being said, there is no reason to build a power delivery system capable of delivering the max current for all servos involved with the understanding that not all of the servos will draw the max at the same time.
However I am not aware of any servo capable of providing the required torque for a biped that has only a 500mA max current draw. Servos capable of providing the torque for a biped have max current draw of more like 2 amps each.
So for the worst case, I need to sum up the max current of all devices, right?
About the servos max amps being more like 2amps. This means that if i used servos like hsr5990 (with 5.2 A), I would need power no less than 100 hAmps to oprate the robot for an houre!!! How to idealy deal with this?
Thanks Jim
True, but no one ever designs humanoid bipeds using the worst case scenario.
No. A humanoid biped made from 5990’s would not draw 100 amps under normal operation. 5.2 amps is the most it could draw at stall. there is a big difference between stall current and normal operating current. A 20 servo biped will draw about 5 amps TOTAL when walking. It may spike to 10 amps if doing a move that is inefficient, such as acrobatics or pushups, etc.
Now if you park your car on the robot and then command ALL of the servos to move, well there’s your 100 amps. 
The Stall current for “Each” hsr5990 is 5.2 amp. Who you assumed that all the 20 servos would consume 5.2 amps all together?
when the specification says that the max is 2 amp, does is mean 2 amp an hour ? or what?
Thanks so much again Jim
I’ve explained it to the best of my ability.
What he said, effectively, was that his experience has shown that bipeds based upon these servos would draw roughly 5A on average when walking and upwards of 10A when doing something agressive.
Now if you were to take such a biped and put it into some situation where multiple servos were stalled, deliberately or otherwise abusing the servos, then could it try to draw 20, 30, 40 or more amperes? Sure. Would that be anything normal? NO.
What has been provided is a ballpark estimate of typical current draw so that you can make an approximation on what size battery pack you need to operate for a certain amount of time. A conservative approach would be to round the 5A up to 6.5 or 7 and then multiply it by the number of hours, and multiply that by 1000 to get the pack size in miliampere-hours which they are typically rated in.
For example: estimate 7A average, 0.5 hour, 3.5 ampere-hour * 1000 = 3500mAH pack. If the biped is just standing around and occasionally walking you will get more than your 0.5 hour. If the biped is doing pushups, cartwheels, and backflips you will likely get less than 0.5 hour.
Clear as mud yet?
