I am new to using servos, I have a small one that I want to hook up to an Adafruit Motor Shield V2. The servo I have is from an Arduino kit, and Arduino shows that it can or should be used with a capacitor across the DC power leads. I understand why.
However, the Adafruit shield connection doesn't seem to have an onboard cap, or is there any reference to one. I assume its because the current limit from the 5V supply from the Uno is like 900 mA or some such, when using a 9 VDC wall wart. In that case, I assume the small servo has all the current is needs at startup.
Can anyone comment on this, and add perspective? Thanks, Steven Lightfoot
Yes agree, its just that I have one and it has nice connections for the servo, so its a clean solution for what I want. The question really is about the need for a cap across the power leads for the servo. Any thoughts on that?
AdaFruit did a big NO-NO with that shield.
They use the 5V output from the Arduino to power the servos.
It will work ONLY for very small servos and ONLY if you power the Arduino via USB.
Don't try it if you power the Arduino via Vin or the power jack
OK, cool. Can you elaborate? I have a small servo SM S2309S that I got from Arduino (in a student kit) and for my demo purposes now its fine. My main question has to do with the need or not for a cap across the power leads (Arduino examples show one, but Adafruit makes no mention for this shield). Also, Adafruit seems to say you can use the servos connected to the shield header with the DC power port, and I was wondering if there was any current limit for servo size using this power.
You indicate here that you should NOT use the DC port on the Arduino, use USB only. I am quite new to this, can you explain more, and do you have any references to explaining good practice when using servos with Arduino? Thanks !!!!! Steven Lightfoot
The shield does have a 100uF capacitor on the 5V going to the servo.
In general it not a good idea to power any servo from the Arduino 5V pin. Even the small ones can draw several hundred milliamps and that can cause the Arduino 5V regulator to overheat.
If you wait a several seconds between servo movements it may be OK but you should check to see if the regulator is getting hot.
If you supply power to the Arduino via the USB input the 5V regulator is bypassed but you may still exceed the usb current limit of 500mA depending on the servo.
When in doubt you should supply the servo with a separate supply
Adafruit warns you (somewhat indirectly) not to use the 5V input but to cut a trace on the shield and use a separate supply
OK, wow, THAT is super helpful. I will study this more based on your comments. For my application I may actually want a stepper motor, so I will think more about what I am specifically trying to do.
Regarding the cap, it wasn't obvious to me that there was one on the Adafruit shield, I looked but I was expecting to see a 'large' barrel type on the board and I didn't see one. I should have checked the Adafruit circuit diagram.
I will eventually report what I do and the results I find.
I will start with the DC power plug (for higher currents) and monitor the Uno temp reg for overheating. I do want to use the servo in such a way that it would be constantly operating for short range movements.
By the way, regarding external power Adafruit scared me with this sentence and the red box: Using an external supply is for advanced users as you can accidentally destroy the servos by connecting a power supply incorrectly!
I would even know how to connect an external supply. Would it be simply connecting the servo +/- leads to, say, a wall wart and keeping the signal line connected to the Ardiuno pin?
Super helpful, amazing. I'd be much obliged with answers to a few questions about the diagram:
So basically this is two independent systems (only connected by the Arduino pin and then a common ground), and the Arduino still needs its own power via USB or VDC?
I assume if the servo needed say 12 VDC, you could simply use a 12 VDC power supply for it and it remains independent of the Arduino (save for the pin and ground)?
If the servo is still 5 VDC spec (so same as Arduino but now independent), but with high current, as long as the servo power supply has enough current headroom, no cap is needed for the servo?
Perfect! I did not know that servos are so current hungry, so this is great info.
I think of caps as charge accumulators, so when connected, they basically kind of dynamically store and release charge to help with providing current when current demand fluctuates. Is this the correct way to look at it?
You have been very helpful, thanks. Steven Lightfoot
Perfect. I am actually a trained engineer (Mechanical) so I have some basic understanding of electronics/electricity and calculus, and I am aware of the RC circuit concept, if you ever want to elaborate further. Cheers.
