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Topic: Servo external power (Read 4718 times) previous topic - next topic

Brian60

Jul 06, 2017, 01:11 pm Last Edit: Jul 06, 2017, 01:25 pm by Brian60
Forgive me if this was asked before but I couldn't find an answer.

As I understand it, to power a servo seperately from the arduino, all that is required is the power source (4.6v nimh in my case) + to the servo, - to servo but also linked in to the gnd pin on the arduino and the signal from the servo to its nominated pin on the arduino.

Well I've done this and the servo will not move at all. Connect it to the arduino and it works fine. The servo is one of the micro g90 types so its not as though there isn't enough battery power for it.
I should also add that if I connect the servo to my r/c receiver with this battery pack it works perfectly.

Can anyone tell me if this is correct or not and offer a solution please.

vinceherman

#1
Jul 06, 2017, 01:40 pm Last Edit: Jul 06, 2017, 01:43 pm by vinceherman
Your description sounds correct.
How was your arduino powered?
Do you have a multi meter?  What voltage is your battery actually providing?
Can you tak a pic of your setup and post it?

edit: your 4.6v is slightly below the specified 4.8v minimum voltage for your servo.  Not sure if this would prevent any movement.  Is that the same pack you use to power your RC receiver?

I'll attach a pic that I frequently use to illustrate the wiring.

Brian60

thanks for the reply. As I was attempting alterations to a sketch the arduino was being powered from the usb port. The batteries are fully charged and showing just a tad more than 4.6v .

vinceherman

Is that the same battery you used when the servo was connected to the RC receiver?
Can you post a pic of your setup?

slipstick

Four NiMH batteries (4.8V nominal) should read around 5.4V, certainly well over 5V, when fully charged. At 4.6V they're pretty well empty. And what size/capacity are they?

OTOH if you're only running 3 cells (3.6V nominal) I'm amazed anything works at all.

Steve

Brian60

Sorry I'm late getting back, I had to go out.

OK it is four 1.2v nimh in a plastic clip same as vinceherman shows. As it has a Hitec type plug on the wires to fit a rx receiver, all I did was use breadboard jumper wires to connect positive to positive on the servo, negative to negative with a third jumper wire from negative to the gnd on the arduino. A fourth jumper wire went independently from the servo to the nominated pin on the arduino.

I'm thinking this could be batteries, but I find it odd that it will drive the servo via the rx receiver which they are also powering, but they won't power the servo by itself. The servo runs fine if I connect it directly to the 5v and gnd pins on the arduino.

these are the batteries, I think I'll have to put them on a charger and try again.

vinceherman


Brian60

OK guys once again thanks for your help. I charged the batteries fully and got readings between 1.31 and 1.4v from the 4 of them so I knew they were fully charged.

However still no luck in operating the servo. I sat down and went through everything again - I had overlooked the battery clip, something as simple as and they never go wrong, well they do! Closer inspection showed what I found in the photo. It seems at some point this clip must have shorted the terminals somehow without me knowing, the result was a melted terminal that wasn't in solid contact with the battery. It didn't show up earlier because I was swapping the batteries to a second clip to use with my current rc equipment.

MarkT

I have an alternative theory - the battery box isn't up to handling large currents.

I have encountered this before - the cheap battery boxes are of riveted construction, and
the contact resistance at the rivet can be large enough to cause local heating, limit the available
current, and rapidly degrade due to oxidation due to the heating.  So I would never use
such a battery box for high currents (> 0.5A) if its possible to avoid it.

Alas the typical plastic used in these battery boxes is not high-temperature rated, so you can't
solder the rivet joints without melting the plastic.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

failsafe7

Why is the ground wire from the battery pack leading back to the Arduino? (in the graphic)

slipstick

Why is the ground wire from the battery pack leading back to the Arduino? (in the graphic)
Because it doesn't work if you don't connect all the grounds together.

Steve

MarkT

failsafe7, this is a long dead thread.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

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