Servo board current limit

All things antweight

Moderators: BeligerAnt, petec, administrator

Post Reply
Danieljr1
Posts: 13
Joined: Wed Sep 04, 2013 5:25 pm

Servo board current limit

Post by Danieljr1 »

Hi,

Whats the current limit on servo boards such as the one in these servos? http://www.ebay.co.uk/itm/MG995-Metal-G ... 20d5a3cead
User avatar
peterwaller
Posts: 3213
Joined: Fri Feb 15, 2002 12:00 am
Location: Aylesbury Bucks
Contact:

Re: Servo board current limit

Post by peterwaller »

Hi
First if all welcome to the forum.
I wasn't sure if you wanted to know how much current the servos draw or what sort of load a servo board could drive.
I couldn't find any figures for the current but most high power servos take around an amp with some going up to 1.5A.
If you run them at a higher voltage they will take more.
If you are looking to use the board to drive a gearmotor you would probably be OK with the standard motors but not the HP types but that's just an educated guess.
Looking around it seems the MG995 have been replaced by the MG996R see below.
http://www.hobbyking.co.uk/hobbyking/st ... ouse_.html
I hope this is of some help.
Danieljr1
Posts: 13
Joined: Wed Sep 04, 2013 5:25 pm

Re: Servo board current limit

Post by Danieljr1 »

hey,

thanks for the reply. the mg996r arent that much more expensive so would probably go for them. Im thinking of four 6v 400rpm gearmotors and 4 servo boards as escs running on 2s lipo. Would need to wire it up so the servo boards bypass the Rx for power. Used to compete in the featherweight class but got a little bored with the costs etc
User avatar
peterwaller
Posts: 3213
Joined: Fri Feb 15, 2002 12:00 am
Location: Aylesbury Bucks
Contact:

Re: Servo board current limit

Post by peterwaller »

Without having tried it I can't say it will definitely work maybe you could get one and try it unless anyone else has tried this ?.
The slight worry is that the servo is only rated at 7.2v but a fully charged two cell lipo although often quoted as 7.2v is 8.4v.
danjr1
Posts: 26
Joined: Fri Sep 03, 2010 11:54 am

Re: Servo board current limit

Post by danjr1 »

just ordered the servo so will give it a go! thanks for your help.
User avatar
BeligerAnt
Posts: 1872
Joined: Wed May 15, 2002 12:00 am
Location: Brighton
Contact:

Re: Servo board current limit

Post by BeligerAnt »

I can only say that I've never had a problem with a board from a standard-size servo driving a 12mm gearmotor. And I've used some really cheap and nasty servos!
They all seem to stand up to 7.2 (8.4) volts as well, even if the original servo was only spec'd to 6V.

Unfortunately the only way to really find out is to try it, ideally without spending too much money! Let us know how it goes...
Gary, Team BeligerAnt
razerdave
Posts: 1562
Joined: Mon Jan 02, 2006 9:40 pm
Location: Carterton, Oxfordshire
Contact:

Re: Servo board current limit

Post by razerdave »

If you need a voltage regulator (to supply 6v to power the receiver) drop me a message, I have a spare 1 or 2.

Since we're talking servo boards, I am having a lot of issues with mine. I'm modding some S3003 boards (old style ones, circa 2002), and at the midpoint the pot registers 1.7kOhms on one side, 1.9kOhms on the other. This with trims central, and motor at a stop. I thought that getting a pair of 1.8kOhms resistors would put it slightly off center, but within the tolerances of the trims. Seems I was wrong, because I have to have the trims right to the end and the stick marginally off center to get them to sit still (mixing off or on) and no amount of trim work seems to fix it. The values I'm getting from the pot are unusual values and can't get them in maplins.

Any advice would be good.

Also, I checked the resistors to make sure I was given the correct ones. On the bench: 1.8KOhms. When its wired: 1.3KOhms. Am I doing something daft ?
User avatar
BeligerAnt
Posts: 1872
Joined: Wed May 15, 2002 12:00 am
Location: Brighton
Contact:

Re: Servo board current limit

Post by BeligerAnt »

Dave, having the "centre" point some way off from the mid-point of the pot' is not unusual. The pot' has much more travel than the servo.

Measuring resistance in-circuit is difficult to impossible depending upon the rest of the circuit. Best procedure is as follows:
0. Connect pot' to servo board using some wires so it's easy to disconnect.
1. Adjust pot' on servo board so that the motor stops with Tx trims set to mid-point.
2. Disconnect pot' from servo board, taking care not to move the shaft.
3. Measure the pot' resistance values.
4. Fit fixed resistors of the values measured above in place of the pot'

If the resistance values are not close to standard values, there are a few things you can do.
First, the ratio is more important than the actual values, so long as the total resistance is in the 5k - 10k range you should be OK.

Second, don't sweat over getting the values closer than a couple of % to the measured value. Standard resistors have at laset +/-1% tolerance, possibly up to 5%.

Third, you can make up odd values by using parallel or series combinations. For example, if you need 2.3k, use 2.2k + 100R. Again, don't sweat over the last few % and the ratio is more important than the exact values.

Hope this helps
Gary, Team BeligerAnt
Post Reply