Changing power input from 9v to 12v stops project working

Drive a 4-wire bipolar stepper motor or a pair of DC motors from your Arduino. [Product page]
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carmo
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Changing power input from 9v to 12v stops project working

Post by carmo » Mon Jan 12, 2015 7:54 am

Hello,
I have a project that uses a H Bridge to drive a couple of lego motors that run at 9V from a battery pack of 6AA batteries. When I connect this and upload the example code from the quickstart page everything runs fine. Note this is with the battery pack connected to the motor power input. In this state everthing will run normal with the VIN jumper on or off and it will run no matter where the current limiting switches are, even on the lowest limit.

So I want to use a SLA battery instead as the motors chew the batteries quick and I also wanted it to have a bit more power and since the SLA batteries I can get only come in 12V or 6V (and the latter doesnt run the motors) it had to be 12V.

So when I make one change, 9V to 12V it stops working.

The 12V battery shows 12.5V on the multimeter. I wondered if it was flat but when I connect it up directly to one of the motors it runs them a lot more strongly than the battery pack. In fact just now I connected both up and it runs both of the motors together in parallel.

The messages coming back via the serial monitor just keep ticking over nicely regardless of the power source, so I would say there is no problems with the Arduino.

carmo
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Joined: Sun Jan 13, 2013 6:21 am

Re: Changing power input from 9v to 12v stops project workin

Post by carmo » Mon Jan 12, 2015 8:24 am

some extra info during further investigation.

say ive had 12v setup to go into it, then I put the 9v back, the project will not work until its been completely powered down. So in case I have the usb connected to the arduino, I can connect the 9v in after the 12v and the serial messages will still continue as if nothings happened but the motors do not drive at all. Only when the usb is disconnected and the whole thing powers down and back up with the 9v successfully work. Pressing reset button does not help.

when you first switch on after connecting up 9v one of the motors turns for half a second then stops then the program starts proper.

When you first switch on after 12v one of the motors (same one) turns but for about a quarter of a second, then nothing.

carmo
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Joined: Sun Jan 13, 2013 6:21 am

Re: Changing power input from 9v to 12v stops project workin

Post by carmo » Mon Jan 12, 2015 8:42 am

when the 9v was connected, i disconnected the pins going to the motors and tested the voltage being applied, it was coming on 9v, -9v, then over to the other motors connection, 9v, -9v and so on.

So I tried this with the 12v and it worked, the voltage was applied. So I reattached one of the motors. straight away that connection no longer had any voltage being applied. When I connected a motor to the other one that one no longer had a voltage being applied.

So I connected up a small motor and that one worked no worries.

Is it the current limiter? Why would something that doesnt exceed 0.6A at 9v exceed 2.0 amps at 12v, is motor current draw non linear??

andrew
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Re: Changing power input from 9v to 12v stops project workin

Post by andrew » Mon Jan 12, 2015 9:15 am

carmo wrote:
Is it the current limiter? Why would something that doesnt exceed 0.6A at 9v exceed 2.0 amps at 12v, is motor current draw non linear??
Depends on the load on the motor at the time. However the LEGO motor may not be happy being given the odd bursts of 12V.
Could you please reply with the LEGO part number for the motor?

carmo
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Re: Changing power input from 9v to 12v stops project workin

Post by carmo » Mon Jan 12, 2015 10:58 am

gday John,

its out of the 8293 power functions kit, i'm pretty sure is an 8883 M motor.

thanks,
carmo

carmo
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Re: Changing power input from 9v to 12v stops project workin

Post by carmo » Mon Jan 12, 2015 11:01 am

also the motors were under no load in all the situations I tested. though it is a geared motor so the gearing would give a bit of a load.

I have operated at 9v successfully in the past on load.

andrew
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Re: Changing power input from 9v to 12v stops project workin

Post by andrew » Mon Jan 12, 2015 9:31 pm

If your other small motor is turning when connected to the HBRIDGE shield I can only say that perhaps something has happened to the LEGO motor. It should only draw 150~300 mA max under load at 9V.

This could be confirmed by connecting it to HBRIDGE shield with 9V motor power and with the current limited to 0.6A max. The chatter in the LEGO community is that the 8883 can run on 12V however will damage it over time.

carmo
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Re: Changing power input from 9v to 12v stops project workin

Post by carmo » Wed Jan 14, 2015 1:11 am

Hmm its a strange one for sure.

I didnt think the motors would draw more than the current limit ( but I dont have any current measuring devices to test) and based on the spec it shouldnt.

I also dont think the motors are damaged by 12V running yet. See bottom.

I have reverted between 9V setup and 12V setup numerous times (using H Bridge). Everytime I go back to 9V it works fine. I have also ran it under 9V in all the current limiting positions numerous times.

I have only run the motors directly wired up to 12V for perhaps 5 seconds in sum total. Each time I only connect it watch it run up and steady and then off again.

Surely a current limitation couldn't be via the batteries. Surely a large 12V SLA battery should be able to supply as much current as a bunch of 6AA's? Again, no current measuring device so can't check.

O well, am looking at other power supply options, trying to see if I can find a 9V SLA battery or a 3V one that I can use with 6V I already have and then see how they go.

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