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Current protection for DC Motor Circuit

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bculver

New Member
Hi Everyone,

I need help trying to find a solution to my robot motor. I purchased a R/C Hummer from Walmart and happily started converting it to a robot with .... some purpose, not sure what yet. Nonetheless, after careful observation I learned the following:
The battery it came with says Nikko Ni-Cd 6V 650mAh
It has a mubashi 6V RC-280RA rear motor
and a simple motor in the front (I know nothing about it).

I want to use this diagram ... and yes :oops: , I have all the parts (why I still want to build it), but after reading all the motor controller related discussions in the forums I feel I need to expand the circuit to read the current and cut it off. I am using an OOPIC to drive this robot. Here is a link to my Hummer Project

Once I am done building the entire thing, it will operate 8 LEDs, a Polaroid 3500 Sonar, 4 IR Sensors, 1 LDR,and 2 Motors. In the future, I may add more to it, to give it purpose and better control it. I am positive this Hummer will be running into things and it worries me that I will burn out the circuits. I thought about a current-limiting resistor but I have no idea :shock: how to implement that ... actually the current sensing is above me too. I am completely new to this with just LED projects behind me so far.

Also, originally I built this thinking I could use it, but then I realized it outputs 12V where I need 6V. PWM Motor Driver with LM18200

I need to figure out how to wire the front and rear motors ... I think the front motor might be a 3V motor ... and its just for turn so its always going to stall ... I'm lost!

Any advice, help in any way is truly appreciated on my part. Thank you in advance. :)
 

bculver

New Member
I found this in Microchip's site "PIC A/D for Smart Cur

If you look a Tip #12, they have a tip on using the
PIC A/D for Smart Current Limiter
... would this work?

Could someone guide me as to how I can integrate it into the circuit?

Thanks
 

seeker

New Member
Hi bculver,

A simple over-current protection like one of these thermal circuit breakers
might work. http://www.hhv.co.uk/ccbkrs_index.html
One of these could be put in line with the battery lead (to protect everything)
or in line with the motor lead only.
To protect the motor only, find the complete model # of your motor and see the max current draw of that motor.(on the mubashi page)
Then find a breaker that trips at a slightly lower current.

I know it aint fancy electronics :) but the breakers are really effective and reliable.

Good luck with your project!.......Tony
 

bculver

New Member
I guess my biggest problem is that I have a 6V motor which under load draws under 2A and at stall under 3A and my driver motor would burn easily before that.
 

ivancho

New Member
I have a couple of ideas for you.

1. Using the already built PWM Motor Driver with LM18200 , you can set your PWM so that the motor only sees 9V. According to the mubashi 6V RC-280RA datasheet, the operational voltage of the motor is between 4.5-9V. Apply 12V to your LM18200 and set the PWM at 70% and the motor will only see 8.4V. One set back is that you are going to need a 12V batery.

2. Use a ST-Micro L6203 with a good heatsink if you go with the 6V. Have into consideration that 6V @ 2A is 12W the MAX is 20W at 90C. So you are good there. I would also use PWM with this IC and increase the voltage to 9V and use PWM at 67% to apply 6V and have enoguh overhead to increase the speed if you wanted to.

R/C cars like the ones found at Walmart and RadioShack have very unefficient motors (not to say cheap) so that current consumption is really bad. Also they don't have much turque. Incerasing the voltage operating your motor will reduce the current consumption, but will be harder on the windings. But if this voltage is in the range of operation stated in the datasheet you are OK.

Both chips have a sense pin to conect a really low resistance to ground. The voltage drop accros the resistor will determine you cut off point for overlaod. Say a 0.5ohms at 2A will set your current limiting at 1V. Using a ADC from the PIC would allow you to see when the current is way past that.

Hope it gives you some ideas

Ivancho
 

bculver

New Member
Those are all great ideas. Out of curiousity, could a zener diode help by pushing the reference voltage up and fooling the 18200.

Just crossed my mind but I don;t know if that is possible but I'd like to hear a comment on it.

Thank you for your help! I am going to try the 18200 approach and also order the other chip just for in case ... :)
 

bculver

New Member
Getting a new motor

After all this, I have to agree ... this motor is definitely going to have to go. I went by Radio Shack today and saw a motor 9V-18V @ 1.98A Max for $5.99 with 18K RPM with no load and 14K RPM with Load ... thats real good and I would work well with the 18200 then. I am probably going to get 2 of the 7.2 R/C batteries and feed everything with those and a 5V Reg for some of the circiuts. 14.4V, 7.2V & 5V should easily run everything.

Thanks for your help!
 
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