Need driver circuit for DC gear motor & suggestion for battery

[aswincool]

The specifications of my motor is:

60RPM 12V DC motors with Metal Gearbox
25000 RPM base motor
6mm shaft diameter
Gearbox diameter 37 mm.
Motor Diameter 28.5 mm
Length 63 mm without shaft
Shaft length 15mm
300gm weight
38kgcm torque
No-load current = 800 mA(Max), Load current = upto 9.5 A(Max)
Since the full load current is 9.5A am in need of a driver circuit,
I have a 12V, 7.2 Ah battery would it be enough to supply power to 2 such motors of above rating and a microcontoller ?

 

[cr0sh]

The microcontroller is likely to be a negligible quantity in answering this question. The loaded current that this spec details (9.5A) is likely for "locked rotor" (as well as startup), running current is going to obviously be somewhere between those two values you have.

The Ah rating of a battery is how many amps in one hour the battery can supply - so a 7.2 Ah battery can supply 7.2 amps for an hour, 14.4 amps for 1/2 hour, or if your current draw is only 3.6 amps, the battery can supply it for 2 hours. In reality this isn't really the case - you would need the spec sheet for the battery to know how much current over how long of time you can max draw - for instance, while in theory you could draw 28.8 amps for 0.25 hours, likely the battery would melt down (or its life would be severely shortened).

The amount of current that will be drawn by each motor will depend on the load on it, ultimately - this isn't answerable in any meaningful manner with the information given. For instance, if you were building a robot platform with two wheels (differential drive/steering scheme), things like wheel diameter, weight of the robot platform, etc - would ultimately figure in to the total current draw. The only way you can answer this truthfully without this information, would be to pretend that "worst case" scenario is at play - that is, both motors locked, plus add some amount for the microcontroller and any other items.

In this case 19.0 amps, plus some extra (let's say 25 amps total). That's the "worst case".

So - in the worst case scenario, you are looking at getting a little more than 15 minutes of run time with a 7.2 Ah battery. Without knowing the maximum allowed current draw from the battery though (which is a part of the spec sheet for the battery), its unknown whether it can support such a draw or not (as noted - maybe it can, but with severly reduced lifetime - up to and including catching on fire).

Hope this helps in some manner...

 

[aswincool]

Thanx dude it really helped me understanding many things
I have attached 3 data sheets of 7.2 ah, 12 ah and 17 ah based on its cost, weight and operational characteristics I have chosen 12 ah.

In the second pdf (12 V,12 ah) discharge ch. If i draw 24 A for 10 mins the voltage would become 11.4 and then drop, I hope such a drop in V will cause microcontroller and other circuits to turn off.

My question : U told about max. current drawn from the battery, which is not mentioned in the datasheet. If i draw 24 A for 10 mins repeatedly after charging will the life of my battery get reduced drastically?

 

[cr0sh]

In the second pdf (12 V,12 ah) discharge ch. If i draw 24 A for 10 mins the voltage would become 11.4 and then drop, I hope such a drop in V will cause microcontroller and other circuits to turn off.

I wouldn't rely on that happening; you may want to monitor voltage, then shut off at a particular trip-point - you probably don't want to go too much lower.

My question : U told about max. current drawn from the battery, which is not mentioned in the datasheet. If i draw 24 A for 10 mins repeatedly after charging will the life of my battery get reduced drastically?

Hmm - I've seen datasheets that have that information before; this one doesn't. You may want to contact the manufacturer; that's a pretty steep and fast rate of discharge. I don't know whether it would shorten the life or not, and if it did, by how much. Its seems extreme to me, but the battery may be designed for it...

 

[ronv]

I think the dashed line on the bottom of the discharge curve reprsents 80% discharged. As long as you stay above this you should be ok.

 

[cr0sh]

I think the dashed line on the bottom of the discharge curve reprsents 80% discharged. As long as you stay above this you should be ok.

I'm not sure what it represents - the datasheet doesn't say, which is why I say contact the manufacturer to verify. If it is 80% discharged, its got a steep drop-off once 11.4 volts is arrived at; I'd say don't go below the 10.8 volt mark...

 

[aswincool]

Skid Steer Relay Drive (10 Amp) [RKI-1006] - Rs.425 : Robokits India, Easy to Use, Versatile Robotics Kits...

can i use this for driving 2 motors of 10A capacity?

Is there any way to trip the circuit after its voltage reduces to 10.8?

 

[cr0sh]

Skid Steer Relay Drive (10 Amp) [RKI-1006] - Rs.425 : Robokits India, Easy to Use, Versatile Robotics Kits...

can i use this for driving 2 motors of 10A capacity?

Well, according to that link it says it can drive such motors of 10A capacity, and your motors are rated at a max of 9.5A - in theory it should work, but it's cutting it a little close. I'd personally look for something closer to around 12A capacity, just to have a little extra room (and to prevent possible heating/welding of relay contacts). Note that it doesn't have any way to control the speed of the motors, so if that's needed, that board won't work...

Is there any way to trip the circuit after its voltage reduces to 10.8?

Well, you could set up a window comparator circuit of some sort, perhaps - but you say you have a microcontroller, right? It should (might?) have on it an ADC or two with analog input pins; since most microcontrollers run on less than 12 volts (usually 5 volts or lower), you would have to reduce your voltage to measure it first, or, if the microcontroller has the feature, it may have a voltage reference pin (VREF) that you can tie to in order to measure higher voltages using the ADC. You would have to look at your datasheet and/or other documentation on your controller. Once you can measure the voltage with the microcontroller, you can then have the microcontroller switch things off when the reading gets too low. Note though, that some microcontroller can't technically shut themselves off - but you can shut off external circuits. If you need a full shutdown, you could have the microcontroller trigger a shutdown timer system (using a 555 wired as a one-shot, perhaps?), which could then turn everything off.