Control of Cordless drill 12V motor

[omkar]

Hi I am building a tracked bot which will use two cordless drill motors (12V to14V). I have managed to hack the torque drive so that it outputs a constant torque. I have two PIC18F4550 microcontrollers which I am planning to use as USB interfaces.
My questions are:
1. How to find out the maximum current drawn by these motors at max load?
2. How to protect the microcontroller from the high currents?
3. I have this attached basic circuit that I am planning to use for the USB interface. The circuit shows a stepper motor which I will replace with the DC servo.

Cheers
Omkar

[waltr]

1. How to find out the maximum current drawn by these motors at max load?

Insert a small value, high wattage resistor between the battery and the motor. Measure the voltage drop across the resistor when the motor is unloaded and with the motor stalled. Ohm's law will give you the current.
Or just measure the DC resistance of the motor windings. The max battery voltage and Ohm's law will give you the current.

2. How to protect the microcontroller from the high currents?

If you use a properly designed motor drive circuit the microcontroller will never see the high currents from the motor. What you will need to consider is the motor brush noise and transients from switching the motor driver. There are a number of threads about these issues. Do a search here on the SoR forums.

[Soeren]

Hi,

Hi I am building a tracked bot which will use two cordless drill motors (12V to14V). I have managed to hack the torque drive so that it outputs a constant torque. I have two PIC18F4550 microcontrollers which I am planning to use as USB interfaces.
My questions are:
1. How to find out the maximum current drawn by these motors at max load?

Do you have a DMM for starters. If not, you need to get one (they can be found for like $5 and up).

2. How to protect the microcontroller from the high currents?

The controller should not handle the motor directly and the driver that goes in-between should be able to handle the load - a protection circuit could be made for protecting the motor against too high currents under locked rotor (stall).

3. I have this attached basic circuit that I am planning to use for the USB interface. The circuit shows a stepper motor which I will replace with the DC servo.

The ULN2803 won't cut it.
First measure the locked rotor current, to know what's needed of the driver (probably what you call "servo"?), then we take it from there.

[omkar]

Hi Soeren... I have measured the stall current for the motor, maximum reading i got is 22 Amps. So my questions (2 and 3) can be combined as: whats the opto coupler that can handle this current? or is there any other way to protect the microcontroller?
Cheers
Omkar

[Soeren]

Hi,

Hi Soeren... I have measured the stall current for the motor, maximum reading i got is 22 Amps. So my questions (2 and 3) can be combined as: whats the opto coupler that can handle this current? or is there any other way to protect the microcontroller?

Optocouplers (alone) won't cut it, as they can only handle way under 1A.
You don't need an opto coupler though, an NPN transistor can provide ample protection.

For this large a current, you'll need to use either a relay or an H-bridge with a suitable current handling, assuming you want to be able to reverse the direction of rotation - if not, a single power Darlington may be the best way to proceed.

To protect the motor(s) from the stall current, the best way is to make a small current sense circuit for cutting the power back before it melts the motor windings.

[omkar]

Yup right you are .... my own research led me to the attached H-bridge circuit. I am planning to replace the MOSFETs with something that can handle the 22 Amps - I am planning to use IRF9540N(P-channel) and IRF1405(N Channel). So this H-bridge will be driven using a microcontroller PIC18F4550. For two motors I will have two sets of similar H-bridges.
One more thing: The IXDN404s will be parallelled as shown in the figure as they cannot handle more than 12 Amps each.

Though for current sensing ... I may need something else. Or May not need current sensing. I wonder if there is a seperate current sensing circuit which I can use to input into the PIC?

[TrickyNekro]

First of all... the IXDN404 are mosfet drivers... Why to parallel them... I assume that they are not designed to drive inductive loads, as in this case the coils of your motors. But I didn't show weather they had any limitation or not about that... Though they are supposed to drive IGBTs and mosfets, meaning that they are better suited for driving capacitive loads... What's the difference... current - voltage phase...

Anyways... It would be better not to parallel them like that... Why?

Mosfet are temp. sensitive devices... The hotter the more current through them... Although this problem is more pronounced with BJT and diodes... You should add an ohmic load for not exceeding the max current rating of the device... Even better it would be, that instead of an ohmic load you add an impedance given that you are switching at a given frequency, an ohm load does anyways...

I'm telling you all these in case you want to parallel transistors, diodes or mosfets.

Best Regards, Lefteris
Greece

[omkar]

i guess you are right. The MOSFET drivers are not going to see a lot of current here as they are only driving the gates of the H-bridge MOSFETs (or am I mistaken), the only reason i thought of paralleling them is to enable large currents. Though I can use lower power MOSFETs (IRLU024Ns) in parallel combo as attached. Thanks for the insight though.
Cheers

[TrickyNekro]

Yup... nothing man... BTW the resistors I've been talking about usually are placed between drain and the device you want to connect to the Mosfet...

In your schematics both R1A and R2B are useless... Mosfets exhibit some hundreds to thousands of megaohms resistance at the gate... 1.5Ohm to infinity is useless....

You can use of course higher current Mosfet instead of... Pay attention to the internal resistance to calculate for heat and needed heat dissipation...

Take a look at these... might help...
N - Channel
http://gr.mouser.com/Search/ProductDetail.aspx?R=FDD3706virtualkey51210000virtualkey512-FDD3706
P - Channel
http://gr.mouser.com/Search/ProductDetail.aspx?R=FDD6637virtualkey51210000virtualkey512-FDD6637

And one final word... If buying mosfets make sure you buy enhanced Mosfets and not depleted....
It's rare to find depletion Mosfets but it's good to check for sure...

Best Regards, Lefteris
Greece

[Soeren]

Hi,

Anyways... It would be better not to parallel them like that... Why?

Mosfet are temp. sensitive devices... The hotter the more current through them...

No, thermal runaway can be a problem in BjT's, but with MOSFET's it's the exact opposite - the higher the temp the less current.

Apart from that... If thermal runaway was a problem, it would lessen with paralleling.

[Soeren]

Hi,

For selecting power-MOSFET's, just go to eg. Fairchild and use their parametric search. All needed info is there.

[TrickyNekro]

Yup, Soeren is right about the temp coefficient... I did mistaken it with the gate voltage threshold... Apart from that...
Anything else ok...

[omkar]

Finally i am some way ahead in this :

Bot turning control and speed control done

Tracked Bot first obstacle trials

Bot climbing 60 deg slope