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### Author Topic: how to protect my motor driver from stall current  (Read 4364 times)

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#### Alia

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##### how to protect my motor driver from stall current
« on: February 25, 2011, 02:46:03 PM »
Hi

I am using baby orangutan 328p to drive a DC motor (70 mA) the problem here that the stall current for this DC motor is 1.6 A and the maximum current this driver can supply is 1 anything more than that will blow up the driver,  and I already did it  .

I want to protect now my second baby orangutan from the stall current. The solutions I had either to use a current limiter (which I did not found any one below 1 A, I want it to be less than 1 A to protect the baby orangutan not to be on the limit ), or to use PTC. I have not use PTC at all but during research I found one which has a maximum current of 250 mA. What I understood from reading about PTC that it will cut the circut if the current is more that the maximum current, but when the DC motor start to work it will draw a current = 1.6 A (short period) so is not that will be a problem at least for the start ?? Any one used it before ? or used the current limiter IC.

Any one has another idea how to protect my motor driver ?

thanx

#### TrickyNekro

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##### Re: how to protect my motor driver from stall current
« Reply #1 on: February 25, 2011, 06:23:49 PM »
I dropped an eye on the datasheets... well...

First of all... the output current is 800mA not 1A...

Secondly... Unfortunately... yes you have to limit current...

The H-bridge is transistor based... And transistors are supposed to work with certain currents...
Higher currents produce large amounts of heat, which can't be dissipated quick enough and the junctions
can go puff in milliseconds...

You must unfortunately place a resistor in series with the motor to limit the current.
At which voltage does the 1.6A stall current occur? Then I should be able to help you out with the values...

BTW... did you buy the same controller or the new one? In the new one, they placed
a MOSFET H-bridge (yes, at least they know that mistakes must be corrected)....
The new one... should operate without problem even close to stall currents...
For whom the interrupts toll...

P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

#### rbtying

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##### Re: how to protect my motor driver from stall current
« Reply #2 on: February 25, 2011, 08:26:26 PM »
The smartest thing to do, of course, is not to stall the motor (it's almost universally bad).  If your motor tends to stall, change the gear ratio to provide more torque and less speed - this is a better solution in most cases (unless the motor way out of spec).

The H-bridge on the B328P, if I remember correctly, can handle short bursts of high-current (up to 3A or something like that): you could try software detection of the stall (via encoders or similar) and cut power to the motors then.

#### Alia

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• Posts: 12
##### Re: how to protect my motor driver from stall current
« Reply #3 on: February 26, 2011, 12:25:34 AM »
thank U guys

TrickyNekro

I bought this one http://www.pololu.com/catalog/product/1220 and I rechecked the output current they wrote (two bidirectional motor ports can deliver ~1 A continuous (3 A peak) per channel ). It has 1.6 A tall @ 6 V but could U told me how I have to calculated it in case I change this value to lower or higher one. Is the resistor is enough ?? Do I need to connected near to the controller of the motor ? and does it have any effect on the speed or any other thing ?

rbtying
In normal situation it does not stall unless it stuck in a place. unfortunately, I can't use any encoder.

Did any one used a IC current limiter or PTC ??

#### waltr

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##### Re: how to protect my motor driver from stall current
« Reply #4 on: February 26, 2011, 04:41:44 PM »
If I remember the L298 data sheet there is a schematic diagram showing a current sense and current limiting circuit.
Basicly a small value resistor connector the ground side to the H-bridge output to ground. The motor current passes through this resistor and produces a voltage drop across the sense resistor. A comparator then is use the turn off the enable of the H-bridge chip it he voltage across the sense resistor exceeds a pre-set value.

#### Alia

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##### Re: how to protect my motor driver from stall current
« Reply #5 on: February 27, 2011, 11:14:53 AM »
hi waltr
I don't have an access to the enable of motor driver in baby orangutan

#### waltr

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##### Re: how to protect my motor driver from stall current
« Reply #6 on: February 27, 2011, 01:39:00 PM »
Oh boy, I goofed that one as I thought you were using the L298 Motor driver.

The baby orangutan uses the Toshiba TB6612FNG Motor Driver which does not have internal over current protection (see the data sheet). The external circuit I mentioned in my post above could be used with the TB6612FNG if you were just using that driver chip without the ATmega already built into the PCB.
The best thing you can do is select a motor that can not exceed the TB6612FNG's maximum current output or select a motor driver that can handle the stall current of your motors.

The TB6612FNG Motor Driver is perfect for Pololu's micro-meter gear motors but not for their larger gear motors. My one Bot does use the micro-metal motors and a TB6612FNG driver connected to a PIC. This combination works very well.
« Last Edit: February 27, 2011, 01:43:50 PM by waltr »

#### Soeren

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##### Re: how to protect my motor driver from stall current
« Reply #7 on: February 27, 2011, 05:17:59 PM »
Hi,

Any one has another idea how to protect my motor driver ?

Since you only have access to the bidirectional motor wires (if you don't wanna hack into the PCB), the easiest solution I can think of is this:

You'll have to experiment a bit with how many windings are needed to trigger the Reed at 1A, as there is a plethora of Reeds and they come in many different sizes. The larger the physical reeds (the arms in the glass tube that gives it its name), the stronger a magnetic field is required. In your case, even the tiniest reed will do.
Wind it on a tight fitting plastic tube and use a wire gauge suitable for the motors.
More windings will make it trigger at a lower current and vv.

Here's what a Reed Switch looks like:

When the current through the motor and hence the coil around the Reed reaches a certain magnitude, the magnetic field inside the coil will trigger the switch and the terminal marked "I/O" will go low.

Then you have to make a routine to alter the PWM to say 30% max. whenever the I/O pin is low.
That way it will still be able to handle the start surge in a reasonable way, which it has to, or it will be very slow starting.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Alia

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##### Re: how to protect my motor driver from stall current
« Reply #8 on: February 28, 2011, 02:10:27 PM »
hi walter

Quote
The TB6612FNG Motor Driver is perfect for Pololu's micro-meter gear motors

I already use both of them but unfortunately I bought 210:1 micro-meter gear motors which has 1.6 A as stall current and I am looking for a solution without change any of them (I have a limited budget). As I told U before I already blow up a baby orangutan  . What is the value of stall current your motor have?

Soeren
Thank U for the idea. I did not know about Reed switch before. I need to read more about it.

#### waltr

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##### Re: how to protect my motor driver from stall current
« Reply #9 on: February 28, 2011, 06:18:17 PM »
360mA. This is the motor:
http://www.pololu.com/catalog/product/1098

Two of these drive a 12Oz Bot.
« Last Edit: February 28, 2011, 09:28:06 PM by waltr »

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