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Dual bridge protection

Posted on 2016-11-10
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Last Modified: 2016-11-23
This is a question about electronics.

I have next bridge board L298N:
L298NFurther info about L298N board is here.  

And next DC motor:
DC motorWith next parameters:
DC motor specsFull datasheet is here and its encoder is here.

The LN298N has 2A max. current for one channel.
The DC motor has 550mA rated current, but 2.8A stall current.
I expect during certain high torque moments the current to go up to 2.8A.

Is the LN298N bridge board protected against the over 2A current?
How is the LN298N bridge board protected against the over 2A current?

The application note of the L298 circuit shows next protection against short-circuits:
Short circuit protection
But looking on the L298N board I cannot see that protection. There are only diodes to protect against the back emf from DC motor or step motor coils.

What do you propose to protect/limit the current to 2A for half of the bridge?
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Question by:viki2000
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by:Dave Baldwin
Dave Baldwin earned 100 total points
ID: 41882148
Here http://www.st.com/en/motor-drivers/l298.html is the datasheet for the chip along with application notes.  The datasheet shows connections for current sensing resistors though most example schematics do not use them.
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by:d-glitch
d-glitch earned 300 total points
ID: 41882163
What is input voltage to the driver board?  What is the applied voltage to get 2.8A of stall current?

The L298 has thermal protection, so you may not have to worry about destroying the device, but the data sheet does not say anything about how it works.

The simplest fix might be to add a current limiting resistor in series with the motor.
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by:viki2000
ID: 41882205
@Dave
I already had that link in "Further info about L298N board is here. " and then you open the Google drive document with all the links. Of course I have read the Application Note of the L298, but there is only suggestion about short-circuit as I posted in the picture above. My problem is to limit the current to 2A, so I do not destroy the bridge due to some high torque moments of the DC motor, because during normal operation is only 0.55A.
So my question is what do you suggest as current limiter for 2A.

@d-glitch
The motor is rated 6Vdc max, but I will provide probably 5Vdc or even 5.5Vdc. Let's say the voltage for the motor would be between 5-6Vdc.
Then the motor will be controlled with PWM to vary its speed. The bridge allows that. Then its average equivalent DC voltage would be lower than max. that I apply.
I do not want to rely on thermal protection. The thermal protection shuts off the circuit, then it needs time to cool down, then it starts automatically again. It is unreliable, out of control. I prefer to limit the current to 2A, of course will be less max. torque, but still enough. In that way the motor may run continuously even if the torque force it around 2A for short periods of time.
The resistor in series is too simple and not efficient, because we have variation of 0.55A to 2A , then is a high power resistor that dissipate heat, then we have voltage drop which is worse disadvantage because it influences the speed. If I have my power supply 6Vdc or 5Vdc then the speed is almost half of it. Then it would imply a higher voltage power supply and the voltage drop will give headaches anyway reflect in RPM of the motor, no matter how good PWM works.
So the 2A voltage limiter must be without voltage drop, or very low.
I have some ideas too, but I do not want to influence the suggestions.
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by:d-glitch
d-glitch earned 300 total points
ID: 41882236
2.8A at 6V indicates a winding resistance of 2.1 ohms, so a 1 ohm resistor will limit the stall current to 1.93A.

When the motor is running at rated speed, this resistor will dissipate  I²*R =(0.55)²*1 = 0.3 watts.
The voltage drop across the resistor will be  550mV.

A 1ohm, 1 watt max resistor will give you adequate protection with a very minor performance penalty.
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by:viki2000
ID: 41882309
When the motor is at stall moment, the rotor does not move, then indeed its coils behave mostly as resistors and dissipate heat, the motor gets hot and can be destroyed. In that situation we get approx. 6Vdc/2.8A=2.14A as you said, by neglecting the inductance of the coils, because we are in DC.
I expect the motor to work with some high torque moments and I need the speed to be constant.
Then let’s assume we limit the current at 2A using the 1ohm resistor.
At 550mA we have 0.55V drop voltage on the resistor.
At 2A we have 2V drop voltage.
If I use a 6.5Vdc supply, then the motor voltage will vary between 6V and 4.5V. I think that is a problem for the speed even if we have PWM=100%.
But when I reduce the PWM for lower speeds, then the current in circuit changes too and the drop  voltage on resistors again changes.
That have influence over the speed and torque of the motor.
So how do I compensate the drop voltage on the resistor?

The resistor starts to dictate current in motor and that is bad for torque and speed. It should be active only at 2A, in the rest of the time to be somehow bypassed.
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by:d-glitch
d-glitch earned 300 total points
ID: 41882533
How are you planning to control the motor?  The L298 is really just switches.  You have mentioned PWM, but how do plan to set it?

If you want to limit current, you have to monitor it.  Typically you use a sense resistor with a comparator and pot to set the limit value.  Some boards have these components.
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by:d-glitch
d-glitch earned 300 total points
ID: 41882649
You have a position encoder.  I don't know what the 14 pole specification means exactly.  Maybe 7 or 14 pulses per revolution.  
If you are using an Arduino, maybe you can be clever in software.

If you limit duty factor of the PWM waveform to 66%, you will never drive more than 2A RMS in the motor at 6V even when it is stalled.
If you monitor the encoder, maybe you can drop the PWM limit when you are sure that the motor is moving.

In practice, you might have a 50 ms watchdog timer that controls the current limit.  Every time you see the encoder advance, you can reset the timer.
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by:viki2000
ID: 41882792
Yes I was thinking at an microcontroller, PIC or Arduino, to generate PWM.
The encoder pulses I will use to monitor the speed.
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by:Malmensa
Malmensa earned 100 total points
ID: 41883160
I notice a 7805 regulator on that board, but no low voltage electronics. Assumedly, this regulator is used to stabilise the motor voltage. Assuming that is correct, then it will limit output current to 500ma. This is a bit low for this motor, it would work, but under load voltage would drop and torque reduce.
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by:viki2000
ID: 41883370
The 7805 is described in the links provided and the schematic diagrams. It is used to generate an output for digital electronics, as power supply for a microcontroller. It has no influence over the bridge or motor.
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by:viki2000
ID: 41883443
Searching the internet I came across next 2A protection ideas:
-      MAX14575C : https://datasheets.maximintegrated.com/en/ds/MAX14575A-MAX14575C.pdf
-      TPS2554: http://www.ti.com/lit/ds/symlink/tps2554.pdf
-      TPS2003C: http://www.ti.com/lit/ds/symlink/tps2002c.pdf
-      LTC1477: https://www.sparkfun.com/datasheets/Components/LTC1477.pdf
-      Microchip (Micrel) MIC2019YM6: http://www.mouser.com/ds/2/268/mic20xx-778478.pdf
-      MOSFET transistor current limit circuit 2A – drop voltage on resistor is variable: http://320volt.com/en/irf4905-ile-5-amper-akim-regulatoru/
-      Bipolar current transistor current limit circuit2A – drop voltage on resistor is variable: http://freecircuitdiagram.com/2008/08/27/variable-adjustable-current-limiter-circuit/
-      Other suggestions:
https://www.precisionmicrodrives.com/tech-blog/2014/03/03/5-methods-circuit-protection-gearmotors-part-1
https://www.precisionmicrodrives.com/tech-blog/2014/03/20/5-methods-circuit-protection-gearmotors-part-2
-      3-terminal voltage regulator (3A) as programmed current limiter for 2A – drop voltage on resistor is variable:
http://diyaudioprojects.com/Technical/Current-Regulator/
http://www.farnell.com/datasheets/76145.pdf
http://www.ti.com/lit/ds/symlink/lm350a.pdf
http://akizukidenshi.com/download/ds/fairchild/LM350.pdf
http://www.st.com/content/ccc/resource/technical/document/datasheet/d3/84/d5/f6/3c/23/40/7b/CD00001883.pdf/files/CD00001883.pdf/jcr:content/translations/en.CD00001883.pdf
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by:d-glitch
d-glitch earned 300 total points
ID: 41883764
Good catch Malmensa.

The max current rating of the 78M05 is 500mA, but it doesn't limit the current, it just fails first.

There are beefier boards using the L298 (look for heat big sinks), and more modern boards with MOSFET drivers.
     http://www.rugged-circuits.com/the-motor-driver-myth/

Since you probably need a new board, you should look for one with current limiting built in.
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by:
d-glitch earned 300 total points
ID: 41884279
Additional thoughts on the L298 board.

The 5V regulator on the board is fine.  It is only intended to run the L298 and maybe some logic or a micro controller.  

The board schematic also a shows a 12V input to the L298 that is actually meant to drive the motor.  

But how can you drive a 5V motor with 12V?  No problem, that is what PWM is for.  You still have to worry about power dissipation and heat sinks.  But if you characterize the motor properly, you may be able to run it safely with just the encoder and microcontroller.
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by:viki2000
ID: 41884506
Until yesterday I only checked the specs of the motor online and the motor was only ordered, but yesterday arrived and I could make some real tests with it in hands.
I have ordered also 2x L298N, but they will arrive next week.
After all, it seems that maybe I do not need any protection at all or maybe only for a special case.
The motor has a small gear box in front.
During the normal run takes under 0.2A.
When I force the output of the gear box with my hand trying to block it then the current increases to 0.7A up to 1.0A.
I realized that for high torque moments that I expect, the current will not go over 1A or 1.2A, and for sure never over 1.5A. This is the case for this particular motor.
If I block the shaft of my gear, due to flexible coupling between my gear and the gear box of the motor, it take 1-2s until the motor goes in that stall state and the rotor does not turn anymore.
Only in that moment the current goes over 2A. But that is very unusual state and I foresee that will never happen or seldom happen and then I can rely on the thermal protection of the L298 as the last type of protection, but with uC I can also do few thing.
I was worried about the high torque moments, but seems that the current will never go over 1-1.2A.
The test and measurement regarding that high current during high torque moments were done at 5Vdc and at 6Vdc. For lower dc voltages, the current is lower.
The rotor turns also at 1Vdc, slowly, without too much output power. Starting with 3Vdc it has already enough power to be used in my application.
In this case, I decided to simplify the circuit and to not add any hardware protection.
Because I must use a microcontroller for different other functions and also for PWM, I think reading the motor encoder is enough for 2 decisions: 1) in case there are no more pulses from encoder then I provide 0% PWM and I stop the motor voltage and 2) in case the speed changes due to high torque moments then I must increase/change the PWM duty to get the same number of pulses from encoder as during the normal operation having the reference a given/programmed value.
I also noticed that at 6Vdc the motor has higher speed and torque compared with 5Vdc – as expected, significant different – as not expected.
I have some regulated power supplies 5Vdc, but not 6Vdc, so that is my next problem.
It seems that I must use 9Vdc or 12Vdc power supply and then a programmable 3-terminal voltage regulator 3A or higher in order to get the 6Vdc.
The next problem is that I want to use that bridge for another small DC motor that I already ordered, because is a dual bridge, for which I have no specs up to now.
It is next one:
http://www.ebay.de/itm/Mini-Elektro-Motor-mit-Getriebe-N20-12V-300-U-min-extra-langes-M3-Gewinde-/152286463677?hash=item2374fb06bd:g:i8sAAOSwd4tULGVC
One L298N can have 4A max. per total, both bridges and 2A max. for one motor.
Even if I will get 2x L298N, makes no sense to use both, beacsue the both motors have under 4A. I assume the 2nd motor will have similar current, but I will test it next week when I will get it. The 2nd motor will not have high torque moments. It will be just start-operation-stop, eventually reverse after stop.
That 2nd motor is also 6Vdc.
So, the 2nd possible problem comes from the current of the power supply attached to the L298N bridge.
If I use a 3-terminal 3A voltage regulator to get the 6Vdc, then maybe is not enough for both motors, but I do not know now.
Then maybe I need to use something like:
http://cds.linear.com/docs/en/datasheet/8027fc.pdf
http://www.ti.com/lit/ds/symlink/lm138.pdf
Then the power supply must be 9Vdc or 12Vdc and at least 4A-5A.

Do you have any other idea how to get in a simple way the 6Vdc (which is not very common) and 4A for the L298N?
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by:viki2000
ID: 41885318
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by:viki2000
ID: 41885976
I measured the encoder signal without any load attached to the motor gear box.
Encoder signal
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by:viki2000
ID: 41888105
The 2nd DC motor arrived today. It is so small, I did not expect, but has a gear box and can do the job that I need.
I measured its current. It takes 28mA when runs normal without any load attached to its gear box output. When is in stall mode, then goes to max. 350mA.
As conclusion L298N does not need any protection.
I have bought also the EasyDriver v4.4 board which may be also suitable for such small motor.
The step down dc-dc converter works fine too and has a shut off protection at 4.5A.
I could get 6Vdc using dc-dc converter from 12Vdc power supply to drive the motors.
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Author Closing Comment

by:viki2000
ID: 41900142
I have no more time to make any tests, but the situation is clear now.
Perhaps in December or January I can play a bit more with it.
Thank you for your thoughts and suggestions.
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