L298 Back EMF

[pittuck]

Hi i have a problem and wanted to check somming.

I am using a L298 to drive 2 DC motors, but i missread/understood the datasheet and did not realise the diodes were needed.

Now my question, would it be possible to 'blow' the L298 without the diodes in?

Half the bridge is not working (well it is able to run a LED but not a motor) and the second half runs the motor very slowly. When i apply the 7.2V to the motor terminsals they work, so its not a motor problem.

The L298 has worked b4 in this setup so its not wiring... Oh and it blew my 18F452, the CCP2 module is broken and logic high is measured as 2V (dont worry i got a new one comming).

any ideas? I am buying a new chip but i want to make sure i have solved the problem.

Regards,

- Martyn

 

[jrz126]

sounds like it is the chip, if you havent changed anything else and it just quit working, and you double checked the motor.

On a side note, I used one of these IC's once and I shorted something out, the chip exploded and the IC and my breadboard caught on fire :shock:

 

[pittuck]

ok well i will put the diodes in and use a cap on the driver ic next time!!!

me = dumb sometimes, but i did not get electricuted, so it was a pretty good day on thw whole.

 

[jrz126]

When I put in my electronics orders, I'll usually buy enough for 2 circuits in case of situations like this. Because I know I'll screw up and be up a tree without a paddle.

 

[fabbie]

just wan to confirm something. Are L298 drivers capable of accepting PWM signals?
Never seem to mention anything in the datasheet

 

[pittuck]

**broken link removed**

**broken link removed**
You use in1 & in2 for direction and enablea for pwm for motor 1
You use in3 & in4 for direction and enableb for pwm for motor 2

Looks like it

I had these links on my scrath pad as i had the same thought yesterday. I noticed that DPRG uses a 1000mF cap on the Vs (motor supply) and GND lines, i will follow. I will also get a 0.1Ohm resistor for the current sense (will have to be wirewound as i cannot find normal 0.1Ohm resistors that are not SMD)

 

[Nigel Goodwin]

I had these links on my scrath pad as i had the same thought yesterday. I noticed that DPRG uses a 1000mF cap on the Vs (motor supply) and GND lines, i will follow. I will also get a 0.1Ohm resistor for the current sense (will have to be wirewound as i cannot find normal 0.1Ohm resistors that are not SMD)

You don't have your location filled in!, but in the UK 0.1 ohm metal film resistors are available from RS Components - I keep them in stock at work!.

 

[pittuck]

well i know RS and farnell do em. But they dont accept solo cards. Doh! I will stick with rapid electronics as they accept solo cards.

 

[fabbie]

I thought of building my own H bridge cct from scratch for the experience but looks like the best way to solve this is to use the IC drivers. sigh. For this L298 driver, the maximum current rating is at 2 Amp per channel. Motor stalling can be rather dangerous for my 12V motor. Any suggestion for protective ccts? or should i just place a buffer interface to act as my fuse.
Also, dont really understand the current sensing pins. When it said limits the current, does this mean that it wont go above the Vs/R or Vs/R constant current? THX ALOT.

 

[Nigel Goodwin]

I thought of building my own H bridge cct from scratch for the experience but looks like the best way to solve this is to use the IC drivers. sigh. For this L298 driver, the maximum current rating is at 2 Amp per channel. Motor stalling can be rather dangerous for my 12V motor. Any suggestion for protective ccts? or should i just place a buffer interface to act as my fuse.
Also, dont really understand the current sensing pins. When it said limits the current, does this mean that it wont go above the Vs/R or Vs/R constant current? THX ALOT.

Presumably the datasheet will explain how it works?, but (as far as I know) the current sensing resistor monitors the current, the chip will current limit depending on the voltage dropped across it.

 

[fabbie]

According to the datasheet, the maximum quiescent supply current,Is to the motor is in mA. Do we need a resistor at the Vs pin then?
I dont find it logical on how does a few mA able to output a maximum current of 2A.

 

[pittuck]

Ok well quiescent current is when its not driving anything. So if u put enable low it will only use 4mA, assuming a resistor of 0.1Ohm is used:

P = iir = 16r = 1.6mW

But at the peak current of 2A

P = iir = 4r = 0.4W

So thats where the numbers come from...

 

[fabbie]

Um...the resistor of 0.1ohm where are u referring to? at the Vs? i dont think it is required or at the current sensing?
By the way, pittuck, pls dont mind teaching me how to use the current sensing. How do u know the value of resistance to place. and what current isit tryin to limit?

 

[pittuck]

its for the current sense pin.

this pin enables u to read the current which is being supplied to the motors. which can be handy...

 

[fabbie]

According to the website you provided,
**broken link removed**
It said here that the current sensing here will control the current according to VS/R and will be provided to the load.

If the Vs is 12V. wouldnt the current be EXTREMELY big? 120Amp.
Mind teaching me how u did u attain the value of 0.1ohm?

 

[fabbie]

I read through the datasheet and went through other tutorials.
Yeah the current sensing is to control the current alright.
but how did u guys get the value ranging from 0.1 ohm to 0.9ohm?? is there somekind of calculation??

 

[Nigel Goodwin]

According to the website you provided,
**broken link removed**
It said here that the current sensing here will control the current according to VS/R and will be provided to the load.

If the Vs is 12V. wouldnt the current be EXTREMELY big? 120Amp.
Mind teaching me how u did u attain the value of 0.1ohm?

VS isn't the supply voltage, it's the sensing voltage - the voltage dropped across the sensing resistor.

I've just had a look at the datasheet for the 298, there's no internal current limiting at all - it just provides a connection pin from the bottom of the H-Bridge to ground. By inserting your own resistor in there you can monitor the motor current, and do whatever you want with it.

By using a 0.1 ohm you would probably have to amplify the voltage to make it to a useable level.

 

[jontromanab]

hiii guys..........

i am a new member...
i am trying to create a mobile robot project.for controlling the dc motors i am using the l298.it will driven from the parallel port of my pc.
this are the corresponding voltages that are going into the l298 pins:---

1. 2.8mv
2.
3.
4. 12.58v
5. -0.7v
6. 5.13v
7. 2.4v
8. 3.1mv
9. 5.13v
10. -0.7v
11. 5.13v
12. 2.5v
13.
14.
15. 3.0v

as the 2,3,13,14 pins are the output pins.......i should be getting 12v as corresponding output for driving the motor.............IS IT OKK??? ......................................but i am getting nothing.

(as the 5,7,10,12 are the input pins i have given an instruction 80 where 2.4 v is the high & -0.7 is the low)

please someone help me getting the outputs as 12v for driving the motor................its really urgent

 

[audioguru]

If the Vs is 12V. wouldnt the current be EXTREMELY big? 120Amp.
Mind teaching me how u did u attain the value of 0.1ohm?

The current would try to be as high as 120A if the IC can pass it and if the motor is a dead short.
The max continuous current for the IC is 2A so a current sense resistor of 0.1 ohms provides a voltage of 0.2V to be detected by another circuit and used to pulse the IC off so that the average current is not higher than 2A.

When will you tell us the max stalled current for the motor? Look on its datasheet or simply measure its resistnce and use Ohm's law.

 

[audioguru]

this are the corresponding voltages that are going into the l298 pins:---
.
.
.

15. 3.0v

Pin 15 needs a low resistor value to ground, or ground pin 15. The motor current goes through this resistor and its voltage drop must be no higher than 2.0V.

as the 2,3,13,14 pins are the output pins.......i should be getting 12v as corresponding output for driving the motor.............IS IT OKK??? ......................................but i am getting nothing.

You will never get 12V from the L298. Its max output high voltage is 2.7V and its max output low voltage is 2.3V. So the max loss is 5.0V plus the voltage dropped across the current-sense resistor.