Let's Make Robots!

Help with Mr Basic

I have assembled Mr Basic with 4xAA bateries, and tried L293D and L298N but with no luck, seems that it needs more power.

I would love to have some help building an H-Bridge with some kind of transistors, I know a lot of websites with lots of schematics, but I don´t know what are the best transistors to use, any help would be appreciated  :-)

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OK, all I can say is give Mr Basic all the power you have :D 

I tried 7.2V battery pack + L298N and I almost couldn´t stop him       LOL

 

 

 

If you are going to use these motor driver chips then I recommend using at least 6V. I will be adding an aditonal 2xAA myself. Here are some things to check if you are having problems based on the datasheets.

L298N:
The power connections for the motor are completely seperate to the power for the logic.
Vss and Gnd are your logic power (+5). The minimum voltage for Vss is 4.5V.
Vs and the current sense pins are the power for your motors.
If you are not using current sensing then tie these pins to ground otherwise use a small resistor value (<1ohm).
With a 1A load, you will loose as much as 3.3V across the drive transistors and 5V at 2A.

L293D:
Vss is your logic supply voltage and has a minimum value of 4.5V.
Vs is your motor supply and should not go below your logic supply voltage.
At 600mA you will loose as much as 3.6V across the drive transistors.

L298N_heatsink.jpg

After reading the data sheets and Riks results for current draw I would not recommend the L293D. It just isn't rated for the current and you will most likely smoke it. If you insist on using it then put as big a heatsink as possible on it and a cooling fan :p

The L298N will work fine if you give it enough power and a suitable heatsink. I have used it before for Pictomatix which had the exact same drive system and it ran on 7.2V with no problems. There is a schematic withPictomatix that shows how to connect it. I did not use flyback diodes although you are supposed to because I was lazy. Do so at your own risk.

The L298N has an unusual pin layout but don't be put off as the long pins are easily moved to fit a breadboard or Mr. Basics project board.

 

 

 

I will move on with the L298...and maybe I will use a 7.2V little pack... but it is a bit of extra weight I guess..

 

If this is Dagu's Mr Basic ..  I imagine Mr. Avanced   :D

If you want to keep your weight down and use just the three batteries in the battery holder provided then I suggest using the IRF7105 Nch/Pch pair. They can handle a few amps and have a lower saturation voltage than most FETs. They are only 50c each US at Futurlec so you can afford to fry a few. The only down side is they are suface mount so they are fiddley to solder. I suggest a drop of super glue under the chip to hold it down while you solder.I would also put a small heatsink on them. Probably glue it on too.

You can also use a technique called stacking where you glue two or more chips on top of each other and connect their pins in parallel. this reduces their on resistance and stops them getting so hot.

7105_Fet_motor_driver_Schematic.jpg

Farnell ( http://uk.farnell.com ) have them in stock. it works out at £10.06 for 25 off including postage. I just ordered :) also have designed a replacement for a l293D using 4 of them :D

Is this circuit just a simplification of what would be really required? Or is it good to go? I mean: I still am prejudiced against FETs. Partly because they are supposed to be damaged really easily. All kinds of protection and precaution circuitry is supposed to be around these, right?

This just looks too easy to be true.

BTW, the two unnamed connectors on the left are digital outputs from the mcu, right? Binary, or maybe PWM too?

It is a good idea to use some protection circuitry reguardless of FETs or BJT, At least pull up and down resistors, but with the right code that would work. I posted that schematic segment of my micro wallracer design to show how to connect them  to the supply and motor.

Looks an easy implementation.. I never worked with SMD before  :)   

I never thought that this motors would have so much power, I think I will use the 7.2V pack because the car becomes really very powerfull, but I need to make some modifications on the car structure to keep the gravity center as low as possible.  :)

You might only get 20 minutes instead of an hour it would reduce the weight considerably. Although it might be best to wait until the rules of the competition are anounced.

There are rules to be anounced???

I thought the rules were already set. 

1 Use the Mr.Basic kit.

2 make sure your robot is way cooler than the other contestants.

3 good luck.