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L293D not working even if its wired correct

hi...

 

I have an L293D driver... I connect logic VSS from regulated 5V and the atmega8 too... the VS is connected through another 5V regulated voltage... it drives a little motor... but it doesn-t drive my tamiya box... everything is set up correctly and tested... why is this? when I measure the voltage on the motor pins it goes 4.95V and -4.95 every 5 seconds as it should go... but after I connect the motors and remove them, I measure again, I got 0.02 and -0.02... whats the problem?

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It sounds like you have a regulated 5V for the logic of the L293D and your atmega8. Then you run this through another 5V regulator to supply the motors through the L293D. Is this correct? There's probably not enough voltage differential across your 2nd regulator to supply the current you need. Perhaps is works under no load, but as soon as you connect the motors...

Remember that the L293D drops some voltage as well.

1A is the regulator...

 

motor eats 470mAh at 3V

Motor winding resistance? you have to overcome the stall current to get the motor turning.  Someone correct me if I'm wrong. I bet the stall current is about 2 A on your motor.

This little L293D peep gives 750mAh... :| so this can't even drive a god damn tamiya box? why are they making this anyway? I need to get some L298N

I recommend Solarbotics RM3 as Tamiya gearbox replacement motors ($1.99) which are nominal 6V:

http://www.pololu.com/catalog/product/604

I run these at 7V with good results.

I have a pair of GM2... but as I told... I can-t move even them... I use 1A regulator...

 

 

should I change it to 7806 for 6V and replace the tamiya motors with GM2 motors?

You can see how they run with 5V, or depending on your voltage input unregulated.  I use rechargeable AA's which have a lower voltage, so six of those can drive that motor well without regulation.

I tried with even two 9V batteries in series... which is... 18V by thw way... still nothing... just peeping motors... maybe the atmega should have some pins fried? I'll try tomorrow with attiny2313 or when I'll have the time

9V batteries are weak current sources, could be part of the problem.  There should be a datasheet for the 9V battery your using.

I should ask what the stall current or winding resistance is of the motor and the max current output of the 5v regulator to the motor is.  The L293D can supply a maximum constant 1A of current.  An L298N should do better if it is an issue with the power requirements of the motor.  This little IC probably isn't very sufficient for the stock FA-130 or whatever the motor is that comes with the Tamiya gearbox.  For me the IC motor driver would overheat in a matter of about 10 seconds, but I was using a SN754410.  All that being said the current bottleneck may be with the regulator, in which case try a big cap, a bigger regulator, or unregulated power.

Cheers