L293B problem with output voltage

[jhonilson]

    Hi. i am using L293B H bridge.
   I am trying one 6v motor (from radio control car), when i connet the suply voltage (5v)  directly to the motor, it work very fine but when i use the l293B the power in the motor is less and not work corretly.
 
   I really need to use the motors with all the power it. The L293B decreases the power of the motors.  This is Normal??? What i can to do for solve the problem???

  Gracias

[karlis]

check shematic of H-bridge, maybe you have wrongly connected something ? h-bridges are like relay's who are basicly switches for bigger power..

i`m looking for some h-bridge's too could you tell me where did you bought it?

[jhonilson]

  I live not in USA.  i am sure that i connect everything ok.
  Here some links of H bridges

http://www.st.com/stonline/books/pdf/docs/1328.pdf

http://www.todomicrostamp.com/pocket_projects.php?proyecto=4

thanks

[izua]

here's a hint: transistors have losses. here's another hint: every power driver has some transistors.
if you want to deliver 100% voltage from the battery to the motors, use a relay. but you won't be able to do speed control.

[bens]

Elaborating a bit more on what izua has said, if you check your datasheet, you'll see a spec called "total voltage drop" or something similar.  This is the voltage drop between your input and your output and it's not something you can make better by any means other than increasing your input voltage.  If you want a smaller voltage drop, look into using MOSFET-based drivers (MOSFETs typically have much smaller drops and they have some other benefits as well).

Looking at the datasheet for your driver, it would appear that the typical high output voltage is VIN - 1.4 and the typical low output voltage is 1.2, so the voltage across your motor is typically somewhere around 2.6 V lower than VIN when the motor is pulling 1 A.  What this means is that when you try to power your L293B using a 5 V power source, your motor is only seeing perhaps 2.4 V across it (maybe more, maybe less, depending on current).

- Ben