# L293 HBridge only outputs 3V?

Hey,
I’m running a Texas Instruments L293DNE H-Bridge off of my Arduino Mini. I’m using it to control a motor that needs 6.5V of power. How can I do that when the H-Bridge only outputs 3V? I feel like I’m missing something.

Thanks,
Mike

You will need an external powersupply for the motor.

The Arduini board will most likely not be able to source the current (or the voltage) needed.

… i’m trying to have a cd-player-tray motor be controllable via arduino / serial.

pretty much the same setup as here, other voltages though:
http://itp.nyu.edu/physcomp/Labs/DCMotorControl

motor and arduino both have external power.

i’m using this standard cd-player-tray motor (5.9V):
RF-310T-11400
http://www.mcmelectronics.com/product/28-1920

i actually have to supply the bridge 12V on pin 8 (VCC2) to make the motor turn ok. if i connect it directly to the power source it has much much more torque.

any idea what i could do?

F

Do you know the stall current of your motors at 12 V? Can your motor driver deliver this current?

One thing you need to note about the L293 is that there is a significant voltage drop between VIN and VOUT. If you look at the datasheet you can see that at 1 A the typical output low voltage is 1.2 V (max of 1.8 V) and the typical output high voltage is Vcc - 1.4 V (min of Vcc - 1.8 V). This means that on average your output voltage is 2.6 V less than your input voltage when you are sourcing or sinking 1 A, and worst case your output voltage is 3.6 V less than your input voltage. This would at least partially explain why you experience less torque when using the driver versus connecting the motor directly to power. I’m not exactly sure how this voltage drop scales with current, though.

One thing you could do is to use a motor driver that doesn’t have such a large voltage drop, or you could increase your input voltage.

• Ben

hmm … so i guess it makes no sense to try to work around the L293’s limits. any idea what other driver might fit my setting? best would be something i can quickly order or pick up here in germany since i’m trying to (have to) get this done this weekend.

many thanks!
F

I’m not really sure what your settings are. The most important thing I need to know is your motor’s stall current at the voltage you plan on running it at (or, if your motors are never driven to stall and their accelerations are low, what is the typical maximum current they draw?).

• Ben

one thing i was unsure about, maybe that’s it: do the arduino and the motor power supply have to share ground? currently the arduino only is connected to the bridge via pins 3 and 4, no gnd at all. should i try to connect motor and arduino grounds (similar to schematic for L298 below)?

http://www.pyroelectro.com/tutorials/l298_control/schematic.html

F

ok, what i’ve found is that the motor has:

"Operates on 1-6 Vdc. 2800 RPM @ 2.5 Vdc @ 0.017 Amps (no-load) "

from another source:

“nominal voltage 2.5, range 1-6V, 0.060 Amps (@ Maximum Efficiency)”

my motor power supply gives 1.5 amps and up to 12V.

F

Unfortunately that’s not so helpful since I presume your motor will actually be under load. Furthermore, it gives the no-load current without specifying the voltage. Is that 17 mA at 1 V or 17 mA at 6 V? 17 mA seems pretty low, so I’d guess the stall current is under an amp at 6 V (maybe even at 12 V), but without knowing for sure it’s hard to confidently suggest a motor driver. Is your L293 driver leading to unacceptable behavior or can you work around the lower torque? It seems like you should be able to get sufficient performance out of it even with the voltage drop if you are using 12 V (assuming it isn’t trying to draw more current than the L293 can deliver).

If you have a digital multimeter you should be able to measure the stall current. What is your power supply, by the way?

• Ben

hm, ok. i can’t seem to be able to find any specs on the motor. … gonna mesure later.
found one that#s pretty similar:
http://www.tedss.com/specs/2099/2099000348.pdf
stall current: 0.22A

the problem with the low torque is that it’s not strong enough to actually move the cd tray. i can stop the motor with my fingertip running it through the bridge. running through the power supply directly it’s closer to a tablesaw, fingertip get’s hot and i can’t stop it. ouch.

power supplys are a 9V-24V wallwart for the arduino and a laboratory power supply for the motor:

thanks for looking into it!
F

one thing i was unsure about, maybe that’s it: do the arduino and the motor power supply have to share ground? currently the arduino only is connected to the bridge via pins 3 and 4, no gnd at all. should i try to connect motor and arduino grounds (similar to schematic for L298 below)?

L298 DC Motor Control - Schematic | PyroElectro - News, Projects & Tutorials

F

Yes Yes Yes the GND must be common otherwise you can’t be sure you are switching the transistors fully on.

perfect, i have it! motor now runs on 5.9V as the specs say it should and everything’s working. yay!

here is my setup:

thanks guys!
(even the breadboard seems to smile …)

best
F

added a ps/2 mouse to read the position and here is the frankenstein mouse tray:

F