DC motor control

stilgar

I am using the propeller to control 2 DC gear motors through a L293D 4-CH driver. I am inputing 9 volts to the driver for the motors but I am only getting a reading of 5-6 volts at the motors. When the bot runs on the desk or floor it seem vary under powered. I I remove the driver and use toggle switches I do get 9 volts to the motors and no problem.

Is this normal for the H bridge chip to have that much voltage drop ??
This is the second driver chip I have tried, the other was a L298N multi-watt dual driver. From it I only got 5 volts to motors.

Do I need to go to a 12 volt battery to get the 9 volts I need ??

thanks,
Stilgar

Duane C. Johnson

Hi Stilgar;

What are the specs on the motor?
How much current do they draw at 9V? and 5V?

Duane J

T Chap

The driver may be current limiting. Meaning that is it not providing a steady dc voltage but a chopped voltage (pwm). So, on a volt meter you may be seeing an average voltage.

stilgar

The motors are pulling about 250mA at full load, don't have any specs on the motors.
The draw is about the same at 9 or 5 volts.

Duane C. Johnson wrote: »

Hi Stilgar;

What are the specs on the motor?
How much current do they draw at 9V? and 5V?

Duane J

There is no PWM at this time, only full on/off, no braking.
Update..
I added 2 more AA cells to bring it up to 12 volts, now I am getting 8.9 volts to motors.
There seems to be a 2.75- 3 volt drop at the H-bridge, running 3.3-9 volts to the control inputs don't matter.

datasheet for L293D is " https://www.jameco.com/Jameco/Products/ProdDS/1341966.pdf "

stilgar

LoopyByteloose

H-Bridges do have internal voltage drops and the 2.75-3.00 volts might even be reasonable. But the L293 doesn't document the actual voltage drops. And yet, the documentation does indicate that the output stages are one Darlington array and one Siklai array.

If you were to use power Darlingtons, such at the TIP120 and TIP125, each would have a documented saturation voltage drop of between 1 and 2 volts per unit.

So in an h-bridge, your drop could be as much as 4 volts, some would argue for less based on 1.4 v drop per unit for a total of 2.8 v- but I am using the actual spec sheet and not a theorhetical 0.7 diode voltage drop. That variation is due to the transistors not being linear. And you may actually get variation dependent on load. So it seem that what you are getting is in the ballpark, unless someone has reverse engineered some really accurate data.

In review, the Darlington can have a 2.0 volt drop, and the Siklai can have another 1.0 volt drop. So together in an h-bridge you can expect 3.0v drop at saturation. With all Darlingtons, this goes up to 4.0v drop at saturation.

Frankly, I think the larger L298 is a better device, but it too has votlage drops. If you have issues with wasted battery, you should be using a MOSfet h-bridge. It is very normal to adjust by going to a higher supply voltage to gain more power.

stilgar

Thanks everyone. Great information on the H-bridge, Loopy Byteloose.