Low voltage operation of H-Bridge L298, etc. Could use some suggestions.

RobotWorkshop

I need to use the Propeller to control a motor rated at 5V 2A. I'd like to use a L298 H-bridge chip to control the motor. It will handle the current but the low voltage (5V) of the motor looks like it may be an issue. If I'm reading the datasheets correctly the logic voltage should at least be 4.5V and the voltage for the motor should be at least 2.5V higher than the logic voltage.

Has anyone use the L298 at low voltages like this?

It looks like I may be able to use a pair of stacked SN754410 chips instead since they should handle the 2A peak of the motor. That may be the way to go. If I get a chance in the next couple evenings I'll try to wire something up and test it.

Robert

Phil Pilgrim (PhiPi)

When using a driver like the L298, the motor supply voltage is pretty irrelevant to the motor's voltage specs, so long as it's at least as high. The reason is that you will want to regulate the current going to the motor windings by PWMing and using the voltage on the current sense resistors as feedback. This has three advantages:

1. At running speed, the motor's voltage will automatically regulate to the proper level.
2. The motor will receive an extra boost for startup.
3. The motor's stall current will be limited to the programmed current.

As long as you regulate the current, you could conceivably power the L298 motor supply from 12V or higher for a so-called "5-volt" motor.

-Phil

RobotWorkshop

Hello Phil,

Thanks for the suggestions. When the motor is being controlled by the Propeller can definitely use PWM. I am also going to use the Propeller to monitor what the pan motor from my camera is putting out. Those will also go to the propeller. The initial thought was to be able to control the motor when I want to and the rest of the time just mirror the signal coming in from the camera to the outputs going to the H-Bridge. I've taken another look at the datasheet for the SN754410 chips and it looks like those should work at 5V. I guess I'll try wiring it up and try it.

This is used for the panning motor for a robot head. It was originally controlled by and RC operated relay. Each end has normally closed limit switches installed with a diode across the contacts. That way the motor can run all the way to the end without damage. When it gets there the limit switch opens the contact with the only electrical path through the diode. It can then run the motor back in the other direction. There shouldn't be any times when the motor would stall. Most of the time I am more concerned with the inrush current when the motor starts up. There is going to be a 2amp fast blow fuse to cover anything unexpected.

I'm going to try adding an encoder to it as well as a sensor for centered position so it will be easier to control.

Robert

GordonMcComb

Is there a reason to use an L298? It's not even the cheapest 2A solution anymore, all things considered. The chip was really designed for use for stepper motor applications, ideally paired with the L297. With steppers it's not uncommon to overvolt them, and then fold the current back to whatever level you want. As Phil sez, you can do the same for a DC motor, but why if all you're wanting is basic motor control.

Pololu has some pretty nice board-level bridges in a similar price range that are designed for 5V motor voltage, and 3.3 V logic. They have a couple in the $15-20 price range based on a Freescale chip that supports 3A continuous, so you won't have to worry if your motor stalls and pulls all 2 amps.

-- Gordon

RobotWorkshop

GordonMcComb wrote: »

Is there a reason to use an L298? It's not even the cheapest 2A solution anymore, all things considered.... -- Gordon

Sure, I have an extra un-used one on a carrier board that I could use. Saves me buying or building one. It will handle the current but once I realized what the motor was really rated at determined that it may not be the best option due to the lower voltage operation. I'm going to try the TI chips since I have those on hand as well. If they won't work I'll definitely check out the Pololu drivers. I forgot about those. Thanks for reminding me. It looks like this one would work well:

http://www.pololu.com/catalog/product/1212

I'm using up some of my leftover robot parts from other projects (where it makes sense) and acquiring/building new parts as needed.

Robert

GordonMcComb

RobotWorkshop wrote: »

It looks like this one would work well:

http://www.pololu.com/catalog/product/1212

I have that driver in the dual package. It's pretty good.

One thing about Pololu breakout boards is that they insist on using #2 holes for everything (when they include mounting holes at all), and they don't always put the holes on opposite corners. Mounting solutions aren't great for something that could use some mechanical stability. They also make their boards the smallest they can possibly be -- I assume to increase the panel yield -- making for cramped space.

That said, the engineering from the motor bridge point of view is very good.

-- Gordon