SparkFun Forums

[jbakse]

Hello,
A friend of mine built a project using Lego Mindstorms that raises and lowers a weight on a string around a shaft turned by the lego motor. It works well, but the lego motor is not strong enough to pull the final load size. He has built an assembly using a 1.5 amp motor that can easily pull the weight. I am trying to help him interface the rcx to the bigger motor.

I'm not sure how much current the RCX can provide, but its certainly not designed to operate in the range of the big motor. Also the rcx seems to put out between 7 and 8 volts, and the big motor is running at 12. I figured that I could just use the rcx to control an H-Bridge that would then drive the motor.

My questions, then, are:
Does this seem like the correct aproach?
Is it fine for the bridge to connect a lower voltage and lower amp input to a higher volt and higher amp output?
What is the most direct route to getting a bridge that can handle this load (ie. can someone recomend a good IC to use?, should i build one from TIP120s, does somewhere like sparkfun have a relatively cheap (<= $30) board for this?)

Any input would help, thanks. Justin

[Vraz]

I am not familiar with the Lego Mindstorms, but it sounds like you need an external h-bridge to run your motor. My suggestion would be the TI 754410 which SFE happens to sell for $1.75.

http://www.sparkfun.com/commerce/produc ... cts_id=315

The 754410 is rated for 1A per driver but since its has four drivers (two needed to create a single h-bridge) you should be able to run them in parallel to get 2A output. The chip has separate logic (5V) and motor (up to 36V) supply pins and its really easy to use and comes in a handy DIP-16 package.

This is one of my favorite microcontroller interface chips and has proven very robust (though I did have one explode when a power supply went bad on me). Not familiar enough with the Mindstorms to know how to interface, but assuming you can get a couple of pins to swing from 0-5v you should be in business.

[jbakse]

Sounds good. I looked at that chip earlier, and was unsure that running the drivers in parallel was alright. I am glad to hear that confirmed.

As far as connecting the mindstoms, they are putting out either 8 volts on one wire or 8 on the other, with a couple of diodes and a voltage divider it seems like I should be able to swing the input pins.

Looking at the tech sheet, it looks like very little is needed to use the chip, they do show some diodes, which i image are to protect against the motor kick. I thought that those would be internal to the chip, am I miss reading the application info? Are external diodes needed?

Thanks. Justin

[Vraz]

Looking at the tech sheet, it looks like very little is needed to use the chip, they do show some diodes, which i image are to protect against the motor kick. I thought that those would be internal to the chip, am I miss reading the application info? Are external diodes needed?

I assume you are looking at figure 3? I don't know why it shows those diodes though my guess is that the diagram was taken from the L293 (which didnt have supression diodes). If you look at the "typical of all outputs" diagram on page 2 shows internal diodes there.

I have never used external diodes myself believing the chip contained them and it has not been a problem so far. Mind you, I have not driven a high load motor with it (though I have a PCB being made right now which will put it to the test).

Once you get it hooked up, just feel the chip to see if its getting really hot under load. If it is, you might need a little clip on heatsink (or you could probably just bend a C from aluminum and glue it to the top). Assuming your use is intermittant, I don't think anything will be required.

[Philba]

There are at least 3 flavors of the L293:
- L293B and L293E do not have diodes
- L293D has diodes.

look at the datasheets for more details.

be careful what you use in your circuit. That beefy a motor can produce some significant BackEMF, especially with a load.

Phil