Voltage regulators

I use an L297/298 in a circuit I built to run steppers for my CNC machine. Trying to use the same power supply for the motors and logic caused me some problems. I think I am getting spikes coupled into the

5v logic supply from the motors that cause logic glitches that make motors sort of twitch when they should be held stationary. My quick fix was to power the 12v with a car battery, and use a bench power supply for the 5v. What I really need to do is mill a new board that has opto-isolators between the L297 and L298, and get a big linear unregulated supply for the motors to keep the two parts of the circut really seperate.

- James B

I *strongly* reccomend using separate power supplies for the motors and the electronics.

My current bot has 12 12 amp-hour SLA batteries, and my next one has 2 35 amp-hour batteries for the motors and 1 70 amp-hour battery for the electonics. Now, if I could only find out how much power a 4 gigaherz P4 board uses... :)

-- D. Jay Newman

What he said. But if you want 5V from 12V, do a google for '7805 voltage regulator'.

It has basically the same functionality as the LM317, but with a build-in resistor divider to set the output to a fixed 5V.

7805 regulators are obsolete. There are better pin-compatible 3-terminal fixed regulators. You can get 3 amps in a TO-220 package today.

A more serious problem is that dropping 12V to 5V in a linear regulator will waste 60% of the power, which comes out as heat.

An alternative is a DC-DC converter, which is more expensive but will only waste about 3% of the power and run cooler.

John Nagle

Ray wrote:

There's nothing wrong with using the same supply. You just need to ensure you have adequate decoupling capacitors for the motors and the logic's regulator. Although I would agree that most people find it easier to just use separate supplies.

Maybe even a resistor, or is that too low tech?

Nothing wrong with the good old 7805 unless you need lots of power.

best regards

Robin G Hewitt

Or very low Iq, or very accurate output voltage, or very high input voltage, or low dropout voltage, or low output current, or automotive transient protection or..

But the 7805/78M05/LM340 is still the best choice for a huge number of applications, which is why it's not obsolete, not obsolescent, but pure JELLYBEAN (with no end in sight).

Best regards, Spehro Pefhany

Must be a big bot to carry that much weight - do you have any photos on your web site or anything?

-Brian

What a nice coincidence! Solarbotics has happened to *just* release a L298 motor driver kit that has a low-dropout regulator built-in to the PCB, with indicator LEDs, all in a small 1.4" package. We're finding it quite useful for in-house robot developments.

If it helps, we're using a LM2937 as the voltage regulator. Same function as the venerable 7805, but able to run with lower voltage "overhead" to maintain 6V.

L298 Compact Motor Driver info available at:

A single battery and a regulator for the logic will work fine so long as the motors never draw so much current as to drop battery voltage below minumum for the regulator. Should the motors or other load overload the battery the regulator may glitch.

The point to using a capacitor is to save charge until you need it, helping out the battery and regulator when they're under strain. Think of a capacitor as a really fast, yet small battery.

The datasheets for most regulators recommend decoupling caps before and after the regulator, especially when used to regulate noisy components. Digital electronics especially are characterized by short periods of high current draw, which can be more quickly and efficiently supplied by a capacitor than a regulator or battery.

There's nothing *wrong* with it. I just consider it a bit dangerous to your logic if you have big motors.

However, you need a lot more regulation to make sure that the motor spikes don't get into the logic.

Frankly, I'd use a good quality DC-DC converter for the logic if I wanted to share the batteries.

-- D. Jay Newman

Ooops.

That's *2* 12 amp-hour SLA batteries.

It's on my web-site and the bot's name is "Stuart".

The body is a modified Walmart 1/5th scale R/C M5 Stuart tank.

12 batteries would allow me to make a *huge* segmented robot! :)

-- D. Jay Newman

Of course. And for small robots that's normally true.

I just like building medium-sized and larger robots.

-- D. Jay Newman

Thanks for the electronics lesson :o)

The point to using a resistor is to drop the volts, reduce the load on the '05 and protect your new Vcc from transients without a monster capacitor on the input side.

best regards

Robin G Hewitt

How big is too big, out of curiosity's sake?

I just finished the motor code for a project of mine last night and I had it racing around the garage floor...

You could always put your motors behind a regulator with current-limiting. That way you can ensure they never hog your system's supply.

I generally consider anything over a couple of amps as "big".

However, whenever possible I do my best to separate power supplies.

As to robots, I think of dogs as a size guideline.

Anything up to a toy poodle is "small".

From there on up to a German Shepherd is "medium".

Anything bigger than that is large.

And yes, this is highly subjective.

-- D. Jay Newman

At about the time of 8/12/2004 10:37 AM, John Nagle stated the following:

And it will make your battery last longer too.