Sanity Check On My Power Supply Plans for Ugly Buster

Bill Chennault

All--

Ugly Buster, my five-processor skid steer robot, is currently driven by two power sources: two parallel lead-acid batteries for a total of 12 volts at 24 amps and two parallel lithium ion batteries for a total of 7.2 volts at 8400 mah. The 12 volt system currently operates only the gear motors driving the tracks and the 7.2 volt system operates the processors and the gear motor encoders. The grounds of both systems are tied together.

The 7.2 volt system drives the breadboard bus directly. The bus feeds 7.2 volts (or whatever) to four BS2-OEM Stamps via Vin. There is a single BS2p40 module that runs off one of the BS2-OEM Stamps' Vdd.

The system operates well but I feel it could be simplified.

For example, why don't I just eliminate the 7.2 volt system and drive everything with the 12 volt system, while continuing to run the BS2p40 off one of the BS2OEMs' Vdd?

Is this a plan or are there big holes in it?

Thanks!

--Bill

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Mike Green

You could certainly run the Stamps off the 12V supply, but you'd want to use a switching regulator to reduce the regulator's power dissipation. You'd also want some extra filtering to reduce the electrical noise from the motors. Typically, this would be an inductor followed by a large capacitor. Look at the design of the Protoboard's servo power filter for one example.

Bill Chennault

Mike--

Thank you for continuing to tolerate my electronic ignorance. Perhaps, one day it will be different. (I doubt it. [noparse][[/noparse]My ignorance, not your tolerance, I hope!])

My web site is down or I would post a picture of the circuit to which I think you refer found at SX 48 Proto Board Schematic (.pdf). Is it the LM2940 circuit, which is the top-right circuit on the page? If so, I have built that simple circuit, but have not implemented it on a permanent basis. I have a bunch of LM2940s laying around, so it would be a simple matter to recreate the circuit.

Or, are you talking about the Propeller Proto Board Manual·and the circuits shown therein, none of which appeared germane. (To ME!) It is not clear to me how the servos are powered on this board in any special manner other than via the inductor and the 1000uF electrolytic capacitor.

I certainly do not doubt your guidance, I would just like to learn a bit.

--Bill

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You are what you write.

Chris Savage

Bill, are you able to measure how much current the 7.2V system is using? With that information you could easily determine if you need to change anything else.

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Chris Savage
Parallax Tech Support

Bill Chennault

Chris--

If I know how to measure current in the first place! Place my multimeter in current reading mode and·make all 7.2 volt power flow through the·meter? That would be easy.

Additionally, all that is driven by the 7.2 volt system are four BS2-OEMs, a BS2p40, the control wires to two HB25s and the two LEDs of the gearmotor encoders.

I'll report back.

--Bill

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You are what you write.

Bill Chennault

Chris--

I must not know how to measure amperage. I placed my "Extech, True RMS MultiMeter" between the power and the logic such that all 7.2 volt current flowed through it. I powered up the robot and the digital meter immediately read 1.4 MICRO amps, then almost immediately dropped to 1.2 MICRO amps. Everything that could draw power was drawing power, including the gear motor LEDs.

By my calculations, I should have read something like 52 milliamps, plus the gear motor encoder LEDs. (3ma per BS2-OEM is 3 x 4 = 12ma and 40ma for the BS2p40.)

Will you tell me how to read amperage? It sure looks like the readings I saw are WAY low.

Thanks!

--Bill

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You are what you write.

Chris Savage

Bill,

Every Multi-Meter I have ever seen requires you to move the positive probe from the normal jack to the Amp jack for current measurement. This jack is usually fuse protected as well. Are you doing this? Once the meter is setup correctly you can disconnect you positive 7.2V supply and connect the Red probe to it. Then connect the Black probe to the wire that used to connect to the supply. I hope this helps.

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Chris Savage
Parallax Tech Support

Bill Chennault

Chris--

Unfortunately, that is exactly what I am doing. I'll debug my Rodeo Shack multimeter.

However, I really appreciate your time and thought to help me.

--Bill

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You are what you write.

Bill Chennault

Chris and Mike and All--

Ha! I found the instruction booklet covering my multimeter! To measure current expected to be UNDER 10 amps, you leave the jacks in their "normal" positions and merely set the dial to the correct position, microamps or milliamps. So, now I have confident readings of current draw.

With all 7.2 volt electronics drawing current, the very first reading is no more than 190 milliamps and then in two or three seconds it settles to 155 milliamps. Although this is certainly more than I figured, I am quite confident it is correct.

Chris, what were your thoughts based on the current draw?

--Bill

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You are what you write.

allanlane5

Basically, the LM2940 is acting as a very fast 'variable resistor' that changes its resistance to insure that the output terminal remains at 5 volts, no matter how much the current flow through it changes.

The power dissapated in the linear regulator (that LM2940 three-terminal device) is related to the voltage drop (12V - 5V == 7 volts) and the current through it.

Power == Current * Voltage. So, 155 mA * 7 volts == 1 watt. A quick trip to the LM2940 data sheet ( http://cache.national.com/ds/LM/LM2940.pdf ) and scrolling to page 11, shows that the TO220 package can dissapate 1 watt all day long without a heat sink at 70 degrees Fahrenheit, and 5 watts with a heat-sink. "With a heat sink" would be better, as it will run cooler. A linear regulator disappating 1 watt without a heat sink will get quite warm.

Bottom line, if these numbers are correct, you CAN do without the 7.2 volt battery. You WILL be "tossing" that one watt as heat, where the 7.2 volt would have less "waste". But with two lead-acid batteries, that "waste" might not be significant.

Bill Chennault

allanlane5--

Thanks. I really liked that explanation. Although I have read such explanations many times, I have never paid much attention beyond the moment as electronics was never my field.

But, maybe your's will stick with me!

I THINK I have some TO220 devices. I will look and perhaps play with the circuit . . . and ask questions before I subject Ugly Buster to it!

I appreciate it.

--Bill

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You are what you write.

Bill Chennault

allanlane5--

I found ten LM2940 devices in an unopened plastic bag. I can smoke several before it gets serious!

--Bill

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You are what you write.

allanlane5

Oh, they're pretty robust. If you over-heat them, there's a "thermal shutdown" that activates and "crow-bars" the output voltage and current to zero until they cool down. Good times!

Chris Savage

Bill,

There’s no reason you shouldn’t be able to use your main power supply for everything. The regulators on the OEM BASIC Stamps can handle 12V no problem. You can power your main board from one of those regulators. As far as dips in the supply causing brownouts, the neat thing here is that with the span between the minimum voltage required by the regulators vs the normal operating voltage, it’s doubtful you will have any issues until the batteries are almost completely exhausted.

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Chris Savage
Parallax Tech Support

Bill Chennault

Chris--

Thanks. I will report back.

In the meantime, I have learned enough to begin experimenting with the LM2940.

--Bill

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You are what you write.

Bill Chennault

All--

Today, Ugly Buster lost 1.5lbs. via the removal of the 7.2 volt battery source. I put about ten minutes of run-time on the 12 volt system, but it was not continuous. However, I did feel each LM2940 regulator and they all felt cold. (Gee. Wonder why I didn't use the temperature probe on that fancy multimeter that I can barely use?)

Thank you to everyone . . . and Ugly Buster thanks you, too! [noparse]:)[/noparse]

--Bill

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You are what you write.