The Pololu Motor Controller and 9.6 volt battery pack

Technic-R-C

Hello,

I am new to electronics and I have a question in regards to the The Pololu Motor Controller that I bought from Parallax.
http://www.pololu.com/products/pololu/0410/

In the pololu user guide (http://www.pololu.com/products/pololu/0410/smc02b_guide.pdf), it states that the motor power supply should stay between 1.8 and 9.0 volts.· Currently I have a 9.6 volt battery pack which I would like to use to power my motors.· Would the 0.6 volt difference·overpower and·destroy·the motor controller or the Basic Stamp? (just in case)

Thanks

Technic-R-C

Mike Green

The user's guide is pretty explicit and says 9.0V, not 9.6V. Even if some controllers can withstand 9.6V for some period of time or under some circumstances, if it breaks, they will tell you it's your fault for exceeding the limit they warned you very clearly about. Also keep in mind that 9.6V battery packs sometimes provide more than 9.6V, particularly right after a charge cycle.

The Basic Stamps can run off higher voltages without damage, but the regulator doesn't have a heatsink and all the extra voltage gets turned into heat. The regulator will turn itself off if it gets too hot and the Stamp will stop working until it cools off.

One solution is to put two power diodes in series with the positive lead from the battery pack. Each diode will drop about 0.6V for a total of 1.2V giving a motor controller supply voltage of 8.4V. The excess voltage will get turned into heat so the diodes will get hot. Use diodes with a current rating at least twice that of the motors. Since the two motors can each draw 1A, pick diodes rated for at least 4A. RadioShack has diodes rated at 50V 6A (#276-1661) for example.

Technic-R-C

Thanks Mike,

I have a couple of questions about decreasing the batteries voltage.

Mike Green
put two power diodes in series with the positive lead from the battery pack

Do the power diodes have leads, or are they non-polar?

Mike Green
pick diodes rated for at least 4A...Each diode will drop about 0.6V

What determines the voltage drop? What is the difference between a 6A and a 4A diode?

Mike Green
RadioShack has diodes rated at 50V 6A (#276-1661)

······What does the 50V mean, is that the maximum voltage that the diodes can handle?

····· Would two of the 50V 6A diodes work well with my battery pack and motors? Would it drop the voltage too much, or do all diodes drop the voltage by 0.6V?

last question.....
Is the attached schematic correct?

I appreciate all your help.

·· Technic-R-C

Steve Joblin

I'll let Mike anwer the questions, but I will suggest that you call or e-mail Pololu directly as to their recommended suggestions... Like Parallax, Pololu has excellent customer service and will be glad to help you with any questions.

Mike Green

The diagram is correct. All diodes have a mark on them to identify the cathode terminal. On most power diodes, this will be a band.
The cathodes face away from the positive battery terminal (towards the Pololu controller).

The 50V rating is the maximum reverse voltage that the diode will withstand.

The particular power diodes I mentioned (and similar ones) come with what are called axial leads. Look at the picture on the
RadioShack website and you'll see.

The voltage drop is determined by the construction of the diode including the materials it's made from. The voltage drop also
varies with the current through the diode. Most diode datasheets will have a graph of this voltage (Vf or forward voltage)
in relation to the current through the diode. 0.6 is a typical minimum.

The current rating of a diode is determined mostly by the construction of the diode including how the leads are attached and
is partly a function of heat and how well and how rapidly it can be dissipated. Diodes (and transistors and integrated circuits)
usually have small leads internally and these will melt like a fuse if they have to carry too much current for their design.

Technic-R-C

Thanks for all the help.

Technic-R-C

J.T.

All,

This Pololu speed control looks like the way to go for controlling motor speeds?

I asked recently about how to control a motor speed, and·am now researching how to use·H-bridges and·L293D??

Why·not this controller?

(and thanks for everyone's help!)
·

Mike Green

The Pololu controller can work fine. It's more expensive than an L293D, but it takes care of most of the work for you.
You won't learn as much about controlling motor speed and direction, but that may not be that important to you.

Technic-R-C

On the contrary, I am trying to learn as much as I can about controlling motor speed and direction.· I have read·a lot of·articles related to the process of controlling DC motors.· To tell you the truth, I have never heard of the L293D chip until this morning.· I have read over the L293D datasheet, and it seems to be·a lot better than the pololu motor controller.· Do you know where I can by the L293D chip at a low price?

Post Edited (Technic-R-C) : 11/12/2007 5:31:56 AM GMT

Mike Green

Mouser Electronics carries the TI L293DNE for under $3 in small quantities. I'm sure DigiKey also would carry it.
I just went to their website and looked up the L293D and went through their list of different variations in vendors
and package types to find a PDIP package available in quantities of 1.

J.T.

The Pololu controller can work fine. It's more expensive than an L293D, but it takes care of most of the work for you.

The only issue I have with the L293D, from what you've explained,·is the constant pulsing.· Does the Pololu also require the constant PWM signal that will tie up the stamp?·I think I am going to need the propeller chip anyway to handle a number of motor speeds, but still would prefer something where I can "set" the motor speed, then come back to it when I need to change it, vs. constantly tasking the chip to be sending pulses.

Thanks!

Mike Green

A couple of options:
1) Use 1/2 of a 556 dual timer to provide the pulses with a digital pot to set the pulse width and the other half to set the pulse repetition rate.

2) Use something like the PWMPAL to independently do the pulsing

3) Use the Pololu.

LilDi

Take a look at the 754410 dual H-Bridge IC. Its suppose to be a new and improved version of the L293D. Its good up to one amp, but you can piggy-back them on top of one another to double and triple the amps.

Technic-R-C

but I will suggest that you call or e-mail Pololu directly as to their recommended suggestions... Like Parallax, Pololu has excellent customer service and will be glad to help you with any questions.

I contacted Pololu and one of their technical·workers said that the 9.6 volt battery should work fine·with the Pololu Motor Controller even though you should try to stay in between the recommended amount of voltage.

Thanks for all the help.

Technic-R-C

Steve Joblin

Technic-R-C: I thought that would be the case! The Pololu products are pretty well designed and can withstand a little bit of "abuse".

J.T.: The entire idea behind the Pololu Dual Serial Motor controller is to make it a "set and forget" device... that is, you send a serial command once to the unit and it will keep doing what you tell it to do until you send it a new command. You can send commands frequent and often, so if you wanted to ramp up the speed of a motor, for example, you could create a "for...next" loop that would slowly increase the speed each time through the loop.

something like:
SPEED VAR Byte
RESET_DSMC CON 0


LOW RESET_DSMC
HIGH RESET_DSMC

FOR SPEED = 0 TO 127
SEROUT 1,32,[noparse][[/noparse]$80,0,0,SPEED]
PAUSE 20
NEXT


If you want it to ramp up quickly, reduce the pause amount; if you want it to ramp up slowly, increase the pause amount.

Bill Chennault

All--

I have used the great little Pololu Dual Serial controller with great success. Plus, I agree with the comments concerning Pololu's support.

--Bill

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