output voltages are different?

J

James Venning

New Member
Hi All,

I have the picaxe 18x, the chi035, and the l293 motor driver.

i have 2 motors connected to the motor output.

the code i am trying to run is:

high 4
high 6

now i run the program, and connect a multimeter across the motor outputs, and motor A reads about 5.5 v, with B reading about 2.5 v.

I have an external 9v voltage supply, and the standard 3 AA batteries controlling the picaxe.

because of the voltage difference, I get a speed difference in the robot I am making...

any suggestions?
 
D

Dippy

Moderator
Post your schematic- as in exactly how your motors are connected. Are the motors the same type. What are their ratings? Do the motors have a load?
And indicate where you are taking the readings as that ain't clear.
 
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J

James Venning

New Member
i don't have a 'schematic' but my motors are connected to the pins at 'A' and 'B' on the CHI035 board. the motors are the same type. not sure their ratings as we pulled apart a toy and just used the base. the toy used to go in a straight line. now because of the voltage difference it goes in a circle...

im measuring the voltage right at the output. ie with the connectors touching the 2 holes at 'A' and then again with them at 'B'...
 
S

slurp

Senior Member
have you commanded the corresponding "low" for the other side of the A & B connections?

I presume that you've followed the manual for the dual voltage modification. Which version board do you have?

best regards,
colin
 
J

James Venning

New Member
no i didn't turn the others on low, since they should already be low by default shouldn't they? if they were high, then the motors wouldn't turn... which they do...

I have the CHI035 board.
 
E

eclectic

Moderator
Strange?

I've got a CHI035 with split power supply.

V1 = 3 x NiMh 3.7v

V2 = 6 x NiMh about 8.2v

Program

Pins = %01010000

Both A and B motor outputs show about 7.8v

First off, disconnect and check ALL the wiring.
Then check the BOTTOM of the board to make sure it's clean.

Then try again please.

e.
 
J

jodicalhon

New Member
I presume that you've followed the manual for the dual voltage modification. Which version board do you have?

Colin's asking if you have CHI035 version 2 or version 3.

Assuming it's version 3 'cos the 9V hasn't fried the picaxe yet:

Put the corresponding 'low' command on the other A and B pins. Explicit is better than implicit. (With apologies to any Pythonistas in the vicinity...)

Check the voltage on pin 8 of the L293D. It should be 9V, or close to it. (9V batteries aren't great for driving motors, you may find the battery voltage has slumped under the load.) Check that the voltage on pin 16 of the L293D is 4.5V - 3 x AA.

Check that the voltages on the Picaxe pins are what they should be. Check that the voltages on the inputs of the L293D are what they should be. Check that the outputs are what they should be.

Build up a picture of what's happening.

Motors from cheap toys usually have a low coil resistance and pull a bit of current.

Edit: ...and what eclectic said...
 
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hippy

hippy

Ex-Staff (retired)
You can also swap the two motors over; does the lower voltage follow the motor or stay with the same board connections ?

If it moves with the motor there's perhaps a problem with that motor, if it stays with the pins it could be a board / programming issue.
 
T

timh2

New Member
I'm working with James on the robot.

I've disconnected the motors from the project board. I used this code to check the voltages across the high power pins 4-7:
Code: 
let pins = %00000000
pause 5000
let pins = %10000000
pause 5000
let pins = %01000000
pause 5000
let pins = %00100000
pause 5000
let pins = %00010000
pause 5000
let pins = %00000000

Using a 7.72 V external power supply and 4.14 V (3 x AA) for PIC control, what I got was:
Code: 
B: -0.04 V	A: 0.00 V  	(B:0,A:0 - what should happen)
B: 0.28 	A: 0.00   	(B:-6,A:0)
B: -6.99 	A: 0.00 	(B:6,A:0
B: -0.04	A: 7.14	(B:0,A:-6)
B: 0.04 	A: -7.17 	(B:0,A:6)
B: - 6.97 	A: 0.00 	(B:0,A:0)
The last one is the most worrying since the pins are turned off.

The bottom of the board looks OK, but there is some shiny glue-like stuff around the pins where wires have been soldered in. Is this damaged?

Also, I tested the motors just using a 7.5V power supply, and they pretty much identical. They draw about 540 mA with no load (except the tracks they run).

I have no idea where to go from here. I kinda expected this to be easier. haha
 
J

jodicalhon

New Member
This'd be easier if we were there, too. :)

Motors disconnected is a good move.

Set pin7 of the Picaxe high. Check the voltage on pin7 of the Picaxe. Is it high?
Check the voltage on the corresponding input of the L293D. Is it high?
Is the corresponding output of the L293D high?
Is this 'high' also at the appropriate A or B pin. Is the other A or B pin 'low'?

Do the same for each pin (4 - 7) of the Picaxe, following the logic path all the way to the appropriate A or B pin.

Check the enables of the L293D are high.

Check the serial download pin of the Picaxe is low.

Maybe try a variation of your sequence by just cycling one motor: off, CW, CCW, off.
See how the voltages on the pins go. Then try the other motor and see how that goes.
Do this with and without motors attached.

Why is your external voltage approx 7V? Are you using a knackered 9V battery? Get some decent Alkaline AA's. 540mA per motor will sorely test the stamina of even a fresh 9V. It's internal resistance will probably drop the output voltage somewhat.

Keep testing and measuring...
 
J

James Venning

New Member
I don't have the picaxe on me so I can't answer those questions, except the one abotu the power supply.

We're using an adjustable transformer doowhitsy. Currently it's on 7.5 V.
 
M

mobents

New Member
The glue-like stuff around your solder joints are probably (almost certainly) solder flux from inside your solder. It makes the solder 'float' much better, and you will get better connections, by my experience.
 
B

BCJKiwi

Senior Member
@JV
I presume you mean one of those wallwart/plugpac DC supplies with a selector switch adjustable output.

Many of these have very poor regulation so what you set is either what you get at no load or an attempt to set set what you get at a nominal load. Test when connected to the the load you are applying.

2. What is its current rating? Ensure it is well above the load - again a check of current under load and voltage drop under load will give you an idea here if the unit is up to the task.

3. Many also are not very clean on their output. Would be much better to set it to 3 to 5 volts above the desired output and feed regulator of the desired output voltage (with caps) to regulate the voltage properly.
 
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