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Potentiometer as divider: How to use both sides of the split voltage?

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jfbrink
#1
Apr21-10, 06:39 PM
P: 4
Hello,

I am building a custom electric vehicle using an existing controller and motor combo.

The controller reads the voltage on two leads to control the speed of the motor and expects the voltages on the two leads to have an inverse relationship.

Thus, at zero throttle, lead A reads 5V and lead B reads 0V.
At half throttle, lead A reads 2.5V and lead B reads 2.5V.
At full throttle, lead A reads 0V and lead B reads 5V.

My question is: How can I achieve these results with a simple rotary potentiometer (and a few other components)?

Unfortunately, my electronics knowledge is more intuitive than actual, although I can read a circuit diagram. And my intuition suggested that putting a voltage input to the wiper and taking output from the two outer pins should work. And it seems to, briefly, but even at a very low current (an alkaline 9V battery), the RadioShack 5K potentiometers I'm using for testing start to smoke (!) pretty quickly.

Any hints, tips or outright answers?

Thanks,

Jesse.
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0xDEADBEEF
#2
Apr22-10, 01:53 AM
P: 825
I suppose that thing is really build with potentiometers in mind. Then you apply your 5V to the end tabs and the voltages are between the center tab and each end.
Studiot
#3
Apr22-10, 04:39 AM
P: 5,462
If I understood you correctly your voltages are measured with respect to another point, probably chassis.

You have a fixed source of +5 volts with respect to chassis.

From this fixed 5V supply you require to generate twin variable outputs of 0 - 5V that vary in antiphase with the rotation of a single control.

These twin oututs will be used by your controller/motor.

In order to satisfy the above you require two potentiomenter ganged on one shaft a bit like ganged volume controls in an audio amplifier. The only difference is that you probably require linear variation laws for the pots.

Practically you need to connect the 0 - 5 supply across the pot terminals for the track ends.
However the 0 and 5 should be reversed on the second pot.
The output is then taken between 0 and the pot wipers terminals.
Depending upon the capabilities of your 5v supply and the drive requirements of your controller values of between 5k and 50k are probably suitable.

There is one more thing.

It is actually poor practice to connect a pot directly as just described. The 5V connection should be wired through a protection resistor of 150 to 500 ohms. This is then in series with the resistance track of the pot.

Good building with your vehicle.

jfbrink
#4
Apr24-10, 01:28 AM
P: 4
Potentiometer as divider: How to use both sides of the split voltage?

Studiot,

Thanks for your reply. I had pondered ganging up a pair of potentiometers, but dismissed it as a "mechanical" solution rather than an "electronic" one. It is certainly a very straightforward approach that is well within my limited abilities.

Still, it seems unlikely to me that this mechanical solution was employed within the OEM throttle (to which I do not have access). Then again, the source-vehicle we are talking about here was a golf-cart, so it's probably not all that sophisticated.

Thanks again for your help. I will give this approach a try.

Best,

Jesse.
Studiot
#5
Apr24-10, 03:15 AM
P: 5,462
It is of course possible to use a single pot followed by an electronic inverter.

Mass production manufacturers also find it economic to employ special pots with special taps, extra outputs etc. They can be a nightmare to replace when servicing.

However I thought it a good idea to confirm my understanding before discussing anything like this. Further information would be required before looking at an electronic inverter as it would require a power supply. It would also be less 'fail safe' than a dual pot.

Did you understand my comment about a series resistor?
jfbrink
#6
May1-10, 05:17 PM
P: 4
StudioT,

I think I know what you mean. The resistor is there to prevent a full 5V at zero resistance from the pot(s)? That would make sense as the actual OEM circuit has a max of 4.87V not a true 5V. The thing I don't know is how to determine the value of that resistor to get my 4.87V, since I don't know the load of the circuit. Adjustable resistor plus trial and error?

I've attached a terrible amateur diagram to show what I think you suggested.



Best,

Jesse.
Studiot
#7
May2-10, 03:12 AM
P: 5,462
The series resistor should be about one tenth the value of the pot. So 150 to 500 ohms would be suitable.

Yes you have the wiring correct.

Look forward to hearing how well it works.
jfbrink
#8
May2-10, 10:36 PM
P: 4
Just so I better understand why I'm doing what I'm doing, why does putting the power to the wiper and getting the inverse split voltages directly from one pot not work?

Is it just too much current through the pot? Even if the pot is fairly high-rated (2W) and the current drawn by the controller leads is seemingly quite small (around 165mA)?



Thanks so much for your help,

Jesse.
vk6kro
#9
May3-10, 02:03 AM
Sci Advisor
P: 4,032
That is important new information.

If the controllers draw 165 mA at 5 volts, they have a resistance of about 30 ohms.
(R (ohms) = 5 volts / 0.165 amps = 30 ohms )

So, neither pot arrangement will work the way you want it to.

What would happen is that each motor would work, but only at one extreme end of the pot travel. This would only happen if the series resistor marked 150-500 was omitted.

You can probably see that even 30 ohms in series with the controller would drop the controller voltage to half, and this would happen right at one end of the pot rotation.

You might need to go to something like this:



It uses a pair of small, power NPN transistors (like a BD139) and you would need a 6 volt power supply to get the full 5 volts out.

Your controllers are shown as 30 ohm resistors.


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