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Potentiometer Questions

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chrisdicko

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How do you know which size of POT to use? Is there any calculations, or anything which is obvious?

For example, when would you use a 1kΩ or 10kΩ POT? How would each work differently, but in the same circuit?

Thanks

In some circuits it would make little difference, in others it is very important. Is this just "in general" or do you have an application in mind?

ken

My main application is for use with Variable Drives/Inverters. Some we have at work use 10k, and others 1k. I just wondered why really, and what effects of putting a 1k POT on in place of a 10k would have.

We also have vibrating bowls (uses a magnet and springs to vibrate), which the operator can adjust how violent the vibration is via a 220Ω rheostat. What effects would putting a 100Ω or 500Ω rheostat have on the intensity of the vibration?

I have asked my manager about the above, but he isn't too knowledgeable.

Thanks

Without know anything about the circuits, my advice is use whatever value was there originally. The manufacturers picked them for a reason. I'm not going to guess. Why are you considering replacing them?

Ken

Oh, I'm not changing them I'm just interested in why different ones are used? I do replace like for like when they break.

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You are asking a reasonable question that doesn't have a reasonable answer.

Potentiometers, rheostats have quite a variety of uses in circuits, which as was stated before can't be boiled down to a general rule.

Let's take a simple control circuit, in which potentiometers are commonly employed for manual variable adjustment. The variable to be controlled could be volume, speed, temperature, direction, frequency, degree of movement, etc. As many variabilities as you can imagine can be listed.

So, lets take your vibrating bowl as an example. You say a 220 ohm rheostat controls how violently it vibrates upon a spring. It used a magnet, but I'm imagining it also uses a coil or transformer to electromagnetically oscillate the magnet within the coil's field, the spring being there to dampen the movement. If it is very simple, it is using mains AC, in the US this would be 120 volts @ 60 Hz.

If the rheostat is controlling the current through the coil, which it likely might be, the amplitude of the oscillation is what is being controlled. A higher current would produce a stronger magnetic field around the coil and more forefully move the magnet. A slighter current would oscillate the magnet less forefully because of the smaller field.

OK, so in a typical circuit the 220 ohm value would be maximum resistance and might set minimum amplitude in the oscillation. The rheostat would go down in value from there, and inversely the amplitude of the oscillation would go up until it is maximum @ close to zero ohms.

Now, if you replaced this with a 100 ohm rheostat, your minimum oscillation wouldn't be as slight. If you replaced it with a 1kOhm rheostat there would be a significant degree of pot shaft rotation that would be effectively "dead" because the resistance becomes too great and the magnetic field is too slight to overcome the spring. In that case, from 220 to 1000 ohms you have little to no effect on the bowl's vibration.

This is an example where the consequence of changing the rheostat isn't dangerous, but it is somewhat foolish because you are potentially reducing the performance of the appliance.

And note, my "description" of the circuit is totally assumed and not implied to be the actual method of operation for your vibrating bowls. Don't go off and say its ok to change anything!

In other applications, and quite possibly this one, the consequence of changing the pot could be damaging if not dangerous.

It's rather like asking "what size resistor do I use?". The value depends upon the application and the circuit requirements.

I see, thanks a lot. That's very helpful.

I understand now that it isn't a clear cut case as I first thought!!

We do have one conveyor which is driven by a variable drive, and is controlled with a 10k POT. A lot of this POT movement is 'dead'.
Now I understand you cannot recommend me a valued POT to replace the current one with, as it could be dangerous. But in theory do you need to lower the value of the 10k POT with something smaller like a 1k POT (as an example?). I would seek advice from the manufacturer before changing anything.

Thanks for the input and helping me understand more!

On the same topic, I just built a small power supply kit, 110 in and 0-20 dc out. The knob style pot is too sensitive. What value in a pot's spec tells you how many turns it offers for finer tuning? Or is there another approach?

(Thanks for answering what might be simple questions ... but not to me.)

On the same topic, I just built a small power supply kit, 110 in and 0-20 dc out. The knob style pot is too sensitive. What value in a pot's spec tells you how many turns it offers for finer tuning? Or is there another approach?

(Thanks for answering what might be simple questions ... but not to me.)

Those cheap chinese pots are for playing it with a DMM, and I suppose you're using them. There are MUCH better models, like the 3296 GZ Electron, where a change in 1 ohm requires a visible turn. Ranges up to 100 KΩ, I think. They're blue colored in plastic, with a very small knob to be turned using jewelry screwdrivers.

If they're not at your local stores, ask about the ones with the generic blue plastic casing. And the price too. Hate to say this, but the pricier, the better.

What if the application uses a potentiometer, but doesn't seem to have any effect if the potentiometer is changed. i.e. 10k pot changed to a 30k pot produces the same operation.

Where can I find information to explain how to tinker with a system that uses a pot in this manor?

For instance I wanted to take a 10k pot that was used on a device, and retask the device to work only over 1/3 of the normal ~270* sweep, so I purchased a 30k pot, and to my surprise it did not change the operation of the device. I thought logicaly it was looking for a 0-10k range and by using a 0-30k pot over 90* I would be able to get the kind of operation I required.

Where can I find information to learn how these devices are used, when this condition exists?

TIA
Been looking for a friendly informative place like this for a while

The resistance value of the pot would affect its operation as you thought only if the pot was used in the two terminal mode as a variable resistor or rheostat.

In the normal potentiometer operation, the wiper output is a percentage of its position of the resistance element, independent of it's resistance value. Thus if the pot is 25% from the bottom (typically the circuit common) then the output will be 25% of the voltage being applied to the top of the resistance element (for a linear pot).

One factor determining the pot resistance value is the loading of the pot by the resistance the pot wiper is driving. You generally want the pot resistance to be significantly less than that. For example if the pot wiper were connected to a 100kΩ load then you would probably want to pot to be 10kΩ or less (the maximum Thevenin pot resistance at the wiper is 1/4 of the pot resistance, so a 10kΩ pot has a maximum equivalent output resistance of 2.5kΩ). The minimum pot resistance is determined by power considerations and the load the pot is presenting to whatever is driving the pot resistance element.

You can make a pot less sensitive to rotation by placing a resistor in series with the pot resistive element. But there's not an easy way to make a pot more sensitive, as I believe you wanted. You would have to add more gain somewhere in the circuit to do that.

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