# Newbie question: How do pots limit current?

From my understanding, a series circuit has a constant current, and when resistance is placed in the circuit it is voltage that changes across the resistors and not current.

Potentiometers, from what I understand, are simply a variable resistors. However, when placed in a circuit they are usually used to control current. I am struggling to understand how what is essentially another type of resistor is suddenly breaking the constant-current rule I have been told. How is this possible?

Thanks for the dumbed-down explanation, I have an engineering background but not in electronics, and I can't find a good explanation for this anywhere else.

• The current in a series circuit changes if you change the total resistance in the circuit. The current through each element is the same, but when you change the overall resistance in the circuit you change the current flowing through the circuit; again, the current through each element is the same, but it's different from what it was before. – Pete Becker May 23 '15 at 18:23
• Ah ha. This makes a lot of sense. So apply Ohm's Law to the circuit as a whole instead of just to individual parts as I've been doing. I think this is the answer I've been looking for. Thank you – Hikonyan May 23 '15 at 21:19
• Start with the circuit as a whole, to figure out the current. Then look at individual parts to see what the voltage drop across each part is. – Pete Becker May 24 '15 at 0:16

A pot doesn't break KCL or KVL, and as you say it's just a variable resistor. Usually it controls current by converting a voltage on the pot to a current. That's just Ohm's law- The current is the voltage across the resistance divided by the (variable) resistance.

There's no law that says a series circuit has a constant current, only that elements in series carry the same current.

• Thanks for the explanation. If "elements in series carry the same current", what is different about a basic series circuit with a light bulb and resistors (where all elements carry the same current), versus a basic series circuit with a potentiometer (where the pot changes the current that the elements carry)? – Hikonyan May 23 '15 at 17:38
• If you alter the circuit by adjusting the pot, it alters the current in the circuit. Since it is still a series ciruit and all elements carried the same current before you made the change, after the change, there is a new current, and all elements now carry that same current. – JRobert May 23 '15 at 17:48
• JRobert, in that case, what is different about a pot versus a standard resistor? To my understanding, a pot is just a variable resistor. Thanks again for the answer, I'm still trying to wrap my head around it – Hikonyan May 23 '15 at 17:50
• @JRobert has it right, there's no law that says the series current can't change, just that all elements in series carry the same current at a given time. The ONLY difference between a standard resistor and a variable resistor is that you can vary the resistance. A potentiometer is a 3 terminal device and not strictly a variable resistor, though it can be configured as one. – John D May 23 '15 at 18:26

A lot of problems in the question: -

• A series circuit has whatever current it demands from the supply and is not normally constant by any means.
• When a resistor is added to a circuit, voltages can rise or fall and current may do the same.
• Potentiometers ARE NOT simply variable resistors. As the name implies they control "potential" i.e. voltage.
• Ohms Law prevails as always - there is NO constant current rule

All this is possible because what you have been taught (or learned) is incorrect.

• Thanks for the explanation. If a pot controls voltage, what is the difference between a basic circuit with a light bulb and other resistors that influence voltage (but don't influence the current the current through each part), and a circuit with a pot (which DOES influence current)? – Hikonyan May 23 '15 at 17:41
• You are still under some sort of misconception. The current in your basic circuit with the light bulb and other resistors IS certainly influenced by the resistors. – John D May 23 '15 at 18:30
• @Hikonyan I think it's better if you produced a small circuit that confuses you rather than trying to generalize this from the beggining. Generalizing is best but on this site, given the limited space to teach from the bottom-up it's somewhat easier to start with a specific circuit. – Andy aka May 23 '15 at 18:44

That is not what happens. If you have a resistor 10ohms , with another resistor 10ohms , and a voltage of 20V then a current of 1A flows through the circuit, and 1A will be generated because of I = V/R = 20 / (10+10) in this case. Here 1A flows through both, and current in both is same.

However if you replace one of the resistors with a pot, then if the value of resistor on being changed becomes 20ohm , then new total resistance R = 30ohm

So new I = V/R = 20/30 = 0.66 A .

Now every time you vary the pot resistance, the entire series resistance will change, so total current will change. But both the Pot resistance and the other resistor will still follow the rule, and both will see 0.66 Amps.

Pots are variable resistors and if the resistance is higher than the rest of the circuit loop it reduces the current. but you must choose wisely so that the power and voltage ratings are not exceeded.

## other info related to current limits

Also, Pots can be made with Wire-wound (WW) for higher current, Cermet ( a resistive carbon-ceramic) material and metal film. (MF)

More commonly today with MF plastic and a metallized coating embedded are used for reduced cost and smooth wiper action.

These MF pots cannot handle much current or power at all and especially if you apply a constant voltage near 0 Ohms where the wiper almost reaches an end terminal.

There are also many kinds of physical pots. Including; Linear, audio-log, sin-cos rotary, sliders and trimmers or trim pots.

With Volume Controls on consumer goods and cars, Pots have been replaced with continuous rotary encoder dials using dual rotary tracks and wipers that are staggered by 90 deg to sense small angular motion digitally.

As always, the current limit is defined by the datasheet specification by the power level or wiper current so that the P=I^2R =V^2/R does not melt the conductor.

If you need to regulate high current then a transistor(s) with high current gain or a FET with voltage controlled transconductance or ON resistance in a Power FET is biased by a pot to become the higher current conductor.