# Prevent Exceeding of Potentiometer Power Rating

I am using a 10k digital potentiometer to adjust the brightness of an LED. However, I need to make sure that when I reduce the resistance on the pot, it will not exceed the maximum power ratings.

The max power the pot can handle is 473mW.

So using P = I^2R, we get Imax = 6.88mA.

The battery is 3.3V

From here I am stuck and unsure how to move forward with those values. I have a circuit like this (10k, not 1k): But according to this answer, that circuit seems to be called the 'pot smoker' How can I make sure that I do not blow out the pot? Is there a circuit I can create to put in between the pot and the LED?

Thank you!

• Add a series resistor that sets a minimum bound on the total series resistance. – uint128_t Apr 8 '16 at 21:09
• @uint128_t great, I was on the right track. I was thinking of putting a 5kohm resistor there, but is there a mathematical way of determining the correct size resistor to put there to ensure I would not exceed power ratings? – Julio Vasquez Apr 8 '16 at 21:11 simulate this circuit – Schematic created using CircuitLab

Figure 1. Current limiting simplicity: add R2.

Let's check your maths: from $P = I^2R$ we can get $I_{MAX} = \sqrt {\frac {P}{R}} = \sqrt {\frac {0.473}{10000}} = \sqrt {0.0000473} = 6.9~mA$, as you correctly calculated. Let's be conservative and round down to 6 mA.

The maximum current for a visible LED will be in the case of a red one as red has the lowest forward voltage drop - typically about 1.8 V at a reasonable brightness. That leaves us with 1.5 V to drop across the series resistor at 6 mA. Mr. Ohm says $R = \frac {V}{I} = \frac {1.5}{6m} = 267~\Omega$ which we will round up again to the nearest value of 270 Ω.

Simply pop this somewhere in the circuit to set the minimum resistance in the circuit to a safe value for the pot.

Thanks for the reference to my pot smoker.

• the pot I am using is 10k not 1k which is where i got the 6.88mA Value. However, plugging that value into your calculations yeilds 1.5V/6mA = 250ohms. (6mA for to be conservative). Thank you so much for your help!! – Julio Vasquez Apr 8 '16 at 21:27
• I missed that in the text and only read the 1 k in my original schematic. I'll edit. – Transistor Apr 8 '16 at 21:30

You should place a series resistor to limit maximum current. Eventually could add an emitter follower to increase maximum current.

• great, I was on the right track. I was thinking of putting a 5kohm resistor there, but is there a mathematical way of determining the correct size resistor to put there to ensure I would not exceed power ratings? – Julio Vasquez Apr 8 '16 at 21:12