# Potentiometer min and max value

I purchased several of these to drive high power LEDs on a custom project : https://www.amazon.com/gp/product/B019W4C5IE/ref=ppx_od_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

These LEDs are to be driver at around max 0.9A around 35V, I can do that easily with these boards. I'm trying to design a better way to use the system now. I'm using the current control potentiometer as a dimmer for the LEDs which works fine. My goal is to find a slider potentiometer to replace those little trimmers. The problem is that right now those trimmers aren't the perfect way to drive the led, because there is a point in adjustment where it will no longer limit the current (I can see it on the ampmeter on my test bench) and the current will just increase as the LED heats, same under a certain value the LED will stay at the minimum 0.2 amps the board specs and won't go lower.

My goal is to find a way to set endpoints on the build so that the physical minimum and maximum values of the potentiometer match the measured values of those two measured endpoints on the trimmer, I don't know if this is possible using additional resitors in series or parallel with the slider potentiometer I'm planning to use.

• Is there a way I can set the minimum and maximum value of a potentiometer with extra resistors or some other solution?
• Do you have a datasheet and schematic for the board? It will be pretty difficult to help without those. – Elliot Alderson Jan 14 '19 at 15:28
• The top trimmer pot is for voltage adjustment and the bottom one current adjustment, I'm just planning to identify min resistance for min current on the pot, and max resistance for max current on the pot, and pick a slider from there, I don't really need to know how the board works, I'm just trying to change the analog way to command it – ulkar Jan 14 '19 at 15:34
• What about the issue you identified starting with "The problem is that..."? Do you not care about that issue now? – Elliot Alderson Jan 14 '19 at 15:38
• Yes I do, basically what I'm trying to do. Let's say for perfect usage I need to have a potentiometer that on the min endpoint gives a reading of 2455 ohms and on the max endpoint is 6555 ohms, right now my only problem is to figure a way to do that with let's say a normal 10 kohm pot, and I have no idea how to do it – ulkar Jan 14 '19 at 15:42
• Don't ever buy electronic components on Amazon without a datasheet. Really. You wouldn't buy a car that doesn't come with someone guaranteeing it goes at least 50 km/h. Don't expect less from electronics. – Marcus Müller Jan 14 '19 at 15:45

## 1 Answer

Well, you kind of obviously can:

simulate this circuit – Schematic created using CircuitLab

If your potentiometer allows values in $$\[a,b]\$$, then your maximum resistance is

\begin{align} R_{\text{max}} &= \frac1{\frac1{R_{\text{series}}+b}+\frac1{R_{\text{parallel}}}} \end{align}

and likewise, your minimum resistance is

\begin{align} R_\text{min} &= \frac1{\frac1{R_\text{series}+a}+\frac1{R_\text{parallel}}} \end{align}

Solving for the different resistors is left as an exercise to the reader.

However, since the minimum value probably should be orders smaller than the maximum value, things simplify to

\begin{align} R_\text{min} &\approx R_\text{series}\\ R_\text{max} &\approx R_\text{parallel}||b \end{align}