Changing a Resistor-Based Temperature Adjustment Range

Overview
Example: Changing the Range of a 5CJ-020
   Using a Multi-turn Potentiometer
   Using a Potentiometer with Range Scaling Resistors
   Using Higher Resistor Values for Setting Temperatuer Range
 
 

If one of our temperature controllers is set using a potentiometer, it generally uses a single turn potentiometer. (The on-board potentiometer of the 5C7-350 is one exception, and it uses a 25 turn potentiometer. The remote potentiometer of the related 5C7-350A is a single-turn potentiometer.) The potentiometer we provide generally covers a fixed temperature range. You can improve the temperature settability anywhere within that range by changing the potentiometer or adding to it. 

Some of our computer compatible controllers also have an input for an optional 5KOhm single-turn potentiometer in order to manually adjust the set point temperature. The methods discussed below for changing the temperature range of the potentiometer apply to these controllers as well.

To make your calculations for an adjustment to the circuit you will need to know the appropriate temperature range. See the specifications on our web site the controller you are using. Some controllers have different temperature ranges depending on the type of sensor used, for example, the 5C7-461 has different ranges depending on the type of thermocouple you use. Those ranges are specified on our web page for that controller.

The methods used to change the temperature range of a potentiometer are presented below in the form of a specific example.
 
 

The Model 5CJ-020 uses a single turn 5 KOhm potentiometer. It has the standard 270 degrees of turn from one end to the other.

With the temperature range of 0°C to 750°C this is a coarse adjustment. Generally, an application for this low cost off-the-shelf controller allows for such a coarse adjustment, but if you need improved temperature set point resolution you can use one of the following methods.
 
 

One way to improve the adjustment resolution is to simply replace the single-turn potentiometer with a multi-turn potentiometer.
 
 

The temperature range in Fahrenheit is 32 F to 1382 F.

If you want a maximum temperature of 275 F we can calculate a ratio of

(275 max desired - 32) / (1382 max - 32) = 18%

and

5 KOhm max * 18% = 900 Ohms

So, a 1 KOhm single-turn potentiometer in series with a 4 K Ohm fixed resistor will come close to what we need by providing an adjustable range of 32 F to 302 F.

See the drawing below.

A 1 KOhm single turn potentiometer with 270 degrees of spin would provide a temperature adjust resolution of about 1 degree Fahrenheit for each degree of turn. Of course, a multi-turn 1 KOhm potentiometer would provide a finer adjustment.
 
 

Actually, the circuit can probably handle higher resitances, and our engineer would probably try using a 20 KOhm resistor with the 5 KOhm potentiometer (instead of the 4 KOhm resistance and 1 KOhm potentiometer that is in the drawing. If that works you can leave the 5 KOhm potentiometer where it is and simply splice in the 20 KOhm fixed resistor in order to get the proper RATIO.

Questions?