# Potentiometer problem

#### Catalyst9

Joined Jun 5, 2005
6
This is thermocouple thermometer circuit..( I attached a pic)
Thermocouple is a kind of temperature sensor that has linear voltage ouput,depending on temperature

First question :

The input voltage from the thermocouple is 40.96uV and the ouput voltage is 10mV when the temperature is 1C.

Therefore,the gain of the non-inverting operational amplifer is 244.14(=10mV/40.96uV)
Does anyone can calculate the 244.14 from the 1K resistor,the 100 ohm potentiometer and the 255K ohm resistor in detail?. I don't understand the role of the potentiometer...

Second question:

I want to change the gain of the non-inverting amplifier to 336.25
what is the value of the resistors and the potentiometer for gain 336.25?

PS. I'm Korean.. my English is poor #### David Bridgen

Joined Feb 10, 2005
278
Originally posted by Catalyst9@Jun 16 2005, 07:54 PM
First question :
Therefore,the gain of the non-inverting operational amplifer is 244.14
I don't understand the role of the potentiometer...
There are no standard resistor values which can be simply combined to give the required gain. The potentiometer is included so that the values in the loop can be adjusted accordingly.

Imagine that the pot is set at mid-travel. The effective value of R3 is now 255k05 and that of R1 is 1k05. The gain would then be 243.9.
So, with a bit of adjustment the required gain may be achieved.
Second question:I want to change the gain of the non-inverting amplifier to 336.25
what is the value of the resistors and the potentiometer for gain 336.25?
I think you may be able to proceed yourself now. You may need to select values from the E96 or E192 series.

#### Catalyst9

Joined Jun 5, 2005
6

but I have some questions ;

Imagine that the pot is set at mid-travel.  The effective value of R3 is now 255k05 and that of R1 is 1k05.  The gain would then be 243.9.
Does 255k05 mean " 255K is multiplied by 0.95"? and does the "05" comes from the pot(50ohm?)?

You may need to select values from the E96 or E192 series.
what is E96 or E192 series?

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by Catalyst9@Jun 17 2005, 01:39 AM
Does 255k05 mean " 255K is multiplied by 0.95"? and does the "05" comes from the pot(50ohm?)?
It means 255.05k.
The preferred way of writing resistor values is to use the multiplier instead of a decimal point because it can reduce the confusion which may arise from poor printing or photo-copying.
And yes, the k05 is 50 ohms - half the value of the pot. The other half is added to the 1k, hence 1k05.

Examples: R047 = 0.047 ohms, R47 = 0.47 ohms, 4R7 = 4.7 ohms, 47R = 47 ohms, 470R = 470 ohms, 4k7 = 4,700 ohms or 4.7k, 4M7 = 4.7Mohms.

what is E96 or E192 series?
You probably already know that resistors are available in "prefered values."
There are several ranges of preferred values depending on the tolerance (accuracy) of the resistors.

Search Google for "standard resistor values" and you will find many sites which give the values in all the ranges.