# How can display resistor value greater than 390 ohms?

hi,

i am using rtd-rpi board, and the maximum resistance actually measures 390 ohms. so when measuring resistances above 390 ohms the results are all around 410 ohms. (for example: I measured the resistance of the Heater about 1057.10ohms ~ 1368.02ohms) .by calculation, how can display resistor value greater than 390 ohms?)
Thanks very much.

this is the command:
{
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return newMsg ;

Not sure this is really a Node-RED question? To get a full range of output from a resistance measurement, you would normally use a voltage divider circuit to match the max voltage output to the capabilities of your GPIO connection - which in turn depends on what device you are using for GPIO.

Voltage Divider Calculator (ohmslawcalculator.com)

If using a Pi for example:

razzpisampler (oreilly.com)

An ADC is better for measuring resistance/voltage though and the Pi doesn't have that I don't think. Most people would use an Arduino or ESP device.

Raspberry Pi Measures 0~5V Voltage via Arduino | UUGear

1 Like

hi TotallyInformation,
thank you for help me.
One quick solution for measuring out of range resistors is to place one known resistor in parallels for lower the total resistance, So what command do I use to show the resistance of the heater.
sorry english is not my native language.

I am not sure exactly what you are asking. Do you mean you do not know what mathematical formula to use to calculate the heater resistance when you add a known resistor in parallel with it? If so then that does not seem to be a node-red question.
Alternatively are you saying that you know the formula you want to apply, but don't know how to do it in node-red. If it is that then tell us the formula you want to apply.

In addition, do you realise that the resistance of a resistive heater will change with temperature? The resistance will rise as it heats up.

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