Adafruit AD8495 Reading Incorrect Temperature

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wgustafson
 
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Adafruit AD8495 Reading Incorrect Temperature

Post by wgustafson »

I am working on a project that involves sensing voltage, current, and temperature of a large barrel of water that is being heated by a resistance heater. I am measuring the voltage using a voltage divider that is read through A0 on the Arduino Uno. I am measuring current using a DC CT that outputs a voltage between 0-5V, which goes into A2 of the Arduino Uno. Lastly I am measuring temperature using the Adafruit AD8495 k type thermocouple amplifier, which outputs a voltage, which can be then translated to a temperature. This voltage from the AD8495 goes to the A4 analog input on the Arduino Uno.

The DC CT and AD 84955, as well as a solid state relay are all drawing power from the 5V pin on the Uno.

The issue I am having is that when the circuit is turned on and I start measuring voltage and current, the temperature reading spikes by about 10 degreesF and maintains this high reading. It seems to be working fairly correctly and gives reasonable temperature readings when the resistance heater is turned off and only the temperature sensor is getting a reading.

I have three pins on the AD8495 wired. Those being 5V to Vcc, GND to GND, and Vout to A4 on the Uno.

The voltages and currents I am sensing are around 30V and 30A DC. Not sure if that makes a difference or not.

Any ideas on why this temperature spike is occurring or how I could avoid this? I would really like to have an accurate temperature reading using one Arduino Uno for all three readings, which I am trying to make simultaneously.

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adafruit_support_bill
 
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Re: Adafruit AD8495 Reading Incorrect Temperature

Post by adafruit_support_bill »

A photo or diagram of how everything is connected might be helpful.
The DC CT and AD 84955, as well as a solid state relay are all drawing power from the 5V pin on the Uno.
Measure the voltage on the 5v pin and see if it changes when the heater comes on. Any change in the analog reference voltage would affect your temperature readings.

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wgustafson
 
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Re: Adafruit AD8495 Reading Incorrect Temperature

Post by wgustafson »

Are you saying that the power and analog reference voltage are one in the same? Also would a change in the amount of supply current affect the readings, even if the supply voltage stayed the same?

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Re: Adafruit AD8495 Reading Incorrect Temperature

Post by adafruit_support_bill »

By default, the UNO used the 5v rail as the analog reference. This can be a problem in applications where there are significant 5v loads. The 3.3v pin tends to be more stable. You can use that as your analog reference - with some tweaks to your temperature calculations.
https://www.arduino.cc/reference/en/lan ... reference/

A dip in the supply voltage usually does not have much impact unless it falls below the dropout voltage of the 5v regulator - which is around 7v.

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