Industrial Control: ADC Chip and Heater

Casey_SCU

I am using the parts from the industrial control kit. I have a heater hooked up and the output of the LM34 temp sensor goes to the ADC chip. I have several other logics gates on the board used to implement a state machine. The problem I am having is that the output of the ADC goes crazy when the heater turns on. If the heater turns off again, then the output goes back to normal.

When I probe the input to the ADC (output of temp sensor) I get the right reading. I have tried a few ADC chips and the same thing happens. I am using the exact circuits in the industrial control book (page 103ish)


Also, I am wondering what the capacitor between pin 2 and ground is on the ADC circuit diagram.


Thanks!!

Franklin

The cap is to filter any ac noise from the line which yours isn't doing a good job of apparently. The heater should be on it's own supply and as well isolated from the adc as possible. I don't know the circuit so I will just say I would use an optoisolator between the stamp and the heater and make sure there is no leakage between the heater and the lm34.

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- Stephen

Martin Hebel

When writing the book, we were tasked to keep it 'simple' and inexpensive and safe.. not easy when dealing with process control issues. Using a 100 ohm resistor for a heater powered from 12V or 9V is for illustration, but it does draw significant current causing loading of the power supply. The fluctuation in the measured temperature were noted, and the capacitor helped with some noise, but it continues to be a problem. You may get a little better reading with a larger power supply,

As Franklin mentions, a seperate supply would help, and even without the opto-isolator. Simply power the resistor to the collector from another power supply's +V, but share a ground between the two. This will help with the loading of the BASIC Stamp's power supply.

Even better, pick up a solid-state relay discussed in the book and use an aquarium and a 120V light bulb as your heater [noparse]:)[/noparse] Just don't try the PID control with this... It won't handle the PWM.

-Martin

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