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I'm in search of a cost-effective solution for monitoring the current draw in a resistive heating element using a PIC.

All of the Hall Effect sensors that I've looked into seem to output the same AC voltage supplied to the heater but with a DC offset, where the offset is proportional to the current draw through the element. I don't think this will be useful to my PIC and, if it is, I don't know how to condition that output to make it so my PIC can understand it. Sure, a current transducer would be perfect, but I'm not going to put a $20 component on a PCB that costs me <$10 to make.

The closest solution I can find is a voltage/current threshold sensor (ACPL-K376) which kind of works but takes away any control I hope to have with the PIC because it wants output a digital signal based upon a predetermined threshold.

Right now I only have two resistive heating elements. One is rated at 1835[W] and the other at 2200[W] on 115VAC both with +/-5% tolerance but I want a solution that would be customizable if other heating elements are introduced in the future.

Does anyone have any recommendations on a better solution that would cost around $5 or less?

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  • \$\begingroup\$ Unrelated to the question, but I hope you're aware that, at least if you're in the US, neither of your heating elements can legally be used at full rated power in any product you hope to sell. (I'm assuming from the low costs you're citing that this isn't the kind of thing you'd be putting on a special high-current circuit.) They'd also trip the breakers on your standard 15-amp household circuit. \$\endgroup\$
    – Hearth
    Jun 8, 2021 at 15:03
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    \$\begingroup\$ Why not use a current sensing resistor? \$\endgroup\$
    – Finbarr
    Jun 8, 2021 at 15:15
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    \$\begingroup\$ So you are asking someone to design a precision rectifier and low-pass filter for you? Have you tried looking up circuits? \$\endgroup\$ Jun 8, 2021 at 15:18
  • \$\begingroup\$ Hearth - Thanks for the comment. I'm actually only pulsing 120V to the heater (3 seconds or less) based upon the feedback of a thermistor. Finbarr - I'm going to have to explore this option more. I understand the concept but will need to look into it further. Thanks for the tip! Spehro Pefhany - I actually looked into that initially but i ended up abandoning it when I was having trouble getting the circuit to function and began exploring pre-made options like Hall Effect sensors. Perhaps I need to revisit this kind of solution. \$\endgroup\$ Jun 8, 2021 at 15:55
  • \$\begingroup\$ Thanks everyone for the comments. \$\endgroup\$ Jun 8, 2021 at 15:57

1 Answer 1

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Use a current sensing resistor. Let's say you choose one with 1 Ohm. For each amp going through it you will read 1 volt. (V=IR) Feed the the voltage across it into an analog in on the PIC and you can set a threshold that will shut off the system when you want. You have to select a current sensing resistor that doesn't interfere much with your system. One ohm is just used as an example.

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    \$\begingroup\$ Kate, your solution completely ignores any safety given that this is a mains monitoring circuit. The OP appears to understand the need for electrical isolation such as provided by a Hall current sensor. -1, I'm afraid. Don't forget you can edit your answer at any time to improve it. \$\endgroup\$
    – Transistor
    Jun 8, 2021 at 16:33
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    \$\begingroup\$ Safety is always my highest priority. You can use appropriate measures to insure safety AND use a current sensing resistor. That's how it is done in industry. Are you saying it is impossible to use a current sensing resistor on mains voltage safely? \$\endgroup\$
    – Kate Moon
    Jun 8, 2021 at 17:20

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