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In my previous question I asked about a simple topology suggestions for regulation of highish currents(around 10A) especially for resistive heating elements in 1...10 Ohm range. I concluded to use a MOSFET controlled via a PWM signal from microcontroller since PWM duty cycle is linear with the power of the heating element(I guess same linearity for the temperature).

And now after I made some readings about the importance of the current being measured, I would like to measure the current through the heating element and send this information back to the microcontroller. So I decided to measure the voltage across it and then divide this to the resistor value(neglecting heatin element's tolerance) and have an estimation for the current so forth and so on.

Before soldering this on a perfboard I wanted to have your opinions for the below circuit I modified so far:

enter image description here

And before going in detail here is LTspice simulation plots for the current through heating element, MOSFET's instantaneous power and average power and the final voltage output to the MCU input:

enter image description here (left-click to see it better)

I want to go from left to the right:

The control signal is a 0...5V 490Hz PWM signal from a microcontroller pin.

A 12V 10A SMPS power supply will be used for a max 0...8A PWM current through the element. C4 and C1 are for noise in the supply.

Q1 Q2 are inverting logic. This is because I want MOSFET to saturate when control signal is ON. I could also use a logic MOSFET but I don't know if IRL3903 is a logic MOSFET from the datasheet. I don't know which parameter makes a MOSFET a logic MOSFET. Is that something to do with gate to source voltage being less than 5V? I have no idea so I decided to use this NPN inverter driver to make it usable for any power MOSFET.

In some examples I saw there is a 20k resistor between the gate of the MOSFET and the ground. In simulation I didn't find it necessary but I'm not sure if needed.

And for the measuring part I decided to use an an LM358 in a difference amplifier configuration(I'm not sure in practice it is a good op amp for this application). The voltage across the heating element is too high for the microcontroller and it is not pulled down to the ground because of the voltage drop across the MOSFET. If I shunt it between the source and the ground then I get less current. So I needed a negative gain meaning that 12V to less than 5V. So I obtain a gain of R4/R1=R3/R2=0.33. The reason I didn't use an inverting op amp configuration is I use a single power supply and inverting op amps have low input impedance. And I also didn't use a non-inverting op amp because as I mentioned then I have to shunt the heating element between the source and the ground.

One of my question is the gain I'm aiming is R4/R1=R3/R2=0.33. But the resistors should be matched. What does that mean? Would 1% tolerance resistors be enough for roughly measurements here(since I'm not measuring mV level small voltages)?

And finally I added a 4.7V zener for precaution to the MCU input. (Would it be good to add a 100Ohm 100n lowpass filter right before this?)

I would be glad to have your opinions or any major mistakes with this circuit.

EDIT:

I was suggested not to use a difference amplifier and a single ended op amp config instead together with a mOhm level low resistor. I tried to use a non-inverting config. since I have a single supply.

But I'm getting distorted output if I use a non-inverting configuration. Here is the modified circuit with LM358: enter image description here And the plots for input(non-inverting input/blue plot) and the output of the op amp: enter image description here

And here is similar issue with another opamp LMC6482:

enter image description here

Basically the gain must be (1 + R3/R7).

But the PWM signal is distorted(like a lowpass filtered pulse) and it doesn't go to ground when the input is zero.

How can I fix this problem?

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    \$\begingroup\$ I would think of putting another very small current sensing resistance to ground, measure over it and then you wouldn't need a differential amplifier \$\endgroup\$ – Claudio Avi Chami Apr 22 '17 at 19:58
  • \$\begingroup\$ Why do you think it is a bad idea to use difference amplifier? Is that because matching will cause trouble? \$\endgroup\$ – HelpMee Apr 22 '17 at 20:00
  • \$\begingroup\$ Because it is always safer to try the simplest solution - KISS - Keep it Simple and Stupid \$\endgroup\$ – Claudio Avi Chami Apr 22 '17 at 20:01
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    \$\begingroup\$ The 'ground' of the measurement will keep jumping from zero to another value, the resistance matching IS sketchy, and why do it difficult when you can just sense from GND? Sensing relatively to GND is done in 99% of the designs I saw. Do not complicate your designs for no reason. Do not underestimate the difficulties of a big (and varying) common mode to your sensing amplifier. \$\endgroup\$ – Claudio Avi Chami Apr 22 '17 at 20:10
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    \$\begingroup\$ @doncarlos then it may just be voltage offset. You may need a two state amplifier. Measuring tiny voltages is hard. \$\endgroup\$ – Trevor_G Apr 23 '17 at 18:41
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I'd change that to switch the high side.

  1. You do not want the wire sitting live when you don't need it to be.
  2. It let's you measure the voltage across the wire single ended.

I'd use a separate micro-ohm shunt resistor to measure the current though, some thing that does not get so warm and change resistance with current. Or for this, a different sensing approach that is not so sensitive to the heat.

I'd also drop that frequency lower, sub 100Hz, even then that wire will sing a bit, but it won't be as noticeable as your current ~1kHz.

That op-amp is not rail to rail either, I'd suggest a LMC6482 or similar.

Zener should be fine provided your op-amp drive is less than the zener current.

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  • \$\begingroup\$ I made an edit. I took the mosfet switch to high side. I tried to use non-inverting configuration with a milliOhm shunt resistor. Please see my edit. I added the details. Output of the opamp in this case is distorted and does not pulled to the ground. Any idea about it? \$\endgroup\$ – HelpMee Apr 23 '17 at 16:49
  • \$\begingroup\$ @doncarlos that likely because the top MOSFET takes longer to turn on and off. That is the issue with top drivers, you need to drive them harder. Also a good reason for a slower clock. 100hz is all you really need for this. \$\endgroup\$ – Trevor_G Apr 23 '17 at 17:09
  • \$\begingroup\$ Yes you are right the distortion was because the PWM freq. was high was set to5kHz. It is sharp now at 100Hz. But I still get around 50mV when switch is off. Output is not totally pulled to ground. You can try it I really dont get why. \$\endgroup\$ – HelpMee Apr 23 '17 at 17:21
  • \$\begingroup\$ @doncarlos in fact in this application, those slower edges are a good thing, they will reduce the EMI generated. Just reduce your frequency so the mosfet does not cook. \$\endgroup\$ – Trevor_G Apr 23 '17 at 17:21
  • \$\begingroup\$ @doncarlos that op-amp is not designed for rail to rail so it will never go to zero. Either power it with a +- supply or switch to a rail-rail op-amp. Stepping out to rake the garden.. back later \$\endgroup\$ – Trevor_G Apr 23 '17 at 17:23

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