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TOPIC: Current Sensing - Best Approach Advice Needed

Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17359

  • Baldor
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I will divide all the way down to arround 3V, and if you want, use the zener as protection. Look what is the logic high for the ESP and set the final voltage a little higer. You will need a 3/1 proportion between up and down resistors.
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Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17360

  • hop
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Baldor wrote:
I will divide all the way down to arround 3V, and if you want, use the zener as protection. Look what is the logic high for the ESP and set the final voltage a little higer. You will need a 3/1 proportion between up and down resistors.

OK well this is what stumps me and forgive my analog newbness...

To do the voltage divider, I pick the ohms of the resistors based on the amount of current I will need for the tap right? So if I am just looking for a reference voltage and not driving anything except an input to a MCU, then I can get away with using... say... 1meg ohm resistors right? At least at the first stage of the divider? Or a higher value? Say 10m ohms? The reason I ask which is probably obvious to you is that I do not want to waste and dissipate any power, or maybe it happens anyway but at a very small level, so can I get away with using 1/8 watt resistors?

You say 3/1 proportion but in my head I see it as 4/1 since 3.3 is 1/4 roughly of 12v. Would it be say 4 1m resistors in series with me tapping after the first resistor from ground? Or should I go two resistors, tap at 6vdc, then go two more and then tap at 3v?

I haven't looked it up, but I am guessing the high threshold of the MCU is around 2v, like 2.1 or so, so 3v should be more than enough. And I will put a 3.3v zener in there for protection to prevent an issue where, for some reason, the source 12v spikes high enough to divide down to over 3.3 at the stages I mentioned.

Thank you Baldor!! I understand that electricity flows like water does, and get most of the theory. I just don't trust my makeshift knowledge of analog.

Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17361

  • Baldor
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Remember you will need some current to drive the input pin high. Very low, but some. See the datashet.

Say you use a 10K Ohm up and 3.3KOhm low for 3V at the "tap". Ohms law, and you see you can have 0.9mA at the "tap", the up resistor will disipate 8mW. The low resistor will disipate 2.7mW. Negligible compared with the pump, and you can use 1/8W resistors. Forget the MOhms. :-)

Once you understand the formulas and concepts, just forget the formulas, and use online calculators.

Voltage divider

Ohms Law
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Last Edit: 4 years 5 days ago by Baldor.
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Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17362

  • hop
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Baldor wrote:
Remember you will need some current to drive the input pin high. Very low, but some. See the datashet.

Say you use a 10K Ohm up and 3.3KOhm low for 3V at the "tap". Ohms law, and you see you can have 0.9mA at the "tap", the up resistor will disipate 8mW. The low resistor will disipate 2.7mW. Negligible compared with the pump, and you can use 1/8W resistors. Forget the MOhms. :-)

Once you understand the formulas and concepts, just forget the formulas, and use online calculators.

Voltage divider

Ohms Law

Right on, and thank you for the online tools!!

Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17363

  • Jon Chandler
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If the motor operates from a regulated supply, which the switches should be, why not a voltage divider calculated to deliver 3.3 volts? You could add a Zener as a "safety" across the resistor if you want to be extra sure. Actually, I've look at the the minimum voltage level considered to be "high" and aim for midway between that and your supply voltage.

Here's an even better approach that might work. Let's say the minimum voltage voltage for a high signal is 2v. If you connect a resistor from the 12v to and LED, and the LED to ground, the LED will clamp the voltage at the plus side of the LED to Vf of the LED – about 2.2 for a red LED at 10mA, higher for a blue or white. Now you have a run indicator and an input to the micro that's clamped to Vf even if the 12v source varies. Note that there is some variation of Vf with temperature; it's good enough for this task, not good enough to be a voltage reference.
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Current Sensing - Best Approach Advice Needed 4 years 5 days ago #17364

  • Baldor
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I like the led option a lot! :-)

Also, wedon't know what protection the motor have. A reverse biased diode across the voltage divider could be a good idea. (The supply/controller of the motor shold have it, but, who knows.)
Aprendiz de mucho, maestro de casi nada.
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