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TOPIC: Current Sensing - Best Approach Advice Needed

Current Sensing - Best Approach Advice Needed 4 years 1 week ago #17365

  • hop
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Thank you gentlemen. I will try all these options tomorrow on my day off. I need to re familiarize myself with some free schematic authoring software so I may post some circuits here for your review. I know you guys are busy, but it would be cool if I saw what you were talking about in a schematic. I am having trouble getting my head around what you said Jon, but I will breadboard it and get out my meter and check it. And Baldor, I thought the zener was inline and not across or parallel with the resistor. Shows you how much I know. :(

Current Sensing - Best Approach Advice Needed 4 years 1 week ago #17366

  • Jon Chandler
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Your wish is my command :) I thought about posting some pictures but I was too sleepy last night.

So here's a basic LED circuit. The voltage across the LED is always the forward voltage of the LED. It varies with chemistry (i.e., color), current and temperature. R is selected so that the current is within the range of what the LED can handle, say 10 mA.



This graph shows the typical voltage vs current curve for a red red. Vf is around 1.9 volts.



This graph shows typical forward voltage vs. color. Check the actual LED to be sure,



The voltage also varies with temperature a little. Probably not enough to matter for this.

Hope this helps.
Last Edit: 4 years 1 week ago by Jon Chandler.
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Current Sensing - Best Approach Advice Needed 4 years 1 week ago #17367

  • Baldor
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Hop, the zener will not be needed if you use a Led as Jon proposes. If you use a resistors voltage divide, the zener goes betwen the "tap" and ground, reverse biased.

But I was talking about adding a normal diode paralel to the voltage divider. The motor is an inductive load, and a dirty one. Inductive loads can do nasty things when you turn them off. The circuitry controlling the pump shuould already have this diode, but, who knows?

Something like this:





(Sorry, the picture is not loading. I will try again later)
Aprendiz de mucho, maestro de casi nada.
Last Edit: 4 years 1 week ago by Baldor.
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Current Sensing - Best Approach Advice Needed 4 years 1 week ago #17368

  • Jon Chandler
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This is what Baldor was trying to show, which I plagiarized off the web. Put the diode as close to the motor terminals as possible.


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Current Sensing - Best Approach Advice Needed 4 years 6 days ago #17369

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It would seem that the IRF540 N-Channel ENH MOSFET has a zener built in. I am curious about this though, since the diode is across the source and drain, where the circuits you guys posted is across the terminals of the motor, or across drain and +V.

What I apparently did, and it worked without blowing anything up, was use a diode inline between drain and the motor ground. :( It was either wired that way in a circuit I found (can't find now) or I made a mistake.

Anyway, I was just curious of the implications of the two wiring schemes. Is external diode even necessary on the mosfet considering that one is built in? And is that protection enough if there is no diode also across the terminals of the motor? As Baldor said, the motor most likely has one built in as part of the control circuitry, but not certain.

Current Sensing - Best Approach Advice Needed 4 years 6 days ago #17370

  • Jon Chandler
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I don't believe the zener in the FET will do anything to protect against the inductive kick generated by the motor when it's switched off. You need to provide a path for current to flow as shown in this diagram.



I think you posted some info about the motor but I don't see it now. Does the motor actually have "control circuitry" or is it just a DC motor? Since you're switching it with a FET, I assume it's not controlled by a logic level input, and it's just a bare motor. Even if it's not, adding the diode directly across its terminals is cheap insurance. Given your belt and suspenders and duct tape and staples approach elsewhere, saving a few cents on a diode doesn't make sense.
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