Hey, All. While testing an IRFZ44N MOSFET to switch a load I tested the current drawn by the load directly from the PS. I got a reading of 1.95A. I then connected the load with the MOSFET and measured the current when the Gate was On with 5V and got 1.75A. Knowing that the IRFZ44N is supposed to have 10V at the Gate, I hit it with 12V at the Gate (the load is supplied with 12V so that voltage was handy). The current didn't change, still 1.75A
This may be a stupid way of checking, I'm not awesome with electronics: I divide 12 by 1.95 to get Ohms...6.15. If I divide 12 by 1.75 the resistance is roughly 6.85 Ohms. That's roughly 0.7 Ohms when the Rds is supposed to be around 17.5 milliohms when On. The load is an LED worklight, if that matters.
Truth in advertising: the IRFZ44N was bought from Amazon because I could get them fast. Perhaps the quality is garbage. Or my understanding is garbage...or the calculations I did were silly.
The IRFZ44N didn't get hot during the few minutes it was powered on and the brightness of the worklight wasn't appreciably reduced. Still, I'd like to understand why I'm seeing a current reduction across the MOSFET which is higher than I believe it should be.
Why am I seeing such current reduction and did I do something stupid?
Second: I'm an idiot. Pretty sure. While it's "only" 2A...there are two DuPont wires in the test setup for the MOSFET. Each of them are approximately 4". I tried to measure their resistance but my DMM only has 2k as the lowest range and reads 0. If I understand Ohm's Law correctly: 12v / 0.2 A (the "lost" current) = 117 Ohms. Cheap, Chinese DuPont wires, roughly 2A current across angel-hair wire in sketchy, mass-produced connections...possible.
I re-connected it all but with cheap, Chinese, angel-hair wire in sketchy alligator clips, eliminating the DuPont wires and breadboard, and the current went up to 1.88A roughly. I believe you nailed it: voltage drops across poor connections. I didn't consider that.
The easy way is to measure volt drop across connections or at both ends of a piece of wire.
That is far more acurate than trying to measure close to zero ohm with a DMM.
Most multimeters are not good at measuring below one Ohm.
Leo..
That would surprise me not one bit. Gotta watch Chinese wire...CCA is I believe how they label it. "Copper clad aluminum". Utter garbage. If it gets wet the clock starts ticking as it goes from "wire" to "dried toothpaste". Pro tip: dried toothpaste is brittle and doesn't conduct electricity. I've had to remove a few runs from trailers when I learned this painful lesson.