I try to switch a 12V non-inductive load drawing 400-500mA with a IRF3708 Logic-Level-MOSFET with the 5V from an Arduino.
I am new to MOSFETs but I simply cannot find my mistake. I think I fried some MOSFETs using the 5V because Rds(on) was too high (3-4Ohm) and this resulting in too much heat which killed the MOSFETs. I hope this conclusion is correct.
But the datasheet states that at 4.5V Ugs Rds(on) should be in a 9.5-13.5mOhm range. Even if it may be optimistic I don't get why it is in the Ohm range and not even near the values in the datasheet.
What am I understanding wrong here?
Schematic:
simulate this circuit – Schematic created using CircuitLab
Circuit:
Calculation of Rds(on)
With a load of 1k my Amperemeter gives me a current of 11.8mA @12V with 5V applied to the gate. This equals a series resistance of 1.0166kOhm. This results in a Rds(on) of 1.66Ohm. Ok it is lower than my wrong measuring but still a lot higher as in the datasheet. If 500mA would flow like in my application this Rds(on) gives an UDS of 0.664V which result in 265.6mW of heat at the MOSFET. Correct?
Best Answer
Your circuit is correct. Your way of measuring the RDSON is not.
This is good to see that the circuit is generally working, but you don't have anywhere near the accuracy to declare the RDSON result you do. How accurate is the supply? How accurate is the resistor? What is the accuracy of the ammeter? What is the resistance of the ammeter.
Note that small errors in the first three cause large errors in the RDSON conclusion. And the resistance of the meter is likely significantly higher than RDSON. Even if the other numbers were accurate enough, all you'd really be doing is measuring the meter resistance.
For example, let's work backwards what some of your measurements would need to be to indicate 0 RDSON. The supply might only be 11.8 V. That's only 1.7% below spec. Is your supply really that accurate? Right. I didn't think so. Or the resistor might be 1.017 kΩ instead of 1.000 kΩ. That's only 1.7 % high. What range of current is really running thru the ammeter when it says 11.8 mA? A combination of errors under 1% is all it takes to make the computed RDSON actually come out negative!
Your measurements don't support your conclusions.