Hej,
is there anything problematic with the schematic (MOSFET, PSU)? Depending on data received via LoRa, I want to change a 104mm long heating pad's temperature between room temperature and 40°C.
Thanks!
Two quick thoughts.
- all four grounds should connect at one physical point near the powersupply
- the 5V dc to the nano must connect to the 5V in, not the vin. (check this I'm not familiar with the nano, it's this way on the pro mini).
JohnRob:
Two quick thoughts.
- all four grounds should connect at one physical point near the powersupply
- the 5V dc to the nano must connect to the 5V in, not the vin. (check this I'm not familiar with the nano, it's this way on the pro mini).
Identical on the Nano!
Paul
Ok, thanks!
The Arduino Nano schematic shows only Vin available for non-USB DC-input, so it should be good, I assume. Physical distance from PSU to Arduino Nano is 70mm so that should be ok for the grounds, it's just a schematic so I know what goes where, by no means a physical layout.
The schematic looks fine but I would think about using a logic level MOSFET like a FQP30N06L or similar designed for a 5 volt logic level gate signal and I would add a 10K resistor gate to ground. This makes sure when it's off it's off. While PWM is not the best solution for heating applications you may find a sweet spot where you get your desired results. Overall use a logic level MOSFET and add 10K Gate to Ground.
Nice schematic! 
Ron
Thanks, yeah, I saw that 10K G-G resistor in some schematics and not in others. Properly off sounds better than "sort of kind of like off". I wanted to use that MOSFET simply because I have it around and it worked well with driving solenoids or PWMing 12V pump motors. Why is it not quite the right thing compared to the one you suggest? Always happy to learn something...
The issue is the device on resistance verses gate voltage relationship. The specified on resistance of the IRFZ44N is 17.5 milliohms with 10 volts on the gate. The device has NO specification for the on resistance at anything below 10 volts. The threshold voltage of 2.0 to 4.0 volts is the range where the device begins to conduct drain to source, it’s not a usable operation point.
In a nutshell, there is no guaranteed way to know what the on resistance of the device will be with 5V on the gate. It’s an ancient part and the five for a dollar units you find on eBay are typically junk surplus or outright counterfeits. They might work with an Arduino at 5 volts of gate drive but if you push any appreciable current through them, they’ll let their magic smoke out.
The power dissipated by a mosfet is equal to I^2 * R. Given that you cannot predict what the R (on resistance) will be, you cannot predict how much power the device will dissipate for your application. Too much and the smoke is released.
If you take your heater of 0.74 amps with the FQP30N60, the on resistance is about 0.031 ohms so it’s dissipating 0.740.740.031=0.017 or 17 milliwatts. Absolutely no heatsink required. But what happens if an IRFZ44N turns on at 1 ohm? That’s 0.54 watts and enough to heat it up. Shove 2 amps through it and now you’ve got 4 watts. That will burn your finger in short order and give it enough time, the smoke will be released.
It’s fairly easy to test the on resistance of mosfets with two lab power supplies, DMM and an appropriate load resistor. One supply controls the gate voltage, one controls the load. You calculate the resistance from the current and the voltage across the known resistance where R = E / I. Run the gate voltage through the expected range and you can watch the effect directly in the Drain-Source current. You manage the power dissipation with the current limit of the D-S power supply so you don’t let the smoke out.
If you’re just knocking about and smoke can be exciting, no worries, have at it. If you’re not about drama and frustration, buy the proper parts, which are logic level mosfets where you can lookup the Ron value from the gate voltage on its datasheet - from a reputable dealer.
Have a look at a logic level device datasheet, specifically figure 3 on page 3.
If you look for something similar for the IRFZ44N, you will not find it.
I do believe that covers it well. Any questions just ask.
Ron
Ok, WhatsThat, thanks for the detailed explanation regarding logic level MOSFETs.
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