arduinomicrocontrollermosfettransistors
I'm looking to drive a magnetic door lock from an Arduino. I've found a question about driving a solenoid from an Arduino, which includes a circuit that looks perfect for this kind of situtation:
What I don't understand is how to select a MOSFET for the job. What properties should I look for, if I know my logic level, device voltage and device current?
In this case it's 5V logic, and the load runs at 12V / 500mA, but it'd be nice to know the general rule.
The Atmega328 provides six power saving modes, ordered from minimal to excellent (estimated current consumptions from this forum post):
Quoting the original question:
I figured I could put
sleepcalls for certain times inside the loop method"
You would need to use sleep_cpu() after setting up the sleep mode you require, from the list above. The Arduino Playground has a useful post about this.
The application needs to be interrupt driven, use the above sleep modes extensively, and wake the processor up on button push, timer overflow and watchdog timer events to actually execute tasks.
Additional power savings can be obtained through the following steps:
Incorporating these suggestions allows for running microcontroller applications for weeks or months on a single CR2032 coin cell, and years on a LR123 type lithium cell. Of course, your mileage may vary depending on what sensors, outputs and actual processing your application requires.
Some useful references:
After receiving IRF540N from a reputable seller I can definitely confirm that the ones I was originally using are counterfeits.
After replacing fake one with a genuine one I got Vds=85mV on the red channel. What I was not expecting though is that the genuine FET got hot after a minute or so. And then I realized that those FETs are not generating much heat themselves but rather get heated up (and quite a lot) from the breadboard and the wires (Connor Wolf mentioned it). Short wires connecting FET's source to GND are screaming hot when this is in full ON state. Moving FETs off the breadboard confirmed that the source of the heat was the breadboard/wires. Fake one was getting hot but I could actually cool it down just by touching it. Genuine one was somewhere between the room temperature and luke warm. Btw. measuring Vds directly on FET pins vs measuring it 1cm away on the breadboard made around 200mV difference (85mV on pins, 300mV on breadboard).
Here are some pictures, fake on the left, genuine on the right and manufacturer's part marking on the bottom:
Although there are more IRF package markings possible as shown in this document I could not find any similar to the fake one (which only supports that this is a counterfeit). Also the cutouts on the top of the back plate are rectangular vs round on the genuine and in the spec.
Thank you guys for all your comments! The circuit now works as expected (PWM included).
Best Answer
You've got a luxury problem: there are thousands of FETs suitable for your job.
1) the logic level. You have 5 V, and probably less than 200 mV or so when off. What you need is \$V_{GS(th)}\$, that's the gate's threshold voltage, at which the FET starts conducting. It's given for a specific current, which you want to keep an eye on too, because it may be different for different FETs. Useful for you could be maximum 3 V @ 250 µA, like for the FDC855N. At 200 mV (or lower) you'll have a leakage current much lower than that.
2) Maximum \$I_D\$ continuous. 6.1 A. OK.
3) the \$I_D / V_{DS}\$ graph:
This one's again for the FDC855N. It shows the current the FET will sink at a given gate voltage. You can see that it's 8 A for a 3.5 V gate voltage, so that's OK for your application.
4) \$R_{DS(ON)}\$. The on-resistance determines the power dissipation. For the FDC855N it's maximum 36 mΩ at 4.5 V gate voltage, at 5 V it will be a little less. At 500 mA that will cause a 9 mW dissipation. That's more than good enough. You can find FETs with better figures, but there's really no need to pay the extra price for them.
5) \$V_{DS}\$. The maximum drain-source voltage. 30 V for the FDC855N, so for your 12 V application OK.
6) package. You may want a PTH package or SMT. The FDC885N comes in a very small SuperSOT-6 package, which is OK, given the low power dissipation.
So the FDC855N will do nicely. If you want you can have a look at Digikey's offering. They have excellent selection tools, and now you know the parameters to look out for.