21–30 of 169 posts
Mechanical relays are not obsolete. They can have lower resistance and capacitance than solid state equivalents. A good relay is a very close approximation to a plain wire when closed. Relays are still commonly used for RF switching.
MOSFETs do have some problems. They can draw large gate currents for the first few nanoseconds of turn-on, which can overload whatever is driving them. The gate input is very vulnerable to electrostatic discharge during handling. Device failure tends to be into the ON state.
I just went through a big struggle with MOSFET selection for a special purpose switching power supply. The big through-hole parts have too much gate capacitance and need too much drive at turn-on. Only in surface-mount could I get something that would work.
That's the real problem with antique parts. The new stuff is surface-mount only.
[1] https://www.fairchildsemi.com/datasheets/FQ/FQP4N20L.pdf
There is nothing wrong with bipolar transistors. They aren't antiquated. FETs have definitely overtaken them in power applications (for good reason) but it seems hard to displace them for small signal applications. Small signal FETs just aren't that plentiful or cheap. There are many instances where a small signal FET makes sense (level translators, etc) but the cost doesn't justify its use. Using a 2n2222 and the li…
The article seems mostly right when it's about interfacing logic circuits with loads that require even moderate currents. FET is king in this field. I've a little aquarium and I've noticed how fish always get scared when lights turn suddenly on or off. So I took an Arduino and wrote a timer app for it, using PWM to slowly drive the output up over 60 minutes (sunrise), keep it at max for a few hours, then drive it dow…
I would certainly use MOSFETs for lighting (a power application). I probably wouldn't design an analog front-end for a specialized sensor with them though.
Earlier quoted context omitted.
The article seems mostly right when it's about interfacing logic circuits with loads that require even moderate currents. FET is king in this field. I've a little aquarium and I've noticed how fish always get scared when lights turn suddenly on or off. So I took an Arduino and wrote a timer app for it, using PWM to slowly drive the output up over 60 minutes (sunrise), keep it at max for a few hours, then drive it dow…
What frequency are you PWMing at, and are you applying any analog filtering? I frequently get annoyed by slow PWMed lighting, and I wonder how other animals with possibly faster vision perceive it.
The frequency is either 500 Hz or 1000 Hz, I forget which (depends on Arduino type). I doubt there are any critters that can tell it's not continuous.
I build a lot of guitar effects, and that community is even worse when it comes to attachement to obsolete components. The discontinuation of J201 JFETs (in TO-92 through hole package at least) by Fairchild has caused a lot of upset. My perspective, not widely shared, is that JFETs in general have been made obsolete by depletion-mode MOSFETs, at least in audio circuits. Part of the reason, I think, is that a lot of "…
JFETs main application these days is for when you need very low noise, usually for precise measurements. A depletion mode MOSFET has similar behavior, but not specs.
Earlier quoted context omitted.
What frequency are you PWMing at, and are you applying any analog filtering? I frequently get annoyed by slow PWMed lighting, and I wonder how other animals with possibly faster vision perceive it.
No analog filtering - that's how you keep things cold all the time. The frequency is either 500 Hz or 1000 Hz, I forget which (depends on Arduino type). I doubt there are any critters that can tell it's not continuous.
Sigh. I like FETs as much as the next engineer but if you're going to write about them, try to not make things worse. In particular, this made me sad: the 10K resistor to ground isn't strictly necessary, but it ensures that the MOSFET remains OFF if the arduino is disconnected or it's pin is not an OUTPUT. I was helping a high school student with a transistor project and he was complaining that "half the FETs he boug…
If you want to switch power, MOSFETs are the way to go today. Voltage drop is near zero. Digi-Key has over a thousand MOSFETs in TO-220 packages. Here's one of the cheapest with a low gate threshold.[1] Probably good enough for most Arduino on/off applications. MOSFETs do have some problems. They can draw large gate currents for the first few nanoseconds of turn-on, which can overload whatever is driving them. The ga…
Let's say you wanted to do a special purpose switching power supply. Digikey doesn't have a column for gate capacitance (for the mosfet), or Q (for the inductor). All that manual work to read datasheets is terrible, especially if your design takes a little while to perfect and some of the components go end-of-life, so you have to replace them.
I personally like to graph the cost-efficiency curve so I actually have to tabulate lots of mosfets, inductors, and caps, then simulate them in a spreadsheet.
I know about the specialized design tools done by each manufacturer that will recommend parts and do a design for you based on your criteria. I want something that does a much broader search for parts though.
Maybe I should write one. :)
Sigh. I like FETs as much as the next engineer but if you're going to write about them, try to not make things worse. In particular, this made me sad: the 10K resistor to ground isn't strictly necessary, but it ensures that the MOSFET remains OFF if the arduino is disconnected or it's pin is not an OUTPUT. I was helping a high school student with a transistor project and he was complaining that "half the FETs he boug…