First PCB design, MOSFET shield for Teensy
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Nathan Roberts 2 days ago
Greetings fellow PCB enthusiasts. My project can be found at [https://oshwlab.com/quickstar77/a-bunch-of-mosfet](https://oshwlab.com/quickstar77/a-bunch-of-mosfet) [https://easyeda\.com/editor\#id=\|bb97d55e8eb845549517a8ae0046d747\|4249db11c9f44a77883ffddf7457e06e\|2f5c6bc133cb48c2809af2b4e8f8a3cb](https://easyeda.com/editor#id=|bb97d55e8eb845549517a8ae0046d747|4249db11c9f44a77883ffddf7457e06e|2f5c6bc133cb48c2809af2b4e8f8a3cb) It's purpose is to allow me to control simple LED lights based on a 3.3V High from a Teensy 4.0. I took a MOSFET circuit example and multiplied it by 12.  Then I added RJ-45 Jacks to act as power cable to a remote LED. My questions are; Am I using the Resistors Correctly? Does a design this simple need to worry about capacitors? For those who've designed functional circuits before does anything jump out as being "wrong"? I have a functional prototype of this design but I used a different FET so I picked out this one based on its similarities to the one I was using but I might've missed a detail. Thank you -Nathan
Comments
andyfierman 2 days ago
"Am I using the Resistors Correctly?" If you are driving the gates from 0V to +3.3V then the effect that the 10k series resistors is to limit the current pulses into and out of the gate capacitances on rising and falling gate voltage transitions. OTOH, this will increase the turn on and turn off times of the MOSFETs. Depending on how much current and from what voltage this is being switched, this could result in momentatry excessive power dissipation in the MOSFEs during these transitions. The 1M resistors will help hold the gate source voltage at zero when the board is unpowered. OTOH, that is largely unnecesary since the gates of mosfets are usually protected against static and voltages from high impedance sources above about +/-20V by internal zener diodes. "Does a design this simple need to worry about capacitors?" That depends on how fast, how much and from what voltage the LED current is being switched. What is supplying VCC? How far away from the shield is it? At what frequency are the LEDs being switched? Is there a possibility that all the LEDs could be switched on or off at the same time? These all feed into considerations of how the supply for VCC will react under these load conditions and how much RF interference the board may export.
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Markus_ee 2 days ago
Hi! It is almost always advisable to insert a "tank" capacitor and a few ceramic 100nF capacitors. Tank capacitor should be placed near the plug connector and 100nF ceramics right beside RJ-45 connectors. I guess 100uF - 220uF electrolytic is enough as a tank capacitor. Regards, Markus Virtanen HW / Electronics Designer
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Nathan Roberts 13 hours ago
Thank you both for your feedback, I will add the capacitors as suggested by Markus.  As I'm only expecting to use about half an amp on any given channel at 12V it doesn't sound like the resistors are even really necessary.  I used as a starting point the schematic of something that Sparkfun sells.
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andyfierman 13 hours ago
@quickstar77, It's the other way round: at 12V and 0.5A there would be no disadvantage in leaving the resistors in the circuit. Since there are 12 channels, if all of them switch on at the same time or off at the same time, that will be a total current change of 6A. Make sure that whatever you use to supply VCC - and the wiring - is able to cope with not just 6A but a possible step transition of 0 to 6A and 6A to 0.
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Nathan Roberts 13 hours ago
@andyfierman Thanks for clarifying, I hate it when I understand things backwards.
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