I put together this basic High Side Switching circuit and got it working at home with a breadboard and an LED (in place of an expensive audio interface - the eventual device: RME Fireface UC).
Before I actually connect it to the real devices I was hoping for a little reassurance that I've chosen the correct components (NPN, MOSFET and resistors) and nothing will get damaged in the process.
This is part of a larger project whose purpose is to manage the on/off status of audio devices for this show I work with. A deeper explanation is found on a recent thread of mine (MOSFET won't turn off as expected) I am here because of the awesome response I got from that question.
Basically the Arduino monitors when the MacMini powers up or powers down, and turns its related audio interface on or off accordingly.
Eventually there'll be another RME interface connected, as well as other sensors, but for now this smaller circuit is the one that I'd like to test at the show in the next couple of days.
I'd like to be sure the audio interface gets plenty of available current when it needs it, so any suggestions as to choosing better MOSFET or tweaking resistor values etc would also be greatly appreciated. I've watched tons of videos and read tons of tutorials in the last couple of days, and that has gotten me this far. Would love to go the next step, if need be.
And if there are any protective circuit ideas, even general ones, that would be great as well. I realize that is more difficult without the whole circuit, but I'll post the entire thing when I get the last few issues ironed out.
Otherwise, what I have here seems to work okay. Just trying to avoid a blue smoke moment.
Thank you in advance! paul
UPDATE: updated schematic is attached to response #2.
You have your MOSFET Gate [Q2] connected directly to +12V -- which means, it will ALWAYS be at +12V.
You need to move it to between R3 and Q1 -- i.e. to the Q1 Collector. Then it will be free to toggle between Ground and 12V.
I would, also, include a 10k resistor from U1 pin20, to Ground -- because when the itsy power's up, the ports will be in a high Impedance state, which means the input of Q1 will be allowed to "float" -- not as critical with a Bi-Polar transistor, but I have seen them conduct a little current through the Emitter-Collector junction, with a floating Base -- probably due to leakage currents.
Everything else looks good to me. The Gate drive is not as critical, because this is VERY low frequency switching.
UPDATE:
Just an FYI, the depiction of the protection diode, in Q2 is backwards. As it is drawn, the Transistor will toggle between it's Channel resistance, and the forward drop of the diode -- in other words, it will never, really, turn off. That doesn't effect the functionally of your circuit, just gives a misleading view of what's really going on.
BTW: That MOSFET is probably overkill, but I'm a fan of that. Almost certainly no need for a heatsink
The only consideration might be, if a less expensive part, that comes closer to the actual requirements, is available, an economical decision might be in order.
Fet is drawn upside down (but labling is ok). See fet datasheet.
Values of R3 and R2 should be swapped.
With the current values gate pull down current is way higher than pull up current.
Leo..
One other thing. I notice you're using a "9V 1A 'wall wart'"
If that's the ol' chunk of iron, unregulated kind of thing, then be aware that it will probably run at voltages higher [probably quite a bit higher] than 9V. Especially, since, the itsy will not come close to drawing 1A. So, there's a slight chance of exceeding the insy's max input voltage. BUT, with a 16V input upper limit, maybe yer OK
Wawa:
Fet is drawn upside down (but labling is ok). See fet datasheet.
Values of R3 and R2 should be swapped.
With the current values gate pull down current is way higher than pull up current.
Leo..
The FET is not upside down. The Drain-Source diode is. Again, not consequential to the thing functioning properly [since, in the actual device, the diode will be the right way around] -- just mildly confusing to those studying your diagram.
The FET is not upside down. The Drain-Source diode is
Sorry about the confusion, the symbol is from the Kicad 5.0 DigiKey library - apparently the diode is drawn incorrectly. I can't for the life of me figure out how to edit it, I should just create a new one. I'll try to do that after the show this evening (midnight NYC time).
BTW: That MOSFET is probably overkill, but I'm a fan of that. Almost certainly no need for a heatsink
The only consideration might be, if a less expensive part, that comes closer to the actual requirements, is available, an economical decision might be in order.
Love that, thank you for those thoughts! I work for a billion dollar company, they'll gladly keep me swimming in hefty MOSFETs if it keeps me happy...
Again, so grateful to you both for sharing your expertise and time.
larryd:
Make the collector resistor 1k, base ~390.
Why 390ohm for the base.
10k (0.43mA base current) is already overkill.
After amplification by the 2N2222 it's at least a collector current of 430mA (assuming a β of 100).
Already way more than the 12mA pull up current from the 1k resistor.
Leo..
I always (well most always) check the switching waveform with a scope.
With similar P MOSFETs and 12 volt supply, I have standardized on these resistance values and have found they cover most situations. There is no major disadvantage of doing so.
Q1 is not used here as a saturated switch, so doesn't have to be driven hard.
Controlling base current also limits collector current (that charges gate capacitance) to a safe value.
A 10k base resistor (4.3mA base current) is more than enough.
Leo..
larryd:
Lowering the collector resistor was the point, so as to turn the MOSFET harder on.
You mean OFF.
The resistor pulls the gate UP (discharges the gate capacitance).
Only <=1.2mA if the resistor is 1k, and <=12mA if the resistor is 1k.
I prefer the latter, for a faster switch-off time of the mosfet.
The transistor pulls the gate DOWN (charges the gate capacitance) with "base current x transistor gain".
A 390ohm base resisor (11mA) would theoretically result in a collector current of 11Amp.
Remember that you're not only pulling the 12mA of the resistor down.
Leo..
larryd:
12mA to turn the P MOSFET on has been quite effective IMO.
Not sure what you mean here.
The transistor turns the mosfet 'on', and it's collector current can only be estimated (Ib * β).
The resistor turns the mosfet 'off', and that final current is 12mA and about 5mA at the turn-off point of the fet.
That current should indeed be enough for an average fet, and if you're not switching too heavy loads.
Leo..
Wawa:
The transistor pulls the gate DOWN (charges the gate capacitance) with "base current x transistor gain".
A 390ohm base resisor (11mA) would theoretically result in a collector current of 11Amp.
Leo..
But, in this case, only 2A needs to be switched. So, in this case you're over designing the thing. But, no harm, no foul..except in cases where, due to limitations, such as battery power, one needs to design to the requirements, not to past successes.
