I'd like your opinion if this is a feasible way to achieve my goal.
I'm powering a load (~1A) with 12V and switching it with a low side logic-level MOSFET connected to an Arduino digital out pin. I have a 220ohm resistor in mind between Arduino and the gate to prevent high currents, and a 10kOhm pull-down resistor to GND to ensure that the MOSFET is off should the Arduino not be powered.
I have the idea of adding a SPDT switch to be able to bypass the Arduino+MOSFET and effectively have the connection be always on. Even if the Arduino is completely removed from my circuit, I still want to be able to switch the load ON. Does this schematic make sense to achieve that effect?
I imagine that while switching, every so briefly the circuit will be disconnected. I don't want my load to shut down so I thought... add a capacitor? If this isn't a naive idea, what capacity would make sense?
Having a capacitor fully charged (the MOSFET is OFF), then the transistor turns on, is like connecting a 12v power supply directly to the drain which might short the transistor.
Convert the transistor circuit into a 2 input OR (NOR) gate and move switch to an input.
You may want to note: "Whereas the conventional bipolar transistor is a current-driven device, the MOSFET is a
voltage-driven device... Since the MOSFET is a voltage-driven device, no DC current flows into the gate... The gate of a MOSFET is composed of a silicon oxide layer. Since the gate is insulated from the
source, an application of a DC voltage to the gate terminal does not theoretically cause a current to
flow in the gate, except in transient periods during which the gate is charged and discharged. In
practice, the gate has a tiny current on the order of a few nanoamperes. When there is no voltage
between the gate and source terminals, no current flows in the drain except leakage current, because
of a very high drain-source impedance".
There is some good advice covering the gate to ground resistor and the gate input resistor. Keep in mind turning on the gate of a MOSFET is a lot like charging a capacitor so resistance numbers are important.
As to your first drawing that's how I would do it using a SPDT On-On switch and I would drop the capacitor unless you figure the transit time of the switch is too slow?
larryd:
Having a capacitor fully charged (the MOSFET is OFF), then the transistor turns on, is like connecting a 12v power supply directly to the drain which might short the transistor.
Convert the transistor circuit into a 2 input OR (NOR) gate and move switch to an input.
Just to entertain the capacitor idea, would it change matters if I added a resistor in series to it to limit the current?
Paul__B:
Just use a SPST switch to bridge the FET. No reason at all to disconnect it.
And the 10k resistor goes from the Arduino output pin to ground, not between the FET gate and ground.
Is this what you mean by bridging the FET, and by the 10k to output pin GND? It might get the job done but seems dangerous regarding the Arduino (12v vs 5v). Or would it then become relevant to introduce a diode after the Arduino output pin, like a Schottky diode because of the low voltage drop?
Ron_Blain:
As to your first drawing that's how I would do it using a SPDT On-On switch and I would drop the capacitor unless you figure the transit time of the switch is too slow?
Thank you for that background info on MOSFETs, and for the feedback on the feasability of using the SPDT switch. The question was asked here because I wasn't sure what "too slow" is The switch itself would be a quick flip-switch so probably like 1/10th of a second if at all. My load is a minipc. I guess a test would show if the switch-flip is too slow or not. But wouldn't want to damage the board over time, even if the switch would only be used rarely.
Aaaanyway, my ramblings are becoming my personal considerations. Thanks for your answer
Ah ok, now I get it. I felt there was something wrong about using that logic gate in a circuit. I understand now, thank you so much! What's the function of the diodes over de load?
Paul__B:
It's not rocket science!
deep, deep sigh :o
So this is bridging? I can see how it's a bridge, bypassing the mosfet. I projected too much into the word. Thanks
MarkT:
What is this mysterious ~1A load? Don't omit details like this, all details can be crucial.
Apologies, I didn't think it would matter too much. It's a mini-itx pc running at 12V and it uses between 0,5A and 1,3A depending on load.
Of course it does matter. I image if it would have been a DC motor then a momentary switch might not matter as much as it might for an operating system.
The switch itself would be a quick flip-switch so probably like 1/10th of a second if at all. My load is a minipc. I guess a test would show if the switch-flip is too slow or not. But wouldn't want to damage the board over time, even if the switch would only be used rarely.
