That makes sense. Makes me wonder why he suggested that I use it in this case. Good point about the consequence of heating up the FET. I'll dig in to this a little more. Thanks for your reply.It simply slows the turn on while the capacitor is charged through R15 and slows the turn off via R9 (1 ms time constant), unless the drive also sinks current, which is likely.
One of the worst reasons in the world to do something is "that's the way it's done"..............
I'm using a MOSFET for a low side switch to turn on LEDs. One of my colleagues told me that I needed a capacitor between the gate and ground. I don't understand why. I asked, but I got the "that's the way it's done" type answer. Can someone explain it please?.......................
Not likely.Is the capacitor there, because the LED ENABLE is intended to be PWM and the cap would serve to average out the pulse voltage?
I do, sometimes... and it gives me a headache when that happens... ugghhhMost people don't see the flicker of 60 Hz fluorescence bulbs.
It is a logic level mosfet. What is the reason for the different values?Is it a logic level gate mosfet, or an ordinary 10-20V mosfet?
I'd use 10k for R9 if the former, and 47k if the latter.
The difference between an ordinary nfet and a mosfet is that the former works by applying a current that flows through the base, while the latter works by applying a voltage to its gate.It is a logic level mosfet. What is the reason for the different values?
Yes, R9 at 1M is fine if he's not planning to use PWM. I wonder, what would the Fet's switch-off time be with that value of R9?Without the capacitor there, it will be hard to tell without an oscilloscope the difference in turn-off time. Use what you have for R9.
John
True, the original diagram doesn't clarify if it's a push-pull or an open collector type of driver.You can get a feeling for that time by calculating the RC time constant using the gate capacitance, the turn off resistor (1M) and the driver's effective sink resistance (unknown) in parallel with that 1M resistor. In this application, I just assumed it didn't matter.
John
The difference between an ordinary nfet and a mosfet is that the former works by applying a current that flows through the base, while the latter works by applying a voltage to its gate.
That is, you need current in order to get the nfet started, and to keep it going. In the case of the mosfet, once you apply a voltage to its gate, it stays that way until you remove it. You could say that the nfet's base behaves like a resistor, while the mosfet's gate behaves like a capacitor.
In simple words: the mosfet's gate is like a capacitor that needs to be charged to a certain potential for it to work. After that, you could disconnect the gate from the circuit, but the gate would stay charged and the mosfet would be active until the gate is discharged again, which in theory, could take forever. That's why you need a resistor between the gate and ground (in the case of an n-mosfet), so that the gate will discharge once the voltage is removed.
That resistor could have almost any value, really. But if it's too high, then the gate will take much longer to discharge and therefore the mosfet will stay on for too long when you try to turn it off (unless you're using a push-pull circuit to switch it, but that's another story). But if the resistor is too low, then your voltage source will need to maintain more current through that resistor than needed to keep it on.
So if you're switching a logic-level gate type of nFet (5V), and you were to use a 10k resistor between the gate and ground, then you'd need to source 5V/10,000Ω = 0.0005A (half a milliamp) to keept it going, which is easy for most MCU's outputs, for instance. But if you were to apply 12V (like most nFets require) to that same configuration, your source would be delivering 1.2 mA instead just to keep it on. And in the long run that would be a strain to some circuits, especially if they're running on batteries. Also 12V*0.0012A = 0.014W. Personally, I think 14 mW is a waste of power on that sort of application. That's why I'd recommend 47K when switching the gate with 12V.
For practical purposes a balance between stay-on current and switch-off time has to be reached.
Also, unless you're doing mid to high frequency PWM to switch the nFet, so as to control the LED's brightness, you don't really need R15. If you're only switching the LEDs on and off over long periods of time (say more than 0.1 sec) then you can do away with that 100Ω resistor.
R15 is there to minimize ringing at the gate due to large variations of current when it's switched at higher frequencies.
Just visualise the nFet's gate as if it were a capacitor (and in a way, it is), and things will become much clearer in your mind.
WHAT??? What is this "nfet" thing of which you speak? Is this some new type of semiconductor device I've never heard of or encountered?The difference between an ordinary nfet and a mosfet is that the former works by applying a current that flows through the base, while the latter works by applying a voltage to its gate.