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Topic: Some kind of electronic switch (Read 2 times) previous topic - next topic

RuggedCircuits

The pulldown is not strictly necessary but it prevents the MOSFET from being damaged should charge accumulate at its gate (easy to do due to electrostatic buildup). MOSFET gates can be damaged by as little as 20V while your accumulation of 1000V on your body of electrostatic charge is barely perceptible when you get "zapped". So it doesn't take much "stray charge" to destroy a MOSFET. The 47k resistor provides a path for that charge to leave the gate. When the Arduino is off or is in reset, the pin driving the gate is floating thus does not pull the gate neither high nor low, which is when bad things can happen (as they can while you are building the circuit, so put the resistor on first :) )

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Beat707: MIDI drum machine / sequencer / groove-box for Arduino

DaveO

Thank You for an excellent explanation.

I'm off to add some resistors to my circuit.

Was the 47k value arrived at by calculating the current required from the pin (after the 1k) to drive the MOSFET ?

RuggedCircuits

No, it was more calculated as a Goldilocks-type thing: not too big, not too small, just right. Too small and the voltage divider formed by it and the 1k resistor doesn't allow the gate voltage to be high enough. Too big and it isn't a good path for charge to drain. The value isn't particularly critical. I think for a 1k series resistor anything between 10k and 100k should work fine.

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The Aussie Shield: breakout all 28 pins to quick-connect terminals

MarkT

The underlying assumption that because there are 4 signal lines and 1 ground line it would be easier to switch/disconnect the ground line is flawed.

The 'signal' really consists of the difference between signal and ground voltage for each of the 4 servos.  Those 4 signals happen to share a common ground but they are 4 signals.  If you disconnect the ground from the signal source but leave all the servo grounds connected together then you still have differences between signal and signal at the receiving end.  You could disconnect each ground separately, but this is still switching 4 wires...
[ I won't respond to messages, use the forum please ]

siraulu


.........I think for a 1k series resistor anything between 10k and 100k should work fine.

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The Aussie Shield: breakout all 28 pins to quick-connect terminals


I disagree. If the 1k resistor is changed to 100k and the other one is 47k, then the current from the output is 34 uA causing only 1.6 V to gate. With 1k, the current is 0.1 mA and gate voltage 4,9 V. Much better.


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