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Author Topic: Depletion mode Transistor for N/C Switch  (Read 690 times)
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Somerville, MA
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Hello all, I have a project to control the movement of 4 automotive motors. These motors area already connected to manual switches, which I need to switch out so that i can do the controlling from Arduino. The design currently has DPDT relays with the manual switches connected to the N/C terminals, but I'm getting tired of listening to the sound of the relays clacking.

I was thinking I could use depletion mode transistors as part of the N/C switch instead. These do not need to switch often, just open for a few seconds while I drive the motor through an H-Bridge, then close and stay closed 99% of the time. Anyway, is this approach a reasonable one?



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It is an option but can you get them at the sort of current rating you want? What is wrong with a simple FET with the gate connected to the supply and pulled down by the arduino output via an opto isolator. That way it will be on all the time except when you want to command it off.
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It is actually challenging to find at the appropriate amp capacity... but I would like to put the Arduino to sleep for most of the time, so i can't be holding the transistor gate high. I want it to fail closed. I guess the best thing is a relay - maybe I can find quieter ones or somehow insulate the sound.
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It is actually challenging to find at the appropriate amp capacity... but I would like to put the Arduino to sleep for most of the time, so i can't be holding the transistor gate high. I want it to fail closed.

Mike's suggestion was to use a pullup to hold the gate high by default. You can use a transistor driven from the Arduino to pull the gate low, that way when the Arduino is powered down or the output pin is low, the gate is pulled high and the mosfet is on. See attached diagram. Btw you've not specified the total current drawn by the motors, so I can't say whether a single IRF540 is up to the job.


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« Last Edit: October 01, 2011, 12:53:12 pm by dc42 » Logged

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PS - I just noticed that you are planning to run the motor via an H-bridge when it is controlled by the Arduino. So switching just one side of the motor supply with a mosfet won't work. You could switch the other side as well using a P-channel mosfet such as the IRF9540. However, can you not connect some CMOS logic between the Arduino and the H-bridge, and connect the manual switches into that logic, such that the motors are always driven via the H-bridge, which takes its commands from either the Arduino or the switches? I can't specify what that logic should be without knowing what the manual switches do.
« Last Edit: October 01, 2011, 12:54:33 pm by dc42 » Logged

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Power FETs are almost all enhancement mode MOSFETs - that's just the way it is.  Its also more convenient if you think about it (control and controlled circuits the same polarity).

There's no problem using a pull-up resistor on the gate if you want it normally HIGH and want to let the Arduino sleep.
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