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Topic: help with MOSFETs (Read 1 time) previous topic - next topic

lowandslow

Jul 28, 2015, 02:47 pm Last Edit: Jul 28, 2015, 02:56 pm by lowandslow
I'm using these MOSFETs,

http://pdf1.alldatasheet.com/datasheet-pdf/view/45079/SIEMENS/BUZ91A.html

When I hook up a simple circuit with a DC motor, the motor begins to spin with nothing attached to the gate. 

Source to (-)neg battery terminal, one side of motor to (+) terminal, and drain to the other side of the motor.  It spins very slowly, nowhere near full power.  The same thing happens with a simple LED and resistor.... 

If I put a diode across the motor, cathode towards positive side, same thing happens..

Why is this happening? 

septillion

Because the gate is floating. And a floating pin can get any charge it can get. So just connect the gate and you are fine. And to stop it from happening if the driver of the gate (for example the Arduino) can float as well (powered down Arduino of pin not set as output yet) it's common to place a resistor between gate and GND. Something between 1k and 500k should work just fine. This will pull the gate down (stop it from getting a charge) so the MOSFET stays off.
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MarkT

Why are you using a 600V device?  Completely inapproprate, its got very high on resistance, 900 milliohms.

Go find something around 30V rating with a ~10 milliohms of on-resistance or you will be
rather disappointed with its performance.

Never put power through a MOSFET with the gate floating, its a very efficient way to break it if the
load is more than the gate-source voltage.   Suggest a 100k resistor between gate and source so
its definitely off until driven.

Also MOSFET gates are static sensitive (no protection circuitry), so observe static-handling
precautions.
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doughboy

#3
Jul 28, 2015, 06:43 pm Last Edit: Jul 28, 2015, 06:44 pm by doughboy
you need resistors.

this page explains it all

http://www.gammon.com.au/motors

lowandslow

#4
Jul 28, 2015, 07:18 pm Last Edit: Jul 28, 2015, 07:24 pm by lowandslow
Thanks everyone.

Quote
Why are you using a 600V device?  Completely inapproprate, its got very high on resistance, 900 milliohms.
I guess I didn't look close enough,  here is the link from where I ordered them..  descriptions states 30V!
Makes sense though because the motor was very sluggish when I got it wired up right.

http://www.amazon.com/BUZ91A-N-channel-Power-MOSFET-Transistors/dp/B0087YK0CO/ref=sr_1_2?ie=UTF8&qid=1438121551&sr=8-2&keywords=mosfet

I think I'll try these MOSFETs,

http://www.amazon.com/gp/product/B00AKTLO4M/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A1THAZDOWP300U

The motor is going to run @12V and draw at very most 30A.  It is highly unlikely it will ever draw that much current though the load is minimal.

What should I be looking for in the schottky diode?

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