Two steppers and and LED - Voltage across MOSFET issue

Other Hardware Motors, Mechanics, Power and CNC
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Hi,
So I have added an LED to my small project and this is working fine.
I have an array of buttons powered by various resistors which is giving me discrete values when doing an analogue Read on A5. I am using these discrete values to drive six 'if' situations. each 'if ' situations does an lcd.print and also activates a motor power pin and a pattern of moves programmed to move to stepper motors ( I am using the stepper.h library).

All is working fine - but I am slightly mystified by the voltages around my MOSFET (IRF520). Here are my observations:

When my power pin (to the gate of my MOSFET) is not active the voltage GND to Gate is 0.06V. This is what I would expect.
When my power pin is active( to the gate of the MOSFET) the voltage is 4.987V. This what I would expect.

Now, when the steppers are active, I am seeing a voltage (+ve of the motor feed to the drain of the MOSFET) of around 6.5 Volts. This is what I would expect.

However, when the steppers are not active, I am still seeing a voltage of 2.6V (+ve of the motor feed to the drain of the MOSFET) . I am seeing a slight dim indication on the stepper control boards.

It seems that the MOSFETs are not completely switching off when no volts are present at the gate??

Now I am quite new to this and this is quite a big step for me. Any help would be appreciated.

Thanks

2

Please provide a schematic showing all components, their part numbers and/or values and power supplies.

An IRF520 MOSFET is not a logic level device so will not fully turn on with 5V on the gate.

1 Like
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@wayneskie, your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with your project :wink: See About the Installation & Troubleshooting category.

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Definitely need your circuit to be sure what's going on.

Where did you get you MOSFETs? There are a lot of counterfeits about, and anyway you need logic-level if you want to drive the gate direct from 5V. Otherwise you'll have to boost the signal to 12V as this is what the IRF520 is designed to work with.

A large power MOSFET will have some drain-source leakage current, measured in microamps, this is normal.

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