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Mosfet Stray Voltage

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MinhNguyen

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Hi,
I am starting learning about the MOSFET. I am using dual Mosfet to control solenoids. I connected pump's positive terminal to 24V power supply, and the negative side connects to Mosfet's Drain terminal, and a diode to prevent back EMF. I have a 10K pull down resistor between gate and source, and able to turn on and off the solenoid by applying 5V to the gate.
The problem i have is, without applying 5V to the gate, i measure the voltage between the solenoid's positive and negative terminal i receive 4.5V, and it is suppose to be 0V. How can i remove this 4.5V?
Thank you much for your explanation.
Minh
 

Hi,

A schematic says more than you can describe with words.
Additionally we need:
* exact mosfet type
* exact solenoid type
* exact diode type

A picture may be helpful, too.
Measure the gate-source-voltage.

Klaus
 

Hi,
Please find my responses below. I cannot attach a picture from my phone. I will add later on today.
Thank you for your help.
Minh
KlausST said:
Hi,

A schematic says more than you can describe with words.
Additionally we need:
* exact mosfet type
I am using BUK9K13-60EX, NxP semiconductor
* exact solenoid type
Biochemfluidic 100P pinch valve
* exact diode type
Bat30SWFILM
A picture may be helpful, too.
Measure the gate-source-voltage.
0V

Klaus
Click to expand...
 

Hi,

A Mosfet has no stray voltage, it has leakage current
--> in your case max 500uA when very hot, usually less than 1uA with your measured 0V V_GS.

Your valve is specified with 24V and 4W, this means about 144 Ohms (170mA)
--> with 1uA of leakage current you may expect less than 1mV (70mV)
But your measured 4.5V means about 30mA

Your diode is 30V, 300mA rated. Double schottky diode...still unclear how you connected them.
30V is a bit low for 24V nominal voltage.

....waiting for the schematic and the pictures....

Klaus
 

Thank you for your detailed analyze. Please see attachment of the schematic.

KlausST said:
Hi,

A Mosfet has no stray voltage, it has leakage current
--> in your case max 500uA when very hot, usually less than 1uA with your measured 0V V_GS.

Your valve is specified with 24V and 4W, this means about 144 Ohms (170mA)
--> with 1uA of leakage current you may expect less than 1mV (70mV)
But your measured 4.5V means about 30mA

Your diode is 30V, 300mA rated. Double schottky diode...still unclear how you connected them.
30V is a bit low for 24V nominal voltage.

....waiting for the schematic and the pictures....

Klaus
Click to expand...
 

Attachments

  • Mosfet Connection.JPG
    Mosfet Connection.JPG
    18.4 KB · Views: 60

Hi,

The schematic is of bad quality. Impossible to see details.

But it seems your LEDs are connected to 5V, so when the solenoids are OFF, then there is up to 20V reverse voltage across the LEDs. Put a series diode (1N4148 or any other standard signal diode) in series with each LED to prevent reverse current.

Klaus
 

Hi Klaus,
Thank you for your advise.
Yes, the LEDs are connected to 5V source. When the solenoid is off, there is a flyback prevention through the path of the diode and back to the 24V power supply. Would this path efficient for preventing any reverse current?
Thanks,
Minh
 

Hi,

follow my recommendation:
Put a series diode (1N4148 or any other standard signal diode) in series with each LED to prevent reverse current.
Click to expand...

Klaus
 

Thank you for your Advice.
In your previous comment about the "30V is a bit low for 24V nominal voltage." May I ask how do you calculate a minimum requirement for Diode voltage rating at a 24V circuit?
Should the emf diode protection be selected base on the current rating of the load?
Thanks,
Minh
 

Hi,

30V diode at 24V:
24V is never pure DC 24.00V.
Some examples*
* A 24V Pb battery will be charged up to 28.8V
* a 24V supply maybe has +/-5% voltage tolearance
* a 24V power supply may overshoot on power up or on overcurrent recovery.
* maybe remaining mains ripple voltage
* spikes caused by switching applications
* induced noise on the cables
...

Klaus
 
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