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I have a N-Channel MOSFET gate connected to a Teensy 3.6 analog pwm pin with the intention of setting the duty cycle on the output of the pin to control the switching of the MOSFET. For reference this is 500 Hz I believe although it is irrelevant.

I have noticed that while the MOSFET works as expected and has low resistance (micro-ohms) when the load voltage (6V) is off as soon as the power supply (12V stepped down to 6V using a buck converter) is turned on the resistance shoots up to its off state. The load is a only 4 ohm resistive heater.

Simple schematic to get the gist of my setup enter image description here

The 5V input to the gate is from a level shifter connecting the pin of the Teensy to the MOSFET's gate. There is a 10K Ohm resistor in parallel between ground and the gate and the voltage to the heater is 6V.

I am new to electronics and any help would be much appreciated.

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closed as unclear what you're asking by Andy aka, Voltage Spike, Autistic, uint128_t, Daniel Grillo Apr 18 '17 at 12:57

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ What is the part number of the mosfet? \$\endgroup\$ – Mike Apr 9 '17 at 21:01
  • \$\begingroup\$ It could be usefull to give information on what MOSFET you are using. The resistance of the heater is likely significantly lower on startup too - what could be happening is that you are triggering some form of protection circuitry. \$\endgroup\$ – Joren Vaes Apr 9 '17 at 21:03
  • \$\begingroup\$ My apologies here is the link to the mosfet I am using. mouser.com/ProductDetail/Nexperia/PHP79NQ08LT127/… \$\endgroup\$ – Crook Apr 9 '17 at 23:25
  • \$\begingroup\$ Are you trying to measure resistance with 6 volts applied? Instead of talking about the resistance you think you see, apply 5 volts to the gate, 6 volts to the heater, and measure the voltage from ground to the bottom of the heater. What voltage do you see? \$\endgroup\$ – WhatRoughBeast Apr 9 '17 at 23:28
  • \$\begingroup\$ I see 6 volts. It may also be worth mentioning I have a calibrated thermistor from which I am reading the temperature of the heater and when I apply the 6 volts the temperature does not increase. \$\endgroup\$ – Crook Apr 10 '17 at 0:01
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4 main choices:

  • MOSFET is dead or misconnected

  • 12V Power supply cannot supply about 12W needed.

  • Buck converter cannot supply 1.5A / 9W needed.

  • Load is not really 4 Ohms.

FET datasheet here

MOSFET is dead or misconnected

Pinout is standard as all getout as per page 2 of data sheet.
On flat surface, metal down, tab away from you legs' towards you connections are gds left to right.

In circuit, power on, short ds with wire.
If result is as when using FET fault is in buck or psu or FET drive.

Apply FET drive.
Measure voltage gs AT FET.
Should be 5V.

12V Power supply cannot supply about 12W needed.

Load 12V with 12 Ohms or less resistor.
Ensure 12V does not droop.
If supply has variable current limit it may be set to below what is needed.

Buck converter cannot supply 1.5A / 9W needed.

Supply 12V to buck converter.
Apply 4 Ohm load AT buck converter outputs across Vout-gnd.
Should supply load.

Load is not really 4 Ohms.

Apply 6V to load.
Measure current in load.
Measure voltage across load to check.
Apply Ohms law.

If all the above fails then you have a misconnection or are using insulating tubing in place of wire.

Remedy any inconsistencies found above.

Your circuit now works.
Q.E.D.

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