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I am experiencing an issue with the N channel MOSFET below:

enter image description here

The MOSFET is not switching at all and I would like to understand why. I have used a similar circuit to control an LED and it always worked fine.

The HEATER_ON signal is either 30V or floating. I am measuring with the scope and I can see 5V at the gate but there is no voltage drop at the load.

What could be the reason? I know I could do this differently but I just would like to understand the reason.

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  • \$\begingroup\$ The load here absolutely should be between 6.5V and the drain. Please edit the question to have the schematic that way around and then base all of your observations off that. It would be good to see the gate-source voltage when the signal is "on" and the voltage across the load at that point too. \$\endgroup\$
    – stefandz
    Sep 2, 2015 at 9:10
  • \$\begingroup\$ You should see about 2 or 3 volts across the 50 ohm load in the source if you have wired the device up correctly. \$\endgroup\$
    – Andy aka
    Sep 2, 2015 at 9:31
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    \$\begingroup\$ Is this an actual circuit or is it being analyzed on a software simulator? Just asking since the GND symbols being used for R4 & D2 are different then that used on the ITO. Possible Analog vs chassis ground issue? \$\endgroup\$
    – Nedd
    Sep 2, 2015 at 9:41
  • \$\begingroup\$ Hi,This is already implemented on a PCB. I have different symbols for GND but they are physically connected... I will edit the question with the correct schematic \$\endgroup\$
    – Bnogart
    Sep 2, 2015 at 12:09
  • \$\begingroup\$ Are you sure your 6.5V source is referenced to GND? \$\endgroup\$
    – Tut
    Sep 2, 2015 at 12:32

2 Answers 2

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What you made is called a "source follower" Its function is to copy the voltage from the gate to the source (and here across your load) with a certain voltage drop. This voltage drop is at least the threshold voltage + some voltage the NMOS needs to turn itself on.

In such a circuit the NMOS does not switch fully on/off !

You need a slightly different circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

In this circuit the 5V will be between gate and source which will turn the NMOS full on which is what you need.

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  • \$\begingroup\$ Read the question - the OP has already tried this configuration he said "I have also tried connecting the load between 6.5V and Drain with similar results". \$\endgroup\$
    – Andy aka
    Sep 2, 2015 at 9:54
  • \$\begingroup\$ I think that it is likely the reason for all this is that the circuit is being simulated, rather than built in real life. The grounds in the question schematic are different and that's probably screwed the simulation pooch. There is no reason that the circuit above will not work correctly - it will drop about 5.5V across the load minimum, according to the 2N7002 datasheet. \$\endgroup\$
    – stefandz
    Sep 2, 2015 at 10:06
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    \$\begingroup\$ @Andyaka although he said he built a common source circuit, it didn't work so obviously he didn't do something right. Just steering him in the right direction. He needs switching so CS is the way to go, not the source follower. \$\endgroup\$
    – Bimpelrekkie
    Sep 2, 2015 at 10:15
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    \$\begingroup\$ @FakeMoustache the OP said he saw "no voltage drop at the load" BUT if he had wired-up the common drain circuit correctly he would have seen about 2 or 3 volts. Your answer is something the op has tried and failed to get working therefore, unless the OP is lying, he must have either a broken transistor or he's wired it up incorrectly. He's tried your circuit and didn't get it working. Your answer is of no help to him (unless instead of lying he just doesn't know how to write in English correctly). \$\endgroup\$
    – Andy aka
    Sep 2, 2015 at 10:27
  • \$\begingroup\$ Well it seems that the transistor just doesn't switch at all... I just wanted to be sure that my assumption was correct and the MOSFTET should switch in that configuration. The issue might be somewhere else then. I have replaced the FET but it seem to have the same issue. I have also connected the load directly to the power supply (to check that the PS was able to provide all the current) and it works just fine. \$\endgroup\$
    – Bnogart
    Sep 2, 2015 at 12:17
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It is possible that you have broken the FET. The 2N7002 datasheet shows an absolute maximum continuous drain current of 115mA, but your circuit could have exceeded that.

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  • \$\begingroup\$ Hi, the datasheet of the 2N7002F says 475mA...2N7002 seems to be a completely different transistor \$\endgroup\$
    – Bnogart
    Sep 2, 2015 at 13:04
  • \$\begingroup\$ @Bnogart - Your right, it is different. \$\endgroup\$
    – HandyHowie
    Sep 2, 2015 at 13:17

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