Microchip 2N7000 Transistor struggles

Semiconductor Discrete Semiconductor Products
1

Firstly, I have no formal training in electrical engineering and only know what I’ve been able to understand from google searches.

Anyway, my issue. I recently ordered several Microchip 2N7000 (n-channel, enhancement-mode MOSFET) transistors for a project I’m working on, and after my initial test for a portion of the project failed, I began testing individual components, and ran into issues with the transistors.

So, the test setup: All assembled on a breadboard, receiving power from an Arduino Uno rated at 5V (though my multimeter read 4V). The source of the transistor was connected to +5V, the drain connected to an LED, which was connected to ground through a 560 Ohm resistor. The gate was connected to a momentary switch which was, in turn, connected to +5V. Upon providing power to the Arduino, the LED immediately lit, and would not go out regardless of the position of the switch.

I tried this on a second, unused transistor with the same results.

Then I tried flipping the source and drain connections on the transistor. In this configuration the LED was initially lit only very dimly, but upon closing the momentary switch the LED lit to full brightness and would not dim again regardless of the position of the switch.

At this point I consulted with a friend who is in Uni for Electrical Engineering, and at his advice adjusted my circuit to have the source connected directly to ground, the drain connected to +5V through the LED and resistor, and the gate still connected to +5V through the momentary switch, but I still got the same result.

I then tried adding a 4.7 KOhm resistor in series with the 560 Ohm resistor to give me a total of roughly 5.1 KOhms (to bring my current draw below the 1mA ‘Zero Gate voltage drain current’ rating in the datasheet, though I will admit I do not know what that rating actually means), but still had the same results.

At each step in this process I also tried using a new and unused transistor, in case I had inadvertently damaged the others in testing. I double checked my circuit on the breadboard multiple times to be sure I had wired it correctly.

Is this just a batch of faulty transistors? or am I doing something seriously wrong?

2

Hi DerpWarrior,

First of all, you are not doing anything seriously wrong. Troubleshooting circuits by testing and observing is the best way to learn.

For an N-FET, the Source needs to be more negative than the Gate to conduct normally. In your first setup the Source was the most positive node, thus the Bulk diode, which is a natural part of FET structure, conducted and made your LED lit constantly.

Your second circuit is almost correct. You can connect the Gate directly to +5V via the switch, but you need to add a resistor (the 4.7k will do) from gate to ground. That ensures, that the very high impedance gate node is drawn to ground potential, when the switch is not ON.

Hope this helps,
Cheers,
Heke
AsamaLab

3

Warning, a reading like that indicates a major problem that must be corrected before going any further.

Worst case you burned out the something on the Arduino PCB. Best case an incorrectly wired circuit overloaded the Arduino power supply and its built-in thermal and current limiting acted to lower the supply voltage. If it’s the best case then once you disconnect the external circuitry the supply voltage will come right back to 5V.

4

Alright, so based on that, I want to have drain connected to +5V with some sort of load between the transistor and the power supply, and then the source is grounded. Because of the logistics of how I’m using the transistors in my final project, I want to be able to connect my main load on the grounded side of the transistor, so I tried this setup with my 560 resistor between +5V and the drain, and then the LED between source and ground, and it seemed to work fine.

Thank you. Adding the resistor to ground fixed my problems. If you don’t mind me asking further, is it then a good generalization that all digital logic inputs should have a connection to ground to properly read a LOW signal?

I’ve been running the Arduino on USB power up till now, which is supposed to be an acceptable way to power it. When I checked it again today it read 4.5V. To be thorough, I connected a 9V battery to the Vin power input and got a measurement of exactly 5.00V from the Arduino, so I don’t think there’s any issues with the voltage regulators or anything like that. Since USB runs on 5V I wouldn’t be at all surprised to see some voltage drop from the components on the board, but it’s odd that Arduino would warn that a power supply under 7V could result in instability due to a sub-5V supply to the 5V pin, but still suggest powering the board with a 5V USB port. :man_shrugging: Maybe I should search the internet for other people encountering this on Arduino boards?..

5

Hi DerpWarrior,

setup with my 560 resistor between +5V and the drain, and then the LED between source and ground,

Yes, that is perfectly OK. You just need to ensure that the drop over the load does not get so large that the FET goes off (E.g. with a blue LED that may happen, as it has large forward voltage drop).

is it then a good generalization that all digital logic inputs should have a connection to ground to properly read a LOW signal?

Yes, that is correct. Or making it a bit more general, CMOS digital inputs should not be left “float”. i.e. should use either pull-down or pull-up resistor in a circuit configuration where the input node may get into undefined state (in other term “high impedance state”).

For your issue with Arduino, the external power supply needs to be at least 7V, otherwise the Arduino draws the power automatically from the USB. The external power supply has a protecting diode and an LDO with voltage drop of 1.2V (which IMHO does not sound like an LDO :), so in order to get stabile 5V, the input has to be around 7V or above. The 4.5V that you get with USB supply may indicate a problem with the +3.3V supply. Try to measure the Arduino’s power connector pin 4 to see if it has +3.3V. If it is much less, the 0.5V drop in the 5V supply is explained by the Arduino’s supply voltage selector FET’s bulk diode drop.

I hope this helps.

Cheers,
Heke
AsamaLab

6

That’s a very different result, definitely nothing to worry about.

Doing this for 50 years has led me to this rule of thumb when checking power supplies:

  • +/-5% expected due to supply and measuring system tolerances
  • +/-10% (4.5V @ 5V) a curiosity but nothing worth slowing down the work with an investigation
  • +/-20% (4V @ 5V) definitely warrants further investigation before going any further
  • +/-25% or higher pull the power plug before a possible catastrophic failure