Author Topic: Very confused by DMM reading of Drain Current on NMOS circuit  (Read 1927 times)

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Offline Mattjd

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Hello I am working on a lab that pertains to the IV characteristics of n type MOSFETS.The lab calls for recording the value of the drain current while increasing VDS/VGS from 0 to 6 volts in steps of 1 volt.

I constructed this circuit



As I increase the voltage for VDS/VGS I am experience I weird issue with the Drain Current readings. It issue is that it is not very stable at all. The current is constantly changing. What I mean by constantly changing is that when I up the voltage from 5 to 6 volts, the current obviously jumps but then quickly plummets close to 40 mA in a matter of seconds, without anything else changing.

I took a video of it.

https://youtu.be/-LMfpRZQWVA

is there anything I can do with the circuit to fix this?
« Last Edit: March 18, 2017, 04:51:39 pm by Mattjd »
 

Offline rstofer

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Re: Very confused by DMM reading of Drain Current on NMOS circuit
« Reply #1 on: March 18, 2017, 04:53:09 pm »
What limits the current (and heating)?
In theory, the MOSFET is conducting as much current as the supply can produce in short circuit.

In any event, it looks like a heating related issue to me.  Does the device start cool and get hotter?
 

Offline Mattjd

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Re: Very confused by DMM reading of Drain Current on NMOS circuit
« Reply #2 on: March 18, 2017, 04:59:31 pm »
What limits the current (and heating)?
In theory, the MOSFET is conducting as much current as the supply can produce in short circuit.

In any event, it looks like a heating related issue to me.  Does the device start cool and get hotter?

Why yes it does. I just left my finger on it for the entire increase from 0 to 6 and it got very hot around 5 volts.

So, to get the most accurate readings, should I wait a certain amount of time in between readings?
 

Online Kleinstein

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Re: Very confused by DMM reading of Drain Current on NMOS circuit
« Reply #3 on: March 18, 2017, 05:04:27 pm »
The circuit might pic up some external noise / Rf signals. Due to nonlinear response this can end up in changing the "DC" reading too. If the drain side load with the meter turns resonant, there is a small chance is might even oscillate.

For the higher currents there might be self heating that can shift the curve. Also air drafts might change the temperature. Waiting a well defined time might help. This could be either very slow or up to pulsed measurement with a faster meter.

If might help to keep the mobile phone away from the circuit or at least switch to flight mode.

It also depends on the voltage supply how stable the driving voltage can be. It does not look that bad.
 

Offline Mattjd

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Re: Very confused by DMM reading of Drain Current on NMOS circuit
« Reply #4 on: March 18, 2017, 05:12:11 pm »
Yeah I was using my second DMM to measure the temp, it reaches 100 Centigrade at 5 volts, rather fast too. Its probably best if I just take the measurements fast and record the temperature also. Then just talk about it in the write up.
 


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