Hi fellows and PAPA,
I followed your guide and made some measurements within a controlled temperature test jig I built to try and measure a bunch of IRFP140 mosfet devices I have on hand. I found a box of around 150pcs. in my move. RL/drain resistor is 47 Ohm/10W. 5%
I am seeing a strange phenomenon when I turn off my power supply my DMM slowing seems to show a "discharge" for lack of better terms. It seems almost related to a internal heating condition as when I quickly switch out another IRFP140 mosfet and plant it on my test jig the value without power almost always recovers exactly to 2.2mV. Is this normal behaviour and what explains it? Sometimes I need to wait a longer time for this value to settle depending on FET?
I am using a Topward DC Power Supply 3303A preset to 15vDC
I made a video here to try and demonstrate the issue:
I followed your guide and made some measurements within a controlled temperature test jig I built to try and measure a bunch of IRFP140 mosfet devices I have on hand. I found a box of around 150pcs. in my move. RL/drain resistor is 47 Ohm/10W. 5%
I am seeing a strange phenomenon when I turn off my power supply my DMM slowing seems to show a "discharge" for lack of better terms. It seems almost related to a internal heating condition as when I quickly switch out another IRFP140 mosfet and plant it on my test jig the value without power almost always recovers exactly to 2.2mV. Is this normal behaviour and what explains it? Sometimes I need to wait a longer time for this value to settle depending on FET?
I am using a Topward DC Power Supply 3303A preset to 15vDC
I made a video here to try and demonstrate the issue:
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That voltage is coming from the power supply, it is just filter cap slowly discharging. If you unplug the banana connection instead you will find it will drop to zero immediately.
I think you are right. Unfortunately; in my worry, I was short sighted and when I unplug the + lead from PSU my meter bounces up to 23mVDC offset probably due to pickup?
There can be no voltage coming from the mosfet; it can only be just varying leakage current from your power supply.
The power button on the PSU only disconnects the mains power; it leaves the power supply output connected, which leaves a lot of filter capacitors and circuitry connected to your mosfet; when the gate-source voltage drops to mV's then the current the mosfet will draw will be in the uA's this will mean it will effectively stop discharging the psu output filtering capacitors, leaving a random residue voltage across your mosfet.
Alternatively, what you can do is:
- Stop worrying about the 12-20mV - it is effectively nothing, at this point you are not measuring the mosfet but the PSU.
- Instead of switching off the PSU, pull a banana plug
- Add a resistor across the PSU to make sure the PSU fully discharges
Alternatively, what you can do is:
- Stop worrying about the 12-20mV - it is effectively nothing, at this point you are not measuring the mosfet but the PSU.
- Instead of switching off the PSU, pull a banana plug
- Add a resistor across the PSU to make sure the PSU fully discharges
I noticed that the RL resistor gets warm during the tests. Should it be heatsinked. I use a 10W 5% 47 ohm resistor. I wonder if it matters if its temperature changes that will skew results?
The mosfets Vgs does depend on the current through the mosfet but with this circuit the resistor values is only partially responsible for the current through the mosfet other factors are the supply voltage as well as the mosfet Vgs.
Is = (Vsupply -Vgs)/R
The drift of your resistor, as it warms up (unless it is a really bad resistor), will be small compared to your psu voltage drift and the mosfet Vgs .
For repeatability I would recommend making sure to measure the mosfets at the same temperature (ie measure the mosfet temp because mosfets drift a lot with temp) and the same supply voltage (ie measure the supply voltage between groups of mosfet tested)
Is = (Vsupply -Vgs)/R
The drift of your resistor, as it warms up (unless it is a really bad resistor), will be small compared to your psu voltage drift and the mosfet Vgs .
For repeatability I would recommend making sure to measure the mosfets at the same temperature (ie measure the mosfet temp because mosfets drift a lot with temp) and the same supply voltage (ie measure the supply voltage between groups of mosfet tested)