Author Topic: none logic level mosfet issue (dummy load)  (Read 5560 times)

taiga

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none logic level mosfet issue (dummy load)
« on: May 14, 2014, 12:00:37 PM »
Hello all,

I just built up a simpler version of the dummy current load that Mr. Lorton made. It's the same as the Mr. Jones on the EEVBlog made.

Anyways very simple circuit but I did run into an issue which means I learned something :)

The issue I had was that after setting the whole thing up it wasn't working - I followed the voltage to the In+ of the second op-amp which is where I found the issue. Being a beginner it took me a while to figure out what was wrong. The mosfet i'm using didn't appear to be letting anything through collector to drain even though the gate had voltage on it?? I double checked the pinout and that was good... I finally came up with the idea that perhaps my mosfet was not a logic level one and I needed a higher supply voltage. Sure enough when I turned up the supply voltage to 10v (was 5v before) the whole thing started working perfect.

Would someone with more knowledge confirm that was indeed the issue? I did use a 100k pot... not sure if that would have caused the issue but I don't think so.
There is nothing worse then thinking you figured it out and it ending up being something else :P



Here are a few pictures on the breadboard and one of a li-ion under test. The li-ion is a 1500mAh and i'm timing how long it'll take to discharge to 3v at 750mA discharge rate.






mariush

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Re: none logic level mosfet issue (dummy load)
« Reply #1 on: May 14, 2014, 01:10:30 PM »
Well, a good start would be to mention the components you used and the values... that would be a good start.

A schematic would also help, even one made in paint or on a sheet of paper.

taiga

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Re: none logic level mosfet issue (dummy load)
« Reply #2 on: May 14, 2014, 02:09:36 PM »
Components used:
- 100k 25 turn trimmer pot
- lm324 opamp
- 2 x 10k ohm resisters
- k2638 N-channel mosfet

Here is the schematic. It doesn't show the ground from the lm324 but it is grounded on my breadboard. I switched the mosfet and the pot for what I had on hand - see above

(sorry for the shitty picture and ignore the little lm317 circuit at the bottom)

mariush

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Re: none logic level mosfet issue (dummy load)
« Reply #3 on: May 15, 2014, 02:12:50 PM »
tip... it's not k2638, it's 2SK2638 ...  that 2S  is like the 1N for the diodes (1n4001, 1n5819 etc)

I think the problem is the Vgs (gate threshold voltage) which is 4v typically (3.5 min - 4.5v max). MTP3055 has a Vgs of about 2.5v..
For my own load i use a fqp50n06 that has minimum 2v maximum 4v and it tops at about 3-4A with the opamp I have (a microchip rail to rail one that works only up to about 6v). I used this one because it has a very low Rds(on) of about 0.022 ohm, which is another disappointing parameter of your mosfet , yours has a somewhat large value at 0.65 ohm max, meaning it will get much hotter than others.
You can also improve your load by replacing the 1 ohm resistor with a smaller one... for example you could get several of those 7w-12w axial resistors (white rectangles, cemented) and put them in parallel).  The only downside is that you won't have that 1:1 relation between the voltage drop on resistor and the current dissipated on the mosfet.

taiga

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Re: none logic level mosfet issue (dummy load)
« Reply #4 on: May 16, 2014, 10:45:55 AM »
Thanks for the reply!
I'm looking into getting a different mosfet with lower resistance and Vgs(th). I also added 20 x 1/2w 10ohm resistors which seems to help the heat there, if need be I can add one or a few of those white block 1ohm resistors like you mentioned.
I'm more interested in getting the Vin lower so I can run it off AAs. I did some more testing and it seems to want 9V in to work properly. Would be nice to get that down to 3v or 4.5.

edit: this is the mosfet I'm looking at as it's available locally: https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf the Vgs(th) is min 1 max 2 and resistance is 0.047ohm
« Last Edit: May 16, 2014, 10:58:47 AM by taiga »

ax2013

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Re: none logic level mosfet issue (dummy load)
« Reply #5 on: June 22, 2015, 09:30:16 AM »
I'm using IRL540 which works in generally fine. Probably not the most ideal (like Rds point of view), but anyway...
I didn't paid enough attention to supply voltage which limits max current to 2.4A (6V Vcc + LM324 + IRL540_Vgs).

Ax.