# Can this be done using a JFET?

Discussion in 'General Electronics Discussion' started by 24Volts, Nov 21, 2012.

1. ### 24Volts

164
0
Mar 21, 2010
Hello,

Please view attachment OPAMP1. Here is a circuit that works exactly as I need it. This circuit was kindly provided to me by some fellows from this site. The last piece of the puzzle is I need to control R5 from my MCU. I would like to know if R5 can be replaced with a JFET as shown in OPAMP2 attachment. Obviously I would add a 5 ohm resistor in series with the FET... (Not shown).

I have not had time to seriously study and learn passives such as opamps JFETS and Triacs yet... I don't know if the calculations to select the correct JFET are complex or even if this circuit can be done with an JFET...

I apologize for asking help like this... but at this point any help is very appreciated...

thanks

Last edited: Dec 4, 2012
2. ### Harald KappModeratorModerator

12,465
2,987
Nov 17, 2011
A FET might be useable, but if will be rather nonlinear.

What do you want to achieve by changing the resistance? Modulate the light intensity? You can do this by changing the 2V...2.5V ramp accordingly.

3. ### 24Volts

164
0
Mar 21, 2010
Hi Harald,

Well, it's not really to modulate the light intensity as this is done by the transistor which its signal is fed from the op amp. The MOSFET (or R5) is really there to provide a maximum range the led can use ... or the maximum current that the led will be allowed to get.

You see, if I set R5 to 5 ohms.... I have enough infra red to bounce off an object 20 feet away.... in certain instances, that's too far. But if I vary R5 to say 50 ohms I can can detect an object
maybe 5ft away max. So basically it doesn't need to be linear... All I need it for is to establish a maximum distance.

In any case you can view the attachment which provides some current proportions if I replace R5 by a 100 ohm pot PT1 (not shown). All I really need is to replace R5 by a MOSFET so that when it receives a voltage at its gate it reacts as PT1 which is:

1V at gate = 5 ohms (approx: 3.8ma)
and 3.3VDC at gate = 105 ohms ((approx: 30ma)

I found this video very interesting ....

As shown in the video, I was planning to use an enhancement Mosfet ... but I was wondering a few things here:

1) Are their any enhancement mosfets with a Vt of say 1V???
2) Which enhancement Mosfet (part number) can I use that would fit my application?
3) If my DAC supplies a 1V to 3.3VDC to the gate of the MOSFET since it is a voltage controlled device can I simply omit an Rb resistor like we used in BJT's?
4) Just one thing the video doesn't get into which is how would I get the MOSFET to act as the required potentiometer in my application which is:

1V at gate = 5 ohms (approx: 3.8ma)
and 3.3VDC at gate = 105 ohms ((approx: 30ma)

confused?

If I was not clear with the specs I provided ... please let me know

Sorry for being such a noob...
All help really appreciated... thanks for your help! Harald Kapp!

24V

Last edited: Dec 4, 2012
4. ### Harald KappModeratorModerator

12,465
2,987
Nov 17, 2011
Using a FET for linearly controlling the current is possible but will certainly require additional circuitry to stabilize the behaviour over temparature and time.

May I suggest two alternatives?
- use the first DAC only and limit the amplitude in SW.
- use analog switches with series resistors instead of the FET. You can set the limit only in discrete steps, but that may be sufficient.

Harald

5. ### 24Volts

164
0
Mar 21, 2010
Hi Harald,

I really don't need it to be linear, its just to provide a maximum current channel for the the Led to use. I explain below in detail!

What you are saying is provide R5 the lowest resistor value according to the maximum range required and use software programming to reduce/increase the voltage fed to the op amp for equal or shorter ranges.

So what I was thinking of doing is to vary the RE resistor to the required maximum range for that particular instance of my program while always producing my 240 steps of precision no matter what the max distance is b/c I will still have my 8 bit digital signal to my dac as 0 to 255 steps all the time.

Also, for my personal learning, I would really like to experiment with a EMOSFET and do the circuit that the guy in the video did!!! (I believe its an N-channel enhancement MOSFET) But I don't even know which MOSFET to begin with... does TI make Mosfets?... does Vishay make MOSFETS? and which one would be a typical type of EMOSFET such as describled at 12:30 of 20:36 of the video?

If you have any EMOSFET suggestions such as a part number that I could begin with ... please let me know !

Thank you so much for your help....
24V

Last edited: Dec 4, 2012
6. ### KrisBlueNZSadly passed away in 2015

8,393
1,272
Nov 28, 2011
Can you describe what you want to do? IN DETAIL?
There may be an easier way to achieve what you want. We're really working in the dark without any description of what you're trying to do. We don't have ESP, you know!
So far I see a failed attempt at a variable current sink, driving some kind of optical device.

7. ### 24Volts

164
0
Mar 21, 2010
Hello KrisBlueNZ,

Yes, I know, it's a little foggy... let me review my circuit and I will get back with better details!

Thanks for replying.

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