# N-channel MOSFET as voltage regulated resistor

Discussion in 'General Electronics Chat' started by newInRobotics, Jun 19, 2011.

1. ### newInRobotics Thread Starter New Member

Jun 19, 2011
2
0
I know that it is possible to use MOSFET as regulated resistor if following conditions are met:

• VGS > Vthreshold
• VGS < VDS
Say I have the circuit as attached below. LED has these specs:

• Vforward = 3.45V
• Iforward = 20mA
To have LED at it's brightest I need to hook up 330 Ohm resistor. However I want to be able to regulate its brightness by varying resistance.

Is it possible to fine tune RDS of a MOSFET so that:

• At 9V from trimmer, RDS = 330Ω
• At 0V from trimmer, RDS = 1kΩ
• Or vice-versa
Is there a formula to calculate RDS?

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2. ### #12 Expert

Nov 30, 2010
16,685
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Better to install the 330 ohm resistor and have the MOSFET control ADDITIONAL resistance.

as for resistance vs. Vgs, look at the datasheet for the MOSFET.

3. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
A MOSFET will only behave as a resistance over a quite restricted range of voltages. However, if you want to control an LED brightness, there is no need for a strictly resistive characteristic. Something closer to a constant-current characteristic would be as good or better.

I would echo the last contributor's suggestion to add the minimum 330 ohms into your circuit, as otherwise it will be easy to have an expensive accident.

Connecting the LED in series with the source will give you quite a large minimum gate voltage to turn the LED on. If you do add a series resistor, this would then need to go in series with the drain, as otherwise the FET will not turn on fully with only the 9V supply.

Why not move the LED in series with the drain? You could then simply put the 330 ohm resistance in series with the drain too, but you might find the turn-on characteristic was too sharp. Perhaps you should put all or part of the resistance in series with the drain, to give a softer adjustment.

4. ### #12 Expert

Nov 30, 2010
16,685
7,322
Awww...party pooper. I was gonna let him find that out for himself

Isn't that why people hook up these circuits? To learn by doing?

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
I think that every nOOb should make a video of his LED burning out.

#12 likes this.
6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
MOSFETs work better for digital circuits. You really need to use a BJT in the constant current mode, or better yet, use PWM (then the MOSFET would be fine).

7. ### newInRobotics Thread Starter New Member

Jun 19, 2011
2
0
The circuit I have provided is just an example application to keep it simple. The real application will require something that changes resistance with a change of voltage or current, hence I thought about MOSFET, as they change RDS with a change of VGS, however I need resistance to be well controlled and well defined.

Are there any other ways to achieve this?

P.S. Thanks for all replies.

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Look above your post, I just showed you one. It works very well, and I believe it does what you want.

If you going to ignore valid input, your level and quality of help will suffer. What don't you like about Post #6?

Simple FETs (not MOSFETs) can make simple constant current regulators (which is what you are talking about though you don't seem to have a handle on it). You can also use a LM317 with one programming resistor.

There is a reason you don't see MOSFETs used like this, while a BJT is practically a constant current source as is.

The problem with what you describe is if the current is very high (for high power LEDs that require 700ma) they will get very, very hot, and are highly inefficient. For lower power LEDs, such as 20ma, it gets simpler.

LEDs, 555s, Flashers, and Light Chasers

Last edited: Jun 20, 2011
9. ### #12 Expert

Nov 30, 2010
16,685
7,322
Go to post #6.
Put a resistor in series with the bases. Somewhere around 330 to 1k.
Put a 1k pot from Vcc to ground.
Connect the wiper to the base resistor you just installed.

Recalculate the 36 ohm resistors so the LEDs don't get too much current at the highest voltage you can dial to base to.

4 instance, if you have 10 volts to work with and the LEDs are 2.2 volts each, figure that the transistors can saturate to .3 volts.
10-6.9 = 3.1V
180 ohms per string should keep the LEDs safe.

10. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
The OP may really be looking for a VVR, for something like a gain control. If so, why didn't he just say so. It would be useful if he would state clearly what he wants, rather than getting us all LED down the garden path!

11. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Something similar to this perhaps...

This is one of those inefficient high current constant current regulators I was telling you about.

Last edited: Jun 20, 2011
12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
This may be true. A FET is still a good answer for a variable resistor, and you can buy digital potentiometers. You can also use CMOS multiplexers, such as the 4051 (a 8 bit multiplexer). It is effectively an 8 position analog switch. Use a binary approach you can get very fine resolution using 8 2 bit multiplexers.

We can only go by what the OP tells us though.