| Electronics > Beginners |
| Mosfet as variable resistor? |
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| aheid:
--- Quote from: FriedMule on January 23, 2019, 08:29:16 pm ---Okay, but lets say that you would build a volume control by using two mosfet (thats the one I am trying to start to understand now) I.e. by building a voltage divider, would that not be doable since the frequency are so wary low? --- End quote --- How would you handle the feedback to ensure that the MOSFETs behave linearly? |
| David Hess:
--- Quote from: FriedMule on January 23, 2019, 08:29:16 pm --- --- Quote from: David Hess on January 23, 2019, 08:02:46 pm ---The capacitance limits the frequency at high impedances. JFETs can be used as voltage variable resistors to at least 100MHz with a dynamic range of only 2.5. At DC, at least 60dB is feasible. At audio or video frequencies, multiple stages might be cascaded for greater dynamic range. --- End quote --- Okay, but lets say that you would build a volume control by using two mosfet (thats the one I am trying to start to understand now) I.e. by building a voltage divider, would that not be doable since the frequency are so wary low? I am thinking that 300M Ohm, as written above has to be like open circuit and 200 Ohm has to be almost like clean copper wire? --- End quote --- It is very workable at audio frequencies but as schmitt trigger points out, capacitance across the MOSFET will limit the maximum practical resistance. A part like the 2N7000 could have a drain capacitance of 30 picofarads which at 20kHz is about 250 kilohms so 300 megohms is not even close to feasible. A practical audio circuit would probably be limited to 1/10th of this maximum or 25 kilohms. |
| Wolfgang:
--- Quote from: FriedMule on January 23, 2019, 12:17:09 pm ---I am trying to understand the Mosfet in general here, so no specific project. I can see that a Mosfet has a open max resistance and a closet min resistance. Lets say 200 Ohm to 300M Ohm. Since the gate is voltage dependent. Could I, If we ignore the non liniarity of the mosfet, adjust the resistance between 200 Ohm and 300M Ohm by changing the voltage to the gate? If so, how fine would such a adjustment be, could it simply be done by turning a single turn pot by hand? --- End quote --- Some points to watch out: - MOSFETs (at least the larger ones, even more at high voltages and temperatures) do leak. 300MOhm is very ambitious - a range of more than 1 to 1 million on a single sweep of a pot ? Practically not, without incredible effort. - you need a current sensing resistor and an op amp. Not an easy choice for your incredible wide range. Questions: - what is the maximum voltage and current you want to handle ? |
| MrAl:
--- Quote from: FriedMule on January 23, 2019, 05:49:21 pm ---Really great answers!! :-) I understand it like, yes you can, you can also sow with a needle while waring boxing gloves, but do not expect a great result:-) Without feedback where we are talking about that maybe 0.1V can make "big" change, the power supply that have ripple and noise would also make it nearly useless, not to talk about heat or runaway heat:-) But how would you make so precise a feedback so that you could maybe make a voltage divider out of two mosfet's, used instead of resistors, I am wondering because we have to consider some value as NUL, because i.e. 20V in with 300M Ohm - 200 Ohm, would give a something like from 19.99V to 0,000013V. --- End quote --- Hi, Already been said. You need to use negative feedback. You can try it without feedback very easy. Use two potentiometers and two mosfets, then adjust each pot and see what you get. Check a few minutes later and see if the reading changes. Get some real current flowing so the mosfets heat up a little and watch to see how much they drift as they heat up. The output voltage will change if they drift. |
| FriedMule:
--- Quote from: Wolfgang on January 23, 2019, 09:15:10 pm --- --- Quote from: FriedMule on January 23, 2019, 12:17:09 pm ---I am trying to understand the Mosfet in general here, so no specific project. I can see that a Mosfet has a open max resistance and a closet min resistance. Lets say 200 Ohm to 300M Ohm. Since the gate is voltage dependent. Could I, If we ignore the non liniarity of the mosfet, adjust the resistance between 200 Ohm and 300M Ohm by changing the voltage to the gate? If so, how fine would such a adjustment be, could it simply be done by turning a single turn pot by hand? --- End quote --- Some points to watch out: - MOSFETs (at least the larger ones, even more at high voltages and temperatures) do leak. 300MOhm is very ambitious - a range of more than 1 to 1 million on a single sweep of a pot ? Practically not, without incredible effort. - you need a current sensing resistor and an op amp. Not an easy choice for your incredible wide range. Questions: - what is the maximum voltage and current you want to handle ? --- End quote --- Now, this is only theoretical, but I think that 6V 200mA would be what I'd expect would be reasonable per mosfet. Lets say we are working with audio, just to redoce the frequency problem, could you not calculate the range that are nessesary for i.e. 2V line input te be reduced to "nothing" = inaudioable even in the speaker after amplification? 2V is full volumen and 0,0000013V is lowest volumen, but the sound has to be gone fare before then? |
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