Wanting an Nmos - Page 1
Forum section: electronics / projects

author: Tenderfoot
author_rank: Contributor
timestamp: September 21, 2025, 05:16:07 pm
content: I'm looking for an Nmos able to handle 24 volts at an amp or so, maybe a TO220 package and easily available. Here's the harder part I think. I want it to turn on 'slowly' I don't mean time-wise but to turn on over a wide range of gate voltage, no specific range in mind, just 'wider than most'. All the FETs I've tried in simulation turn on sharply over a half volt or so on the gate, most are used as switches I guess. I'm hoping for something more linear. Any suggestions? (yes I know it'll heat up but that's ok)

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2025, 06:19:53 pm
content: Quote from: Tenderfoot on September 21, 2025, 05:16:07 pmYou'll find that all MOSFETs do that. The circuit you're using is the wrong type. You need to use negative feedback to monitor the current through the MOSFET or voltage across it to gain the desired turn-on characteristic.

author: temperance
author_rank: Super Contributor
timestamp: September 21, 2025, 07:24:25 pm
content: You are probably trying to solve something the wrong way around.

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2025, 08:03:12 pm
content: The behaviour described by the original poster is intrinsic to all FETs, whether they're NMOS, PMOS, J-FET or P-FET. The polarity and magnitude of voltages and currents may differer but the curves are similar.Simulation comparing the response of a small MOSFET vs to a JFET. The X-axis is the get voltage which is swept between 5V and 5V and the Y-axis is the drain current. Note the curves are the same shape.

author: Tenderfoot
author_rank: Contributor
timestamp: September 21, 2025, 08:21:06 pm
content: Quote from: Zero999 on September 21, 2025, 08:03:12 pmThat's a very small voltage sweep!Thanks for your above replies. I'm just asking a question, wondering if there is a mosfet to replace a Darlington pair used as an emitter follower. That gives me two diode drops in a working circuit where I would rather not loose that voltage. I know at one time there were... vertical? fets used for audio and I had assumed they might be more 'linear', though mostly obsolete now.

author: NE666
author_rank: Frequent Contributor
timestamp: September 21, 2025, 08:32:52 pm
content: How deep are your pockets?https://www.digikey.com/en/products/detail/littelfuse-gsd/IXTH2N150L/5810057

author: Benta
author_rank: Super Contributor
timestamp: September 21, 2025, 08:41:59 pm
content: Quote from: Tenderfoot on September 21, 2025, 08:21:06 pmThe VFETs are legendary and were produced (and used) by Sony and Yamaha around 1980 for their audio power amps.Unobtainium today, and not really suited for your use.As you seen from Zero999's (green) plot, you won't gain anything from using a MOSFET, rather the opposite. Instead of 1.4 V drop you'll have >2.5 V instead.Might be better to look for a high-gain single transistor instead of a Darlington. Another option could be a Sziklai pair.

author: Tenderfoot
author_rank: Contributor
timestamp: September 21, 2025, 08:53:16 pm
content: Quote from: NE666 on September 21, 2025, 08:32:52 pmNot that deep 'specially when delivery is added.Quote from: Benta on September 21, 2025, 08:41:59 pmI believe a Sziklai pair has a saturation of around two volts? EDit..I am wrong, don't know where I got that from.(I'd missed that a fet might have around the same voltage loss)

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2025, 09:36:26 pm
content: Quote from: Tenderfoot on September 21, 2025, 08:21:06 pmI meant -5V to 5V. I know I messed up the text, but look at the plot.A Darlington pair also has a fairly steep on voltage characteristic. Here's a similar plot, but with a Darlington pair, rather than FET. S1 and I1 are models for the simulator to change V1 to a current source, i.e. limit the current, when the base-emitter junctions start to conduct. I could have simply used a resistor, but this illustrates the sharp turn-on more clearly.Darlington Vb Ic curve.asc(1.18 kB - downloaded 5 times.)Why do you want to replace a Darlington pair with a MOSFET?

author: amyk
author_rank: Super Contributor
timestamp: September 21, 2025, 09:55:42 pm
content: It sounds like what you want isn't a MOSFET, but an amplifier.

author: Benta
author_rank: Super Contributor
timestamp: September 21, 2025, 10:57:18 pm
content: Well, we're talking emitter followers here (according to post #4), so the "steepness" isn't really an issue, but an advantage for signal fidelity.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 21, 2025, 11:28:34 pm
content: Quote from: Tenderfoot on September 21, 2025, 08:21:06 pmSo on one hand you want a transistor whose control voltage to output current curve is "slow", but you also want something like a darlington with very low voltage drop. These two things contradict each other. You should probably give more detail on what you're actually trying to do, otherwise it sounds like you're going to get unhelpful feedback.Quote from: Benta on September 21, 2025, 08:41:59 pmI think you've got it mixed up; nearly all power MOSFETs are vertical, lateral MOSFETs used to be popular for audio applications (and others requiring operation in saturation) but are very rare nowadays. I think a couple companies like Exicon still make them for audiophiles though.

author: Benta
author_rank: Super Contributor
timestamp: September 22, 2025, 12:00:08 am
content: Quote from: mtwieg on September 21, 2025, 11:28:34 pmI've got nothing mixed up here, but it seems you're out of your depth.The VFETs made by Yamaha and Sony can be characterised as almost perfect triodes or "power JFETs".VFET is the marketing term, the scientific name is SIT or "static induction transistor".Only two complementary sets were ever made commercially, Sony's were 2SK82 and 2SJ28, Yamaha's I don't remember.https://en.wikipedia.org/wiki/Static_induction_transistorVery expensive to produce, which is why they disappeared in favour of planar, trench-gate etc. technology.

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2025, 01:22:27 am
content: Shunt feedback, with a resistor from the drain to the gate, will create a gradual turn-on characteristic. This works just as well with a bipolar transistor.

author: Tenderfoot
author_rank: Contributor
timestamp: September 22, 2025, 11:20:05 am
content: Time to own up, I made a silly mistake. The circuit is nothing to do with audio, nor does it need negative feedback. It's simply a fade up and fade down of some LEDs. The circuit uses a R/C and Darlington as a driver. I made it a couple of years ago and it's been working fine till it got water damaged. I thought I'd rebuild it maybe with a FET. Drew up a simple circuit in MicroCap simulator and got a strange result. Well what I'd done without thinking was use the FET as low-side switch, (NMOS is always low side switching, right) forgetting in the original I had used a high-side Darlington. This of course gave a result completely different to the way I knew the original circuit worked. I'll include a picture of what I saw, the FET result is in green, original circuit in red. Moral... Less drinking, more thinking.

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2025, 11:50:58 am
content: Quote from: Tenderfoot on September 22, 2025, 11:20:05 amAgain, why not simply replace it with another Darlington?Please post a schematic.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 22, 2025, 12:13:12 pm
content: Quote from: Tenderfoot on September 21, 2025, 05:16:07 pmAll bipolar transistors turn on even sharper over a 0.1V on the base.I suppose the vacuum tubes were turning on less sharply.

author: Tenderfoot
author_rank: Contributor
timestamp: September 22, 2025, 01:11:44 pm
content: Quote from: Zero999 on September 22, 2025, 11:50:58 amWhy replace the Darlington? Why not, who knows I might even learn something.A schematic is hardly worth it. It's like I said, a R/C into a Darlington, now changed to Sziklai driving some LEDs. But ok

author: exe
author_rank: Unknown
timestamp: September 22, 2025, 01:24:13 pm
content: How much is more linear? You can drive gate via a resistive divider... Though without a feedback (with e.g., opamp) it's prone to temperature drift and device-to-device variation.

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2025, 01:38:49 pm
content: Quote from: Tenderfoot on September 22, 2025, 01:11:44 pmBecause there is no need to. A MOSFET is an inferiour choice in that circuit.QuoteThe schematic is worth it. No one knew what you were on about before you posted it.You mentioned using the MOSFET on the low side. This is why it gave a sharp turn on. It had nothing to do with the type of transistor. The difference is in your schematic, you have the transistor configured as a follower, which has a voltage gain of just below 1, which buffers the capacitor voltage to the output, minus the threshold voltage. Moving the transistor to the low side, converts it into a common source/emitter amplifier with a high voltage gain, hence the large suddent change in voltage across the LEDs for a small change in voltage across the capacitor.You could swap the Sziklai for a low threshold voltage MOSFET and it would give a similar response, but it will still have a greater voltage drop, due to the threshold voltage being slightly higher than the Sziklai pair.If you really want to learn something, then redesign the whole circuit to use a microcontroller and dim the LEDs with PWM, using a MOSFET.

author: Tenderfoot
author_rank: Contributor
timestamp: September 22, 2025, 02:27:39 pm
content: Quote from: Zero999 on September 22, 2025, 01:38:49 pmThat would teach me nothing, I was programming in assembly decades ago, and using PICs shortly after. Using a microcontroller and code to replace one cap and one resistor is, I think, pointless. You of course may disagree.Regarding your comment about using a low side transistor, I agree, as you would see if you read back.Thanks for everyone's comments and suggestions (and I do mean everyone). I'm guessing this post has come to it's natural conclusion at this point?

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2025, 03:02:50 pm
content: Quote from: PGPG on September 22, 2025, 12:13:12 pmThe turn-on characteristic for MOSFETs and bipolar transistors is defined by their transconductance, which is the change in output current for a change in input voltage at the gate or base. Bipolar transistors have higher transconductance than MOSFETs, so they switch more abruptly.

author: MariuszD
author_rank: Regular Contributor
timestamp: September 22, 2025, 03:30:53 pm
content: Using just the MOSFET will be difficult, but a two-transistor amplifier will allow you to achieve the desired characteristics like low dropout voltage and better linearity.My circuit uses PMOS and NPN transistors, and it can be easily converted to NMOS and PNP.This circuit used to control LED strips has one drawback: current only flows thru the diodes above 18V. This is an area for further improvements.

author: mawyatt
author_rank: Super Contributor
timestamp: September 22, 2025, 04:11:51 pm
content: Quote from: David Hess on September 22, 2025, 03:02:50 pmTheoretically a MOSFETcanhave ahigher transconductancethan a bipolar at a specific current since the transconductance is proportional to W/L and a bipolar transconductance is fixed and not dependent on size or anydevice parametersother than Ic/Vt, where Vt is the thermal voltage kT/q and thus the bipolar device is not dependent on anything but collector current, temperature and 2 physical constants. On the other hand the MOSFET gm is directly size (W/L) dependent and can be scaled for a specific transconductance, of course for a normal size die area the bipolar has the higher gm.Another interesting aspect of MOSFETs is theThreshold Voltagecan be controlled so they conduct well below the bipolar 0.6~0.7 volts and can be created as Depletion or Enhancement mode devices.Interestingly way back ~2000 when we were working with IBM SiGe bipolar devices a discussion was to create a Depletion Mode Bipolar SiGe Device!! The device physics indicated that a specific Ge profile grading within the SiGe bipolar base could create a sufficient electric field to cause significant collector current to flow without any base connection even at low collector potentials and somewhat independent of collector potential. Altho this idea gained some traction we couldn't get funding to continue and the Depletion Mode Bipolar Transistor was abandoned.Best

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2025, 04:45:20 pm
content: Quote from: Tenderfoot on September 22, 2025, 02:27:39 pmAre you sure? Have you done it recently? Things have moved on a bit since then. How about programming in C?QuoteThat wasn't the idea. There are other reasons why it might be a good idea. The main one is PWM means the transistor can run cooler so a smaller device can be used i.e. TO-92, rather than TO-220.QuoteFor future reference, please post the schematic in the first place. A lot of time has been wasted guessing.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 22, 2025, 08:09:42 pm
content: I like to control backlighting LEDs (at less than half a watt of power) using current control, instead of PWM'ing, because of an utterly unreasonable hate towards flickering. As my enclosures are small (palm-sized to smaller than a shoe box), the problem with linear current control is the waste heat. Thus, I've switched my focus to current-mode buck and boost controllers; specifically those that adjust the feedback voltage threshold using a filtered PWM input, likeTI TPS92201A. (That particular .one is a buck/step-down controller limited to 5.5V or less, so is not suitable for OP.)I wonder if such a buck or boost controller (depending on what kind of input voltage OP has available) could be a better solution for OP too?For example,TI TPS922051D2you could control using a potentiometer in a voltage divider configuration. These are cheap, too, this one less than 1€ apiece in lots of ten from Mouser or Digikey, although you'll likely spend more on a good inductor to use with it. (I haven't used this one in particular, and only bring it up as an example to start your IC search/selection from.)

author: Tenderfoot
author_rank: Contributor
timestamp: September 23, 2025, 12:19:03 pm
content: Quote from: Zero999 on September 22, 2025, 04:45:20 pmFor future reference, please post the schematic in the first place. A lot of time has been wasted guessing.Re-read my original post. I asked about the availability of a component with particular characteristics. People were free to say 'Such a thing doesn't exist, you are an idiot' or possibly 'Have you tried this one'. There was no need of a Schematic. Schematics seem to lead to more arguments and off-track comments. That is why I purposefully did not show one. Now regarding using some kind microcontroller or whatever. I have zero interest in that. Electronics for me now is a hobby I dabble in it for enjoyment, no other reason. Taking a circuit from some application note, or buying a module from China and then getting chatgpt to write the code is not what I want to do. Digital is the future that's obvious, but some people like to tinker with old cars, I like 'old' electronics. So I will stick with analogue thank you. Maybe I'm weird, but there is no law against that.

author: Tenderfoot
author_rank: Contributor
timestamp: September 23, 2025, 12:22:50 pm
content: That looks an interesting circuit, I'll give it a try as soon as I have time

author: Zero999
author_rank: Super Contributor
timestamp: September 23, 2025, 12:28:26 pm
content: Quote from: Tenderfoot on September 23, 2025, 12:19:03 pmI have read your original post several times.On the contrary, the lack of schematic resulted in off-track comments. Once you posted the schematic, it became abundantly clear, the turn on characteristics, had nothing to do with the transisor type but how it was configured: follower vs common source.QuoteFair enough. No one is telling you what to do, merely offering alternative solutions.I also suggest a buck converter, similar to Nominal Animal's suggestion. It would improve efficiency because power woudn't be wasted in current limiting resistors, but if it doesn't interest you, then no problem.

author: temperance
author_rank: Super Contributor
timestamp: September 24, 2025, 09:07:29 am
content: For light dimming, you need a logarithmic dimming curve. You can create this dimming curve with a PN junction "for free". The same can be achieved with MOSFET operated below the threshold voltage.

author: Zero999
author_rank: Super Contributor
timestamp: September 24, 2025, 11:15:17 am
content: Quote from: temperance on September 24, 2025, 09:07:29 amThe OP's circuit already acomplishes this with an RC circuit.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 25, 2025, 12:11:55 am
content: The shape of the LED current ramp isn't that important to human perception, it is the maximum slew rate — fastest rate of change — that really matters.Seeing that the OP has around six chains of LEDs they want to turn on and off "slowly", I think I'd use aTI TPS922050D2DGNper chain, so that each chain has a 24V input, GND, and a 5V active-high enable signal. One could use e.g. shielded phone cable, two twisted pairs within a common shield, for longer runs.The active-high enable signal would be connected to a 2.5V regulator enable pin, sayAnalog ADP121, whose output would be connected to a low pass filter (say, three 33kΩ resistors in series, each followed by a 1µF capacitor to ground, for about one-second turn-on/turn-off time), and to the analog DIM pin of the TPS922050D2.The LED control circuitry costs about 2€ per LED chain — TPS922050D2 is 0.435€ apiece in lots of ten at Digikey, 0.836€ apiece in lots of ten at Mouser; the inductor depends on the current each LED chain needs, with the calculation procedure described in the datasheet linked to above. Because you can adjust the turn-on/off time per chain, you can do stuff like have them turn on/off almost sequentially, for an even more pleasing effect. With a bit of care in the design of the control PCBs, you can have them made and assembled in sets of ten, leaving out just the parts that vary (like the RC low-pass filter and the sense resistor setting the maximum current). After populating and testing the board and soldering the wires (to connectors) to it, use UV-curing nail polish to protect the PCB and components from moisture.As you can see, one can definitely use "modern" ICs to get this kind of stuff done pretty effectively, without any microcontrollers or PWM at all. Also, TPS92205xD2 are definitely not the only LED drivers with an analog input control pin you can find; there are many, many alternatives from several manufacturers that will work well. You only need a microcontroller if you want to use the PWM controlled one LED drivers and a programmable ramp; I recommend the 3·(t/T)² - 2·(t/T)³ one for turn-on during 0≤t≤T, and 1 - 3·(t/T)² + 2×(t/T)³ for turn-off during 0≤t≤Tin that case, which I find particularly pleasing. Note that many of the cheap (~0.5€ in singles) microcontrollers easily programmed in the Arduino environment can achieve this, so it really would not affect the price much.