Forums

Low Threshold MOSFET

Started by rickman March 20, 2017
I was looking to order some low current MOSFETs and realized all the 
popular items had thresholds of 3 volts and higher.  I would like to use 
something that turns on pretty well at 3 volts so it won't need a 5 volt 
PSU.  Any suggestions?  I'm looking for a TO-92 case preferably but 
surface mount will be ok too.

I could have sworn some years ago I tracked down a number of low 
threshold voltage FETs.  But maybe those devices operated under 5 volts 
rather than under 3.3 volts.

-- 

Rick C
Am 20.03.2017 um 05:34 schrieb rickman:
> I was looking to order some low current MOSFETs and realized all the > popular items had thresholds of 3 volts and higher. I would like to use > something that turns on pretty well at 3 volts so it won't need a 5 volt > PSU. Any suggestions? I'm looking for a TO-92 case preferably but > surface mount will be ok too. > > I could have sworn some years ago I tracked down a number of low > threshold voltage FETs. But maybe those devices operated under 5 volts > rather than under 3.3 volts. >
The 2N7000 (200 mAmps, 60 Volts) has Gate Threshold Voltage of 0.8V min to 3.0V max <https://www.onsemi.com/pub/Collateral/2N7000-D.PDF> Jorgen
On Mon, 20 Mar 2017 00:34:21 -0400, rickman wrote:

> I was looking to order some low current MOSFETs and realized all the > popular items had thresholds of 3 volts and higher. I would like to use > something that turns on pretty well at 3 volts so it won't need a 5 volt > PSU. Any suggestions? I'm looking for a TO-92 case preferably but > surface mount will be ok too. > > I could have sworn some years ago I tracked down a number of low > threshold voltage FETs. But maybe those devices operated under 5 volts > rather than under 3.3 volts.
I assume you want N channel and that "low current" means up to a few hundred mA. I use a lot of BSS138 (VGS(th) 0.5 to 1.5V) when driving from 3.3V logic, and DMG1012T (VGS(th) 0.5 to 1.0V) when driving from 1.8V logic. Both are cheap and readily available. Regards, Allan
On 20/03/2017 04:34, rickman wrote:
> I was looking to order some low current MOSFETs and realized all the > popular items had thresholds of 3 volts and higher. I would like to use > something that turns on pretty well at 3 volts so it won't need a 5 volt > PSU. Any suggestions? I'm looking for a TO-92 case preferably but > surface mount will be ok too. > > I could have sworn some years ago I tracked down a number of low > threshold voltage FETs. But maybe those devices operated under 5 volts > rather than under 3.3 volts.
'logic level mosfet' is a good search term. Cheers -- Clive
Allan Herriman wrote...
> > ... DMG1012T (VGS(th) 0.5 to 1.0V) when driving from 1.8V logic.
The DMG1012UW is in a bit larger package, 1.3mm vs 1mm lead spacing. The DMG1013UW is a p-channel equivalent. -- Thanks, - Win
On Mon, 20 Mar 2017 00:34:21 -0400, rickman wrote:

> I was looking to order some low current MOSFETs and realized all the > popular items had thresholds of 3 volts and higher. I would like to use > something that turns on pretty well at 3 volts so it won't need a 5 volt > PSU. Any suggestions? I'm looking for a TO-92 case preferably but > surface mount will be ok too. > > I could have sworn some years ago I tracked down a number of low > threshold voltage FETs. But maybe those devices operated under 5 volts > rather than under 3.3 volts.
Go DigiKey searching on FETs. Choose "logic level" as a category. If I remember, you can also choose the gate drive voltage -- choose 3.3V. You should get some candidates to ponder over. Alternately, maybe a bipolar makes sense? Zetex invented some pretty awesome little bipolar transistors before they got bought out by Diodes, Inc. -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
On 3/20/2017 8:30 AM, Lund-Nielsen, Jorgen wrote:
> Am 20.03.2017 um 05:34 schrieb rickman: >> I was looking to order some low current MOSFETs and realized all the >> popular items had thresholds of 3 volts and higher. I would like to use >> something that turns on pretty well at 3 volts so it won't need a 5 volt >> PSU. Any suggestions? I'm looking for a TO-92 case preferably but >> surface mount will be ok too. >> >> I could have sworn some years ago I tracked down a number of low >> threshold voltage FETs. But maybe those devices operated under 5 volts >> rather than under 3.3 volts. >> > > The 2N7000 (200 mAmps, 60 Volts) > > has Gate Threshold Voltage of 0.8V min to 3.0V max > > <https://www.onsemi.com/pub/Collateral/2N7000-D.PDF>
That's the threshold voltage where drain current begins essentially. Looking at the drain current vs. drain-gate voltage typical curves the current at 3.0 volts is well under 100 mA. I suppose I could put two or three in parallel. I didn't want to use an NPN because the base current goes through the emitter resistor which I'm using to measure the current, but when I did a simulation that current is pretty small, so I suppose it's not really a problem. The resistors are only 1% accurate. I've got NPNs on hand, so I'll use one for now. -- Rick C
Fairchild (whoever now) has a nice n channel device speced, if I remember 
correctly, at 1 volt max turn on. It served as a voltage follower op amp 
with lots of offset. They came from Mouser.

Hul

rickman <gnuarm@gmail.com> wrote:
> On 3/20/2017 8:30 AM, Lund-Nielsen, Jorgen wrote: > > Am 20.03.2017 um 05:34 schrieb rickman: > >> I was looking to order some low current MOSFETs and realized all the > >> popular items had thresholds of 3 volts and higher. I would like to use > >> something that turns on pretty well at 3 volts so it won't need a 5 volt > >> PSU. Any suggestions? I'm looking for a TO-92 case preferably but > >> surface mount will be ok too. > >> > >> I could have sworn some years ago I tracked down a number of low > >> threshold voltage FETs. But maybe those devices operated under 5 volts > >> rather than under 3.3 volts. > >> > > > > The 2N7000 (200 mAmps, 60 Volts) > > > > has Gate Threshold Voltage of 0.8V min to 3.0V max > > > > <https://www.onsemi.com/pub/Collateral/2N7000-D.PDF>
> That's the threshold voltage where drain current begins essentially. > Looking at the drain current vs. drain-gate voltage typical curves the > current at 3.0 volts is well under 100 mA.
> I suppose I could put two or three in parallel.
> I didn't want to use an NPN because the base current goes through the > emitter resistor which I'm using to measure the current, but when I did > a simulation that current is pretty small, so I suppose it's not really > a problem. The resistors are only 1% accurate.
> I've got NPNs on hand, so I'll use one for now.
> --
> Rick C
On 2017-03-19 21:34, rickman wrote:
> I was looking to order some low current MOSFETs and realized all the > popular items had thresholds of 3 volts and higher. I would like to use > something that turns on pretty well at 3 volts so it won't need a 5 volt > PSU. Any suggestions? I'm looking for a TO-92 case preferably but > surface mount will be ok too. > > I could have sworn some years ago I tracked down a number of low > threshold voltage FETs. But maybe those devices operated under 5 volts > rather than under 3.3 volts. >
Assuming you want to switch power to some circuit and thus need P-channel: http://www.diodes.com/assets/Datasheets/DMP2100U.pdf Key is that Rdson is guaranteed at your min drive voltage, not just in the graphs but in the tables. N-channel example: http://www.diodes.com/assets/Datasheets/DMP2100U.pdf A beefier one: http://www.diodes.com/assets/Datasheets/ds31837.pdf -- Regards, Joerg http://www.analogconsultants.com/
On 3/20/2017 6:47 PM, Hul Tytus wrote:
> Fairchild (whoever now) has a nice n channel device speced, if I remember > correctly, at 1 volt max turn on. It served as a voltage follower op amp > with lots of offset. They came from Mouser.
Any more info than that? Threshold voltage is not easy to search on because everyone specs it at a different current. Also the threshold voltage can be a *lot* lower than the fully on voltage which is the important one. Or is that what you mean? Still, it takes a lot of data sheet searching to find a part that is optimal for low voltage control and that is why I asked here. I am hoping to benefit from other's experience. -- Rick C