Looking for the right MOSFET transistor

[Charlie Kelly]

I need a MOSFET transistor that will turn off anywhere between 2.5-2.7V and stay on when above that voltage, looking through the datasheets of these transistors, what spec should I be looking for?

Here are some I've found that I think may work:
https://uk.rs-online.com/web/p/mosfet-transistors/6887178/
https://uk.rs-online.com/web/p/mosfet-transistors/9047348/

I think these will work for me based off of the "max/min gate threshold voltage" but I'm worried that may be incorrect.

 

[Baluncore]

What maximum voltage when off? What maximum current when on?
Turn on and off times depend on how it is driven. How fast must the MOSFET switch?
On and off are not exact. The pinch-off voltage range can be predicted and listed as max/min, but the switch resistance will fall as gate voltage increases above that critical point.
How much gate drive voltage and current do you have?
Are you switching an inductive load such as a motor? If so look for an internal Schottky diode.

 

[Tom.G]

Neither meet your stated requirements, the Minimum Gate Threshold Voltage is too low. The first one may turn on at 1.4V and the second one at 2.4V. Keep looking.

 

[Charlie Kelly]

What maximum voltage when off? What maximum current when on?
Turn on and off times depend on how it is driven. How fast must the MOSFET switch?
On and off are not exact. The pinch-off voltage range can be predicted and listed as max/min, but the switch resistance will fall as gate voltage increases above that critical point.
How much gate drive voltage and current do you have?
Are you switching an inductive load such as a motor? If so look for an internal Schottky diode.

Im using it to power LEDs than run at very low current, about 20mA and the max voltage will be only 5.5V. It is not necessary for the MOSFET to switch instantly but would be better if it did. I am making a small crank light and I want the capacitors to go straight to the LEDs until the voltage drops and the LEDs get too dim, then for the MOSFET to switch off and the power to go through the joule thief to drain the capacitors completely while still providing bright light.

 

[Charlie Kelly]

Neither meet your stated requirements, the Minimum Gate Threshold Voltage is too low. The first one may turn on at 1.4V and the second one at 2.4V. Keep looking.

Okay, so the min gate threshold voltage is when the resistance in the mosfet will become low enough to start letting some current through but as the voltage becomes higher, the resistance in the mosfet will keep dropping until it reaches the max gate threshold voltage where the resistance in the mosfet will be as low as it can be, correct? Or am I understanding it incorrectly.

 

[Tom.G]

Well... close. The device will start to turn on somewhere between the Min and the Max Gate Threshold Voltage; it won't be the same for every transistor you get. The spec sheet says only that every device they supply wil turn on somewhere between the Min and Max. Since these devices are rated in 19Amps and above, there won't be much Gate voltage difference between 'barely on' versus 'full on' for your 10mA or so current.

 

[Baluncore]

digikey.com stock the Infineon BSR802N for less than $1. It is small but should do the job OK. Data sheet here;
https://www.infineon.com/dgdl/Infin...n.pdf?fileId=db3a30431b0626df011b129302297bc1