# How to choose the correct N channel Enhancement MOSFETs?

Having little to no experience on MOSFETS i am having trouble part sourcing the correct one, one that will work for my applications.

But before i go with the terms let me show the circuit first

simulate this circuit – Schematic created using CircuitLab

I am creating a inverted LED indicator, the initial plan was to use a 3904 instead of MOSFET, but notice R3 before the switch, the switch that i have may OFTEN HAVE a 11.3k contact resistance when closed meaning the led will remain on even if switch is closed.

Following the recommendation of a previous question and my own reading the net, I already know that i will be using a N-channel enhancement MOSFET. So i went to digikey to source possible candidates. There i was presented with filters that are way over me.

Here are the filters, Please correct me if weather i am right or wrong:

Drain to Source Voltage (Vdss) - I guess this filter means that the maximum voltage in the drain and source, basically the maximum voltage the mosfet can function.

Current - Continuous Drain (Id) @ 25°C - The maximum continuous current that the mosfet can handle. but on the choices there are some letters in parenthesis that are odd to me. e.g. 115mA*(tc)* or 115mA*(tj)* or 115mA*(ta)* . What are these ta tc tj.

Drive Voltage (Max Rds On, Min Rds On) - The voltage in the gate for the mosfet to turn on, since the mosfet act like a variable resistor the max Rds On is giving out the maximum resistance and min rds on is giving the smallest possible resistance e.g. 1.8,2.5 at below 1.8volts the mosfets is at maximum resistance and at 2.5v it is at minimum resistance

Rds On (Max) @ Id, Vgs - im a litle bit confused on this one i think this is supposed to be maximum resistance the mosfet but the values are in milli range e.g. 22mOhm @ 6.5A 4.5v

Vgs(th) (Max) @ Id - ???

Gate Charge (Qg) (Max) @ Vgs - ???

Vgs (Max) - ???

Input Capacitance (Ciss) (Max) @ Vds - ???

are there filters that i should not mind ?

• At such a low supply voltage I suggest you first check the gate On-Voltage. Many a designer has grabbed a NMOS to connect to their 3V3 CPU to find they need about 4-5V to switch on. – Oldfart Nov 5 '19 at 13:22
• i thought that is the drive voltage, if it is not then i have to apply the filter first and look through the datasheet of whatever is reaming for the On-voltage – Jake quin Nov 5 '19 at 13:31
• tj = temperature of junction (solder joints) , ta = temperature of ambient (i.e air around part) – vicatcu Nov 5 '19 at 13:48
• @vicatcu thank you for that, that is one thing clarified :) – Jake quin Nov 5 '19 at 13:51
• Sorry, yes I am talking about the drive voltage. – Oldfart Nov 5 '19 at 14:10