# Best way to lower current flow through MOSFET

amateur here... I want to switch a load through a MOSFET, approximately 200vdc with a 3amp resistive load, . If the source voltage drops I would also like to step down the current, ie if it hits 100v, I may want to drop current to 1.5amp. I dont need it to be linear, but rather just select a desired step level in my sketch, ie at 125v step to 2amp, at 100v 1.5amp, etc, in 2-3 steps.

I plan to drive the 10v gate with an optoisolator, with a 1k resistor across gate/source.

What is the best way to step my current down?

Should I PWM it, or is there a concern of MOSFET switching speed or gate heating?

Or can I lower the gate voltage to get a lower source current (I think this may increase resistance/heat on the gate=bad)?

Or do I need to build a separate circuit leg for each current level I desire, ie 3 current levels means I need to use 3 mosfets of 3 different io ports and switch one off, then the next step gets switched on. The switching speed does not matter to me, and the circuit can be paused/interrupted while switching to ensure complete discharge of the drain.

A requirement is to lower the source current, I do not really care what the load is seeing for current, so I do not want to burn off current as heat, that would still be using similar source current but showing less to the load, this is not what I want.

The mosfet I have chosen is IRFP460C, spec sheet: https://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/IRFP460C.pdf

thank you kindly for any pointers.

If the load is resistive, obeying Ohm's law, then the load current will drop as the source voltage drops according to I = V/R. You can reduce the [u]average[/u] current further by switching off the voltage source with PWM, but the peak current will be determined by the load and remains the same.

I do not want to burn off current as heat

Electrical power is converted into heat production. The current is everywhere the same in the circuit.

What is the best way to step my current down? = what for/why ? For low voltage I am using NDS 9430 - low resistance MOSFET, in you case 3A power dissipation on transistor will be less than 0.5W.

Use several mosfets with different loads (lightbulbs). Two mosfets = three different discharge currents.