Q:
To power the LED module with MOSFET, what does it mean by "emits (a) constant current"?
I've got this simple circuit:
It's basically a constant current power supply (9V), just powered by a MOSFET.
When I put a LED in the circuit and turn the MOSFET on, it works fine. But I'm not sure what's the meaning of "emits (a) constant current". The image below is what the schematic said, what does it mean by that?
The circuit is powered by 9V through diode, C5 & D1. The MOSFET's gate is connected to 9V through C5 & D1, and GND. What does it mean by "emits (a) constant current"?
A:
All MOSFETs are capable of switching a load with a high peak current - that's the point of them. This includes BJTs and even MOSFETs in the low-power'science-project' category.
However, the load current normally comes from a battery or other voltage source and so is not high peak.
So, when driving a low load current such as a diode or other LED, the load voltage is close to ground (due to the (usually) high internal resistance of the diode) and so the MOSFET 'opens up' quickly, and only needs to handle a short spike of current.
However, when the MOSFET is switching the load, it does need to handle the entire load current. So, to keep it operating for any length of time with a switched-load, it needs to be either a high current drive MOSFET (probably an IGBT) or a low-power MOSFET such as a STP72 or STP99, which have a much lower internal resistance.
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