Hello, I have a couple question regarding these transistors...
I understand that MOSFETS are controlled by voltage and BJT are controlled by current, but in simple terms explain when I might use one or the other and why?
Also, i know how to use my n channel MOSFET in linear mode to dim an led, could I essentially do the same with a BJT?
You can dim leds with transistors but not high power leds.
As you do projects always look at the data sheets for the parts you use. The more you do this the clearer things become over time. At some point parts of them that made you glaze over before suddenly become very interesting. Since this is an Arduino forum let's start there. The UNO is fairly robust compared to some of the 3.3V models when it comes to current. When I started playing with a Beagle Bone Black current became a bigger pin issue. 3 to 4 mA per pin.
Always do the math and measurements so you know what your circuit is doing and the components start to sort themselves out.
It really comes down to application. If mosfets were the do-all solution, bjt's would cease to be produced. Just by the nature of each, mosfets being voltage controlled and bjt's being current controlled, is a factor in deciding.
Bjt's have diode drops, fet's do not. Mosfets are easily controlled by PWM since the gate requires no current, and the only current is the pulldown resistor of the gate, which can be readily controlled with a single value resistor. Bjt's on the other hand, are easier to use in amplifying sinusoidal signals.
Yes, adjusting led current was done by analog means in the past, before mosfets, before the flood of digital chips. Engines used to have a hand crank on them too.
so what would be the best way to power and dim LEDs without the use of microcontrollers?
Found this with Google, one of thousands...
tjones9163:
so what would be the best way to power and dim LEDs without the use of microcontrollers?
An adjustable Constant Current module
MOSFET gates require a lot of current(*) when being PWM'd, but only in brief spikes. A MOSFET gate behaves
very like a capacitor. No net DC current, but the AC component can be large.
(*) 100's of mA is quite common. More current allows faster PWM without large switching losses due
to sluggish transitions from on <-> off.
Noobian:
You can dim leds with transistors but not high power leds.
Sure you can, it's just harder.
- I understand that MOSFETS are controlled by voltage and BJT are controlled by current, but in simple terms explain when I might use one or the other and why?
Low power stuff in the hobbyist space tends to be dominated by BJTs since they're easier to get and deal with than logic level MOSFETs. There's also far more example circuits with 2N2222, 2N3904, or BC548 than something like a 2N7000.
High power stuff though (up in the amps range or higher) is almost exclusively done with MOSFETs through, because of lower power wastage. The gate voltage needed by these MOSFETs is usually much more than an Arduino can supply (usually 10V or more), requiring external level-shifting circuitry that a Darlington transistor wouldn't need.
"Easier to get and deal with" means "are through-hole components available from hobby suppliers"
If you go to an electronics distributor and are happy with surface-mount components, FETs are extremely
easy to get in a vast variety (many thousands of lines) - as are surface mount BJTs, since commercial
electronics uses as few through-hole components as possible (more expensive to manufacture).
Simple way to dim standard leds (in series, up to supply limits of ~15 volts). IMO, a 555, two resistors and a cap.
Sure, there are other ways, some "better". For me it, its nostalgic, the 555 was the first non-opamp linear that I did anything useful with in 1973. Still have my original Signetics linear data book dated 1976. The year of the US bicentennial.