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If the MOSFET can handle the excess power (and is mounted on a suitable heatsink), a small signal transistor and a resistor can set the max current and then you just use PWM to dim it.
I'd recommend a totem pole circuit for that... Don't forget there is a gate-drain capacitance that can cause excessive problems, especially on the transistor ( of course you still need schottky diodes)Not that Sören's idea is wrong. By any means No.... But using totem poll is just a little bit finer for the expense of complexity.
Don't forget that it would be operated in a combination of switching and linear modes.
The transistor I suggested doesn't determine whether you use a totem pole or not, as the "pole" would be for amplifying the current to the gate, while what I suggested is just pulling the gate down (so would just have more work to do) and since it needs to ride in the MOSFETs linear range to be used for current limiting anyway, I'd suggest using a gate resistor of 220 Ohm (and no totem pole, but if one is at hand, a smaller device would be my choice).
And where do you want to use Schottky diodes??
BTW. Do you have a Swedish or German keyboard? The letter "ö" (o-umlaut) is used in Sweden and Germany), but the Danish equivalent is "ø" (0-slash)
How can he ride the Mosfet to the lineal region without being able to determine the voltage at the gate? To drive it to the lineal region he would need an RC circuit and PWM. He probably has pwm, but no RC?
If he's driving the MOSFET with a direct PWM signal then he's using it in the switching region.
And why such a high gate resistor, it's generally most needed to have on, but I usually use one at the 18 - 50 Ohm range.
Hmmm, Imagine the totem poll circuit driving a motor (the one lead), don't you usually place Schottky there.
I'm talking about these Schottky. Just before the gate resistor to protect the driving circuit, from the evil capacitive load.
With a 220Ohm resistor though, he may not need to use them.
Mein Freund lerne ich Deutch, weil ich werde nach Bremen gehen. Und ich werde Robotik für sechs Monate dort studieren.Erasmus, oh wunderbar, Erasmus!!!
"Usually" depends on where you look Without a (small-ish) gate resistor (or a ferrite pearl), the MOSFET may start oscillating at HF in some instances.For switching use, it should be as low a value that keeps the oscillating from happening (at any time).In this case, I consider it more important to keep the I/O pin that controls it from going into latch-up and to keep the stress on the BjT low.220 Ohm won't hurt the controller in any case, but this is a starting point and any circuit, however simple, can (and should be) be experimented with if the proper tools (like an oscilloscope) is at hand.The comparatively small losses due to the slow turn on won't matter, as we're introducing loss as the goal of the exercise.
(As a side note/FYI, if you write either "220 Ohm" or "220ohm", it's much easier not to read it as 2k2).
Quote from: TrickyNekro on May 11, 2011, 08:37:54 AMMein Freund lerne ich Deutch, weil ich werde nach Bremen gehen. Und ich werde Robotik für sechs Monate dort studieren.Erasmus, oh wunderbar, Erasmus!!! Hehe - you're better at electronics than at German (My "sister in law" is married to a German and lives in Germany about 150 km NE of Bremen and while I speak Danish to their kids, they usually answer me in German - and spoken German is not exactly my strong side )So, how did you get on the Erasmus?
The resistance unit, Ohm, is always capitalized since it is a proper noun named after the German physicist Georg Ohm. This goes for any of the other units is electronics (and other sciences). TrickyNekro correctly capitalized Schottky and Ohm.
You can build a very very nice current source with a microcontroller, a power transistor, two resistors and one capacitor.If you need I'll upload a circuit.
What is the forward voltage of your leds? Knowing this I can answer
What do you think about this:http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step8/a-little-micro-makes-all-the-difference/
From what I gather the following are the parts for it:R1: approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)R3: R3 = 0.5 / 0.7 = 0.71 ohms. closest standard resistor is 0.75 ohms. Current set resistor.Q1: small NPN transistor (such as: Fairchild 2N5088BU)Q2: large N-channel FET (such as: Fairchild FQP50N06L)Z1: use a 4.7 or 5.1 volt zener diode (such as: 1N4732A or 1N4733A)
Give me some time, I'll have it ready ;-)
That's exactly what I was talking about (minus R1 and the zener diode).
Loose R1 and Z1.You need a resistor between I/O pin and gate (the 220R I mentioned).Q1 can be just about any small signal transistor (BC547, 2N3904 etc.)Q2 could be rated lower, but will do fine - it will need a heatsink!R3 should be ~0.65V/ILED(0.65V is the approx. base-emitter voltage drop for an average BjT).
The collector of the BjT goes directly to the gate of the MOSFET.
Couple of questions:1. Does the circuit I attached make sense assuming I only want 1000mA of current going to the LEDs?
2. Is the value of Rc correct? Checked up on DigiKey and the closest to 0.65ohm was 0.68ohm
3. What should the watt ratings for Rg and Rc be? I'm assuming Rg=1/4Watt and Rc=5W
As for this, should I use the solid line as it was originally connected or use the dotted line that I put in?