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Hello. Here I have attached an image. The image mainly contain the constant current driver circuit for 10 w led. Now I want to switch it on and off (not PWM). I am considering to connect an npn transistor which is shown in dotted box to switch on and off. The first arrow point at the cathode terminals of the diode. Whenever this point gives zero then only the Led should turn on. O/w off. Am I going right or I will burn any other components. Plz guide me and give lil bit explaination of the working. Thank you. The Input is 12V.
Looks good to me. The MOSFET will need a heat sink.
How many volts will be across the LED? Probably 3V 3A.
The top of the LED is at 12V. The bottom of the LED is at 9V.
There will be 0.7V across R2. (lets say 1V so the math is easy)
This puts 8V across the MOSFET. 8V x 3A = HOT.
Just for fun lets drive two 10W LEDs in series.
So now there is not 3V but 6V across the LEDs.
The MOSFET voltage will be only 5V.
5V x 3A = HOT but better.
Lets try 3 LEDs.
LED voltage = 9V (more or less, varies with voltage,)
MOSFET voltage will be about 2V.
2V x 3A = 6w Ok that is also hot but better yet.
Led fwd vtg is 9-10V and current 1A. I have found this constant current circuit from a blog. Without the transistor in the dotted box. My question was Can I switch this led by adding that "dotted box transistor". Will this affect the const current circuit or whole circuit will overcurrent the led.
I'm really curious as to which LED you have choosen. Post a link to its data sheet.
I have found this constant current circuit from a blog. Without the transistor in the dotted box. My question was Can I switch this led by adding that "dotted box transistor". Will this affect the const current circuit or whole circuit will overcurrent the led.
I got the led from a local shop and have measured fwd vtg by myself. I don't have its datasheet.
And no I didn't understand the simulation. Sorry for that. Can you plz explain.
For your turn off transistor to work you should add a resistor from it's base to ground. maybe 10k. But there may be other problems with the circuit depending on the other parts you are using - the FET for example may not have enough voltage to get to 1 amp. So maybe you can add the rest of the values???
V2 in the simulation of Post #2 represents your control signal, presumably coming from a micro (Arduino?) port pin. It is the independent variable of the simulation. We are curious at what voltage from V2 does the LED turn on/off. V(V2) is plotted along the x-axis of the plot and in the simulation goes from 0V (logic zero) to 5V (logic one). The current through the LED I(D1) is plotted on the y-axis, so you can read the LED current for every input voltage V(V2).
As I set up the circuit, the added transistor Q2 turns on when V(v2) is about 2.5V (about halfway between a logic zero and a logic one). You can have multiple inputs creating a diode or-gate by connecting the cathode of the added diode to the cathode of the existing D2.
I am also showing the effect of rising temperature on the current through D1 (the LED). Note that the current decreases at elevated temperatures, meaning that the LED is somewhat protected from thermal runaway due to self-heating. Note that this LED MUST be bolted to a large heatsink, as it will be dissipating a lot of heat. So will the NFET M1.
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