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Step 7Constant current source tweaks: #2 and #3
these slight modifications on circuit #1 address the voltage limitation of the first circuit. we need to keep the NFET Gate (G pin) below 20V if we want to use a power source greater than 20V. it turns out we also want to do this so we can interface this circuit with a microcontroller or computer.
in circuit #2, i added R2, while in #3 i replaced R2 with Z1, a zener diode.
circuit #3 is the best one, but i included #2 since it's a quick hack if you don't have the right value of zener diode.
we want to set the G-pin voltage to about 5 volts - use a 4.7 or 5.1 volt zener diode (such as: 1N4732A or 1N4733A) - any lower and Q2 won't be able to turn all the way on, any higher and it won't work with most microcontrollers. if your input voltage is below 10V, switch R1 for a 22k-ohm resistor, the zener diode doesn't work unless there is 10uA going through it.
after this modification, the circuit will handle 60V with the parts listed, and you can find a higher-voltage Q2 easily if needed.
in circuit #2, i added R2, while in #3 i replaced R2 with Z1, a zener diode.
circuit #3 is the best one, but i included #2 since it's a quick hack if you don't have the right value of zener diode.
we want to set the G-pin voltage to about 5 volts - use a 4.7 or 5.1 volt zener diode (such as: 1N4732A or 1N4733A) - any lower and Q2 won't be able to turn all the way on, any higher and it won't work with most microcontrollers. if your input voltage is below 10V, switch R1 for a 22k-ohm resistor, the zener diode doesn't work unless there is 10uA going through it.
after this modification, the circuit will handle 60V with the parts listed, and you can find a higher-voltage Q2 easily if needed.
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And if you would 'use this with a microcontroller' (though I do not understand how a microcontroller would deliver such high currents) the Voltage would most likely be 5 V to begin with. Or am I missing something?
First of all, I couldn't understand why did you insert the R2... Since I needed more input power, I just verified my NFET datasheet (IRF260) and Q1 (BC548) components and I noticed Q2 supports until 200V. So, I connected all the circuit at 180V, (an local 110V plus half-wave retific and filtered voltage). This power in drives a string of 40 5mm white leds (40x3.1=124V).
I know I could put more leds until reach near 180V, but I preferred put 3 paralel strings of 40 LEDs (a resistor was added in series on each LED string in order to fit all the strings to the same current). Of course, NFET will waste the extra voltage, but it doesn't matter to me, the important is to sustain the same 60mA constant current (3 strings of 20mA LEDS) and for this, the circuit is perfect.
So far, so good but, I have questions to ask for the group: what would happen with my 120 LEDs if Q1 openned its base or other else problems with this component ? There is a way to introduce a protection for them?
and more: How this circuit would behave if I strobe the LEDs feed with any flip-flop? it seems that the voltage delay a bit to fall when I just insert the leds... so, in a pulsed condition I belive the leds power will rise a bit.
Thanks
I use a group of 32 Cree MC-E leds(4 in series x8 in parallel), mounting in a XLarge heatsink with fans.
An IRF3710(Vd=100V,Rds=23mΩ, Ιd=57Α) drives these leds.
All work fine, except that leds open/close "unevenly" during first/last steps(Sunrise/sunset mode)
(it isn't a PIC problem, cause i tested with four 5mm simple leds that worked "smoothly")
Q: Can i replace the two resistors(100R, 10K) with your circuit and drive safely the leds?
*Vtot=12,5Volts/Itot=3,9Amperes
*See IRF.sch below.
Here's the problem:
a constant current source works by changing voltage to stay at set current.
because of this, you have to make sure each led shares current evenly, as they each have different turn on voltages. you can do this by putting a small resistor in series with each one. (10 ohms) The ideal solution would be a separate driver for each, but that might be expensive.....
After testing this i noticed that ATMEL doesn't minimizes the light at all, plus remote ctrl step to step cliks are different.
So with a digital voltmeter, i deside to measure the PWM output steps.
1)My PIC diy dimmer steps: 0.00>0.01>0.03>0.05>0.07>.....>5.02 Volts
2)ebay Atmel dimmer steps: 0.02>0.04>0.08>........>4.98 Volts
That's the answer:
the different voltage PWM steps in each microcontroller's software.
A smooth opening light mode should have 0.00>0.005>0.01>0.015>0.02>0.25>....voltage steps(maybe this will be done with an analog potentiometer).
I guess that there is no solution, except someone re-programs the .hex file. Cause i dont "speak" the PIC-language, my circuit will stay as is.
Anyway thanks for answers.
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*To help others:
A 10x10cm IRF heatsink gone useless. IRF+heatsink were TOO HOT!
I cooled the burning IRF, mounting it to a cheap 5x5cm PC VGA heatsink fan!
IRF now has the room temperature!
firstly thanks for your project!it's great, i have try it few days ago. and it works with one led.But the NFET heating too much . Do you know something about it? i've try it with 5v zener. and when i tried the circuit with three power led it wasn't heating but the leds were not bright enough.actually too dead.. ?
There is also one that doesn't specify a Vf... Which one should I get?