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High Power LED Mood Lamp


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Hi Gramo,
You have read the datasheet for the PIC. You saw that it does not have a higher pulsed max current rating like a motor and a power rectifier do. Absolute maximum means absolute maximum, not "sometimes".

Read the forum of the author of our Mood Lamp project. He forgot to use base resistors and some people did fry their PIC. Then the author used 10k resistors with a value much too high for our project. Kade used 100 ohms which is too low.

You didn't answer my question about what is the max PWM duty-cycle in the PIC's code. The author's max peak current measurement is far from the max current rating of the expensive LEDs.

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You are right. I did not answer that question. You are telling me how it works, but at the same time, asking me how it works.

At this point I see that I have allowed this thread to get away from being useful to members of the community. Thus, I will withdraw from the debate. I encourage you both to continue your education and to take a few chances on parameters that are not listed on the data sheet.
When a spec is not listed on a datasheet, this does not mean it is not allowed. A datasheet is only a guide or starting point. It would be a fairly large book if the manufacturer listed all possible test parameters. It only gives you the basics. From there, you must use your own knowledge and experience, or perform your own testing.


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Hi Gramo,
I started this thread because the value of the base resistors is too high to allow the transistors to conduct the high currents needed for the expensive tri-colour LED.
I also noticed that the value of the current-limiting resistor for the red LED was too low and would cause the current to be too high and the value for the blue LED was too high.

The author made his first Mood Lamp using the transistors as emitter-followers that did not need base resistors.
Then he made this one with the transistors as common-emitter type but did not include base resistors. People on his forum complained that their PICs were frying. I have attached his schematic here.
He added 10k base resistors to our project without calculating their value properly.
Kade in his forum also mis-calculated and used 100 ohm base resistors.

I don't know if it is the switching speed of the transistors or the program PWM that limits the currents in the LED too much. Maybe the author burned a few LEDs because his heatsink is too small. Then maybe he reduced the PWM max duty-cycle to reduce the average current for less heating.


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Hi ante,
The BC517 darlington transistor has a typical base-emitter voltage of 1.4V at the current used by the LED. When a PIC output goes high then a base resistor is needed to limit the current to less than 25mA.
The max current in this project is much lower than the 350mA rated current of each of the 3 colours at  max brightness simultaneously. I don't know why the current is so low.

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Hi Ante,
The absolute max output current allowed for a PIC is only 25mA.
The absolute max output current allowed for a 74HCxx is also only 25ma.
I think a PIC has the same Mosfets as 74HCxx ICs.

My graph of output currents for 74HCxx ICs shows that with a 4.5V supply and a transistor's base holding an output at 0.7V then the typical output current is 41mA and with a 6V supply it is 68mA. Average the two at 55mA for a 5V supply.
The current reduces a little as the output Mosfet gets hot.

Base resistors as low as 172 ohms would limit the current to 25mA.
100 ohms is too low because then the current is too high.
10k ohms is much too high unless the transistor's load current is very low or the transistor is a high gain darlington.

This circuit is fine when the proper base resistor value is used.

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  • 2 weeks later...

Hi there

i found your project here, but was really shocked when i found out
you did not recommend a heatsink as a musthave

if you use the 3W zpower led like in your tutorial
you MUST use a heatsink.

with no heatsink you will shorten the life of your led very extremly!!

i quote the datasheet of the original 3W zpower led:

Please do not drive a rated current more than 5 sec. without proper heat sink

so, to the author please add that to your tutorial

some suggestion:

i used an old pentium 100 heatsink in my moodlamp:


my complete worklog could be found here:

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Hi Harald,
Welcome to our forum.
The author of out Mood Lamp project used a heatsink on the first one he made and it is shown on his website. But for our project he says you can use a heatsink if you want.

The 10k base resistors have a value that is way too high for the transistors to conduct high current. The base resistors should be 680 ohms. Then the LEDs will be much brighter if the program allows it. Then a heatsink must be used.

Here is a pic of Toon's heatsink:


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  • 1 month later...

Hi Blueroom,
An old IRF510 Mosfet has a max threshold voltage of 4.0V and it conducts only 250uA. So if its gate voltage is 5.0V then it conducts only about 2.5mA. It turns on well when its gate voltage is 10V.

An expensive logic-level Mosfet like an IRF3711Z has a max resistance of 7.3 ohms when its gate voltage is 4.5V. Then its voltage loss is 2.6V which is much higher than the saturation voltage of an ordinary cheap transistor.

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I've got a Vgs of 20v on the datasheet

That is the highest voltage that can be used without breaking it.

and a Vgs MAX of 4.0 on the same datasheet.

That is the max threshold voltage for some of them to barely turn on when they conduct only 250uA. The curves show that a typical one conducts 350mA when its Vgs is 4.5V.
You cannot buy a sensitive one. They are random.
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Ok is a 2N2222A a workable transistor? or a 2N4401?

The 2N4401 has identical spec's to a BC337, a max saturation voltage of 0.7V with a collector current of 500mA and a base current of 50mA. Their pins are reversed.
The 2N2222A has a max saturation voltage of 1.0V at the same currents.
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