High Power LED Mood Lamp

[ante1]

Hi AG,

So, perhaps we need to change the series resistor values to compensate for the BC517 drop then. I believe the current draw of the BC 517 base will never get anywhere near 25mA!

 

[audioguru2]

Hi Ante,
The base current of the BC517 darlington transistor must be limited by a resistor. The PIC has high-speed powerful Cmos transistors like the 74HCxx series, so when the PIC output goes high but it is held to only 1.4V then its current will be typically 55mA.

View attachment 40414

 

[ante1]

So, the outputs are not protected? I did not get anything from the diagram. Why not completely redesign or just use mosfets instead?

 

[audioguru2]

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.

 

[ante1]

Yeah all right, no mosfets!  8)

 

[DirtyH]

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:

--------------------------Caution--------------------------
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:
http://freenet-homepage.de/haraldschmid/bml/

 

[audioguru2]

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:

View attachment 40425

 

[blueroomelectronics]

On MOSFETs like the IRF510 which had an on resistance of 0.54 ohm won't even get warm at 350ma. No gate resistor is required and the PICs outputs will drive em fine at 5V. They are expensive though, much more than a 2N2222A + 680ohm resistor.

 

[audioguru2]

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.

 

[blueroomelectronics]

I've got a Vgs of 20v on the datasheet, and a Vgs MAX of 4.0 on the same datasheet...
http://www.aoc.nrao.edu/~pharden/ref/_IRF510.pdf

Ahh, I see, the MAX 4V is the minimum it needs to switch on...

 

[audioguru2]

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.

 

[blueroomelectronics]

Ok is a 2N2222A a workable transistor? or a 2N4401?

 

[audioguru2]

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.

 

[blueroomelectronics]

Ahh thank you, I'll use the 2N4401

Do you know the 2N equivalent of the BC548 & BC558?

 

[audioguru2]

Do you know the 2N equivalent of the BC548 & BC558?

A 2N3904 is NPN like the BC548 and a 2N3906 is PNP like a BC558. They have their pins opposite to the European ones.