Choosing a transistor

[The Master]

Ive been trying some more values in the search at digikey and its started to give me no results for what i want. I had a quick look through the transistors at rapid and found one that looks like it might work. 2N3702 40V TO92 PNP GEN PURPOSE TRANS RC. The most current the LEDs would draw should be 510mA. The datasheet for this transistor says "Collector Current - Continuous 500 mA". It wont be used continuously. It will only be on for 1/4 of the time at the most. It also says "Collector-Emitter Voltage 25 V". I was planning to use 12V for the LEDs (depending on further tests). I am using 12V at the moment but it might be because im using NPN transistors. When i put the correct PNPs in then 12V might be too much for them. The datasheet doesnt mention darlington pair so i assume its just a normal transistor. I think that means that the output might be slightly lower than if it were a darlington pair. That might be a good thing if the 12V is too much.

Edit: That one is also in a TO92 package. It doesnt matter too much but those would fit best on the PCB. Things like the TO220s that ive seen are way too tall. There is also a height limit on the PCB of about 8mm

[Luciano]

Hi,

Use a power MOSFET with logic-level Gate Drive.
(You can drive it directly with the I/O of the microcontroller).

For my Stereo microscope ring light (warm white LED's) I use
IRLR/U2905 mosfets with I-Pak TO-251AA case. The microcontroller
and the LED's use the same power supply of 3.25V.
(20mA @ 3.25V for the used LED). The LED's are driven with PWM.

IRLR/U2905 datasheet:
http://www.irf.com/product-info/data...a/irlr2905.pdf

Best regards,

Luciano

[The Master]

(You can drive it directly with the I/O of the microcontroller).

Should i not be doing that with normal transistors?

I cant find that type of mosfet on rapid's site but they do have a mosfet category.

Would i be right in saying the following
Drain-source voltage is the max voltage it can output to the load
Drain current (continuous) is the max amps it can output to the load
Drain current (pulsed) same as above but relevant when using PWM.

Im not so sure about the Drain-gate and Gate-source voltages. Looking at a few datasheets i would guess that the Gate-source voltage is the max voltage that the PIC chip can output to it. Does it matter if its a lot lower? One datasheet im looking at says +-18V. Does that need 18V to make the LED as bright as possible or will about 3V do the same job?

I know how many volts and amps the circuit will be using so do i just have to look at Drain-source voltage and Drain current?

Im planning a slightly bigger light too so i will need to know things like this. The bigger one will use seperate LEDs for each color so i can get the really bright ones. That means i can wire some in series to use less current too.

Edit: I see there are N-type and P-type mosfets. Do i need a P-type?

[Luciano]

Hi,

Use a power MOSFET with logic-level gate drive.

This is what I use for my lamp:


If you use a 12V supply use that:

Best regards,

Luciano

EDIT: For the 12V schematic you can also connect groups of the 4 or 5 LED's in series.

[The Master]

Ive found some logic level ones in rapid. They all seem to be n-type though. Does that matter? The circuit im making has to have the mosfet and LEDs the oposite way round. The multicolor LEDs have a common ground so its the positive side that needs to be switched

[The Master]

Would it be better if i redesigned this small light to use seperate LEDs for each color? I cant add anymore LEDs because of space so it would be like dividing the amount of LEDs by 4. I could also connect them in series etc and group their anodes together so i can use either an NPN transistor or the same type of mosfet that you showed in your diagrams.

The only problem with using seperate LEDs is the color mixing. The idea is that they combine to make loads of colors. When the light is shining at an object then its not so bad but these small ones are designed to be pointed at people. I think from a distance the colors should mix alright

[The Master]

Ive been playing around with the circuit design and using single color LEDs ive managed to fit 27 more of them on the board! Im not quite sure how i did it either :S I think this way might work better. I can use brighter LEDs now and instead of 2 blues i can use 1 blue and use the 4th chanel for white. The price of the light has about doubled by using more LEDs but it should be worth it. Is 35,000mcd bright? It sounds like quite a high number.

Can i just double check that a 4V LED will be fine running on 12V if i use PWM? The average would end up being 3V. Im sure it was someone from here who said that would work but i dont remember who.

The alternative is to wire the LEDs in series-parallel and not use PWM when they should be at full brightness. The only problem i see here is that the light would use 4 times a smuch current because all 4 sets of LEDs would be on at the same time