Choosing a transistor

[Lajko]

I think a question is what you want to do with the transistor. If for driving something, there are tons of IC drivers for driving high or low and very high current DC or AC loads - even opto-isolated drivers. There are even opto isolated triacs (I've used them to directly drive up to 25 watts of lights for disco effects! 64 channnels for $1 a channel!)
So fill us in and maybe everyone can help with a driver IC or how to select your transistor.

[The Master]

This is actually for disco lights. Im making some that use some full color LEDs. I checked the datasheet of the LEDs and it appears the Red, Green and Blue use 30mA, 25mA and 30mA respectively. The datasheet for my PIC says each IO pin can only take 25mA. I will be using 17 of these LEDs on each light and they have to be wired in parallel so thats way too much current for the PIC to handle. I decided to add some transistors in and they seem to work although im using NPNs at the moment when i should be using PNPs. About 2.5V seems to get the LEDs to the correct brightness without overheating but to reduce the overall current needed im flashing them very fast between Red, Green and 2 Blues so each color is on for 1/4 of the time. Someone on here told me that means i can use 4 times the voltage because it averages out to what it was origionally. I tried that and it seems to work fine.

I would like to use transistors rather than a chip because the PCB is going to be extremely small and transistors can be dotted about where they are needed (2 on each side). I definately have room for 4 transistors but i cant make the PCB any bigger. To make things even more interesting its a round PCB!

Im pretty sure i would want a darlington pair type of transistor. Thats what im using at the moment for other projects and they do the job pretty well. There are 2 problems with them though. The first is that they are NPN and the second is that Rapid have suddenly stopped selling them. I would like to use NPN transistors but the LEDs i have are common cathode

[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