Choosing a transistor

[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

[skimask]

Is 35,000mcd bright? It sounds like quite a high number.

Depends on the viewing angle.
If you've got a 35kmcd LED with a viewing angle of 170 degrees, then, yes, that's a seriously bright LED. If that same 35kmcd LED only has a viewing angle (1/2 theta) of 10 degrees, then it won't be so bright.

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.

LEDs don't run on voltage...get that idea out of your head. Yes, they need voltage...'nuff said.
It's CURRENT that makes an LED work. Yes, you have to have enough voltage to get the current required, which is why you also sometimes need a current limiting resistor. You start pumping 12v thru a single LED (unless it's rated for 12v of course), and you WILL burn it up sooner rather than later. If you've got 12v PWM running at 25% duty cycle, yes, the average would be about 3v-ish, but you're still hitting the LED with 12v. Again, if it's rated for that 12v, then I suppose you'll be fine. Otherwise, look to have a bunch of open LEDs eventually.
Do yourself a favor and get a copy of 'The Art of Electronics' and do a bit of reading. It's my 'bible', great book, will teach you a lot about a lot of stuff.

[The Master]

I had a look for that book. £48.45 on Amazon! It wants to be good for that price. Ill add it to my x-mas list

The LEDs im looking at are 15 degrees i guess that puts them towards the dimmer end.

So, using LEDs on 12V in parallel is definately out of the question. Looks like ill be using a series-parallel type circuit. That way each of the 4 sets of LEDs should only use 200mA of current.

If i only need 200mA now is it still better to use a mosfet instead of an NPN transistor? Yes i can use NPN now with these other LEDs

[The Master]

Luciano, I have seen the max forward voltage. I thought it meant on average.

Im using more LEDs now. Ive re-arranged the circuit a little so that theres hardly even room for the 2 screws and ive got 48 LEDs on it. Its a bit less cramped on the track side because each LED only has 2 legs instead of 6. There are 12 LEDs in each color. The datasheet says 4.2V max for the white ones. If i have groups of them in parrallel (4 LEDs in each group) and connect the groups in series then i should end up with 4V per LED. I think that means they would only use 150mA per color too

[skimask]

I had a look for that book. £48.45 on Amazon! It wants to be good for that price. Ill add it to my x-mas list

Obviously, not everything in that book applies, but the first bunch of chapters (i.e. 1/3 of the book) breaks down the basics of transistors/mosfets, and the bulk of the discrete components needed to put together a complete circuit. And it really helps to have a few examples of each laying around, along with some spare parts and a meter (and/or 'scope) to play with pieces/parts and see what actually happens in practice vs. just reading all about it.

The LEDs im looking at are 15 degrees i guess that puts them towards the dimmer end.

Not necessarily. Look at the datasheets for those LEDs, look at the distribution, then compare that LED with the cheaper and more expensive types.

If i only need 200mA now is it still better to use a mosfet instead of an NPN transistor? Yes i can use NPN now with these other LEDs

I think you're better off using a MOSFET over a BJT anyday. The 'bible' mentioned above shows why and I don't remember exactly why. But it's got something to do with the forward bias voltage. A BJT gets hot, forward bias voltage drops a smidge, current flow rises, BJT gets hotter, forward bias voltage drops a smidge, current flow rises, it's a nasty circle of death for the BJT. With a MOSFET, it effectively chokes itself when it gets hot. I don't remember the exact mechanism by which this happens, but it supposedly can 'save itself' in certain situation whereas a BJT would go up in smoke.

[Luciano]

Hi,

The choice of using 17 LED's is not going to help you
if you connect groups of LED's in series.

* * *

With the PWM you can control the brightness of the LED's.
With or without PWM the maximum forward voltage is the same.
The maximum forward voltage is visible in the datasheet of the LED.

Best regards,

Luciano