Help needed on LED driver

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him27hk

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I hv just found a LED driver from the instructables.

**broken link removed**
The components are:
R1: approximately 100k-ohm resistor
R3: current set resistor - (0.5/R=desired I)
Q1: small NPN transistor (such as: Fairchild 2N5088BU)
Q2: large N-channel FET (such as: Fairchild FQP50N06L)
LED: Luxeon

the Q2 isn't avaliable in my country
Wt other tansistors can i use?
eg. 2SX series
Can i use power mosfet instead of it?

Thx everyone~
 
hi him,

Ref your diagram, if my notes are correct the drawing of the MOSFET is incorrect, its showing a P channel device, is this the original drawing.

You require a 60v 50amp equivalent enhancement mode, MOSFET N channel.
The S2X series looks suitable.

Whats the specification of the LED's ?

EricG
 
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Eric, I think the symbol is correct. You were probably thinking of the variant where the substrate junction is shown on the body. See
**broken link removed**
 
Eric,
Here's the article in question. I found it a week or so back whilst looking for something else. Can't say that I've read it yet.


**broken link removed**
Circuits for using High Power LED's
 
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There is no way some LEDs in series need to be driven with a 50A Mosfet! Any N-channel power Mosfet will work or an NPN darlington power transistor will also work.

A 5W Luxeon LED has a max current of less than only 2A.

This regulated current circuit works well:
 

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The main difference is that a MOSFET will have marginally more headroom (work with a slightly lower supply), and the bigger the MOSFET, the lower the voltage can go. Smaller MOSFETs require higher gate voltage for a given current. This would probably only be an issue if you were using just one LED. Smaller MOSFETs also have higher IR drop.
I realize that these factors are usually unimportant, but could be in some circumstances.
 
hi RonH,

The FQP50N06 which the OP has in parts list is a MOSFET N channel, enhancement device.

According to the datasheet gate arrow' should be pointing left.

As you say possibly a variant. ???

EricG
EDIT:
arrow indicating the substrate-to-source and substrate-to-drain junction direction
 
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ericgibbs said:
hi RonH,

The FQP50N06 which the OP has in parts list is a MOSFET N channel, enhancement device.

According to the datasheet gate arrow' should be pointing left.

As you say possibly a variant. ???

EricG
Click to expand...
Eric, it's actually an arrow indicating the substrate-to-source and substrate-to-drain junction direction. Below is a drawing from the link I had in my previous post.
 

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hi Ron,
Perhaps my more poor wording was not technically accurate, I shouldn't be so sloppy.
Should read as per RonH'
arrow indicating the substrate-to-source and substrate-to-drain junction direction

Hi HouseOfWax.
Thanks for the link, had a look thru, can't understand the OP's difficulty in choosing an alternative.
If he can't source the original device at least he must have its datasheet.

The 2SX series has a similar spec.

Eric
Edited:
 
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ericgibbs said:
hi Ron,
Perhaps my more poor wording was not technically accurate, I shouldn't be so sloppy.
Should read as per RonH'
arrow indicating the substrate-to-source and substrate-to-drain junction direction
Click to expand...
Eric, I wasn't nitpicking your wording. I was pointing out that the symbol in the OP's schematic is correct. Did you look at the drawing I posted? If you have evidence that this is wrong, can you post it?
 
hi Ron,
Relooked at your dwg, agree top right MOS shows, N channel enhancement MOSFET.

As does my 'copy/paste' from the datasheet for the FQP50N064L, which is listed in the OP's parts list.

Its just that the drawing posted by the OP, shows a 'variant' in that the arrow is in the other direction.
[or am going crazy, (answers on a postcard only, please)]

Ron, I know that you are not nit picking, I welcome our technical interactive posts, hope to have many more!.

Regards
Eric

Comparing the orginal dwg,with your diagrams, 'suggests' a P type depletion, which it cant be???
 
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I agree with Audioguru... why a power mosfet is required? a bipolar should drive a few leds no problem....
 
One of us is going nuts. Below is a comparison of the various symbols in question. All are of N-channel MOSFETs. All are correct (AFAIK).
I can't see how you equated the OP's symbol with P type depletion.
 

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Ron H said:
I can't see how you equated the OP's symbol with P type depletion.
Click to expand...

Maybe he did the same thing I did. I saw the symbol on the data sheet, noticed the arrow was different, and guessed the hand-drawn symbol was supposed to represent a P-type. Thanks for clearing this up, I think.
 
@him27hk:
While members debate the proper symbol:--

Yes you can use a different N-channel power MOSFET.

Tell use more about the current/power you need and maybe give some examples of examples of what you can find easily..

regards,
Mike
 
mneary said:
@him27hk:
While members debate the proper symbol:--

Yes you can use a different N-channel power MOSFET.

Tell use more about the current/power you need and maybe give some examples of examples of what you can find easily..

regards,
Mike
Click to expand...

Thx everyone,

I am going to using the circuit to drive Luxeon V with batteries

Vf.6.8v, @750ma

I hv some IRF510 mosfets,can i use it?(Is any other better subsitution?)

Forgotten to mention, the circuit have a drop out of about 0.6v.
thx
 
him27hk said:
Thx everyone,

I am going to using the circuit to drive Luxeon V with batteries

Vf.6.8v, @750ma

I hv some IRF510 mosfets,can i use it?(Is any other better subsitution?)

Forgotten to mention, the circuit have a drop out of about 0.6v.
thx
Click to expand...
What do you mean by this?
the circuit have a drop out of about 0.6v.
Click to expand...
 
hi him

From what we can 'deduce', you driving 3 off, series connected 6.8Vfwd high power LED's from a constant current source of 700mA.

So the total volts drop across the Luxeon V, 3 LED's is 3 * 6.8= 20.4 Volts.

If you are powering the system from say 24Vdc and you have a current limiting resistor of about 0.67hm: ,
then the volts drop across the resistor is 0.47V, leaving about 3V across the MOSFET. [when its switched hard on]

So MOSFET, P= 3 * 0.7 = 2.1Watts.

So if you drive a MOSFET or Power transistor hard enough to minimise the switching times.
Then you should have no problem finding a suitable device.

hi RonH, check the 'sums'. Keep well Eric

EDIT: the IRF510 should be OK.
 
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For driving LEDs up to 50 mA, I use LR-12 Packaged in SO-8. constant current mode. Look at supertex datasheet. Use a 24 Ohm feedback resistor.
 
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