Author Topic: Help please with Commanda`s led driver design  (Read 9637 times)

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Hilltopgrange

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Help please with Commanda`s led driver design
« on: January 23, 2011, 09:10:11 PM »
Hi all,
          I need a little help from the electronic gurus please!

 I am planning on building some led lights based on Commanda's design but for 48v. I was going to use a high voltage regulator(TL783CKS) and resistor for each light or a DC-DC converter to drop the voltage but as these are rather inefficient I have decided if possible to use Commanda's design. The snag is that whilst I can build a circuit from a plan I am not so hot when it comes to tweaking a circuit.

Has anybody already done this for 48v? if so would you be willing to share?

I have searched, honest!
I have also read Amanda's write up so many times I almost know it word for word.

I should be able to figure out the resistors, my main problem is finding alternatives for the transistor BC547 http://www.datasheetcatalog.org/datasheet/MicroElectronics/mXuwzwr.pdf I think I may be able to use BC546 (see same spec sheet) but I am probably wrong!
The other problem is the fet 2N7000  http://www.fairchildsemi.com/ds/2N/2N7000.pdf I cant find a higher voltage version! maybe the 2n7000 would be fine< I just dint know!

If This design can be modified for 48v I would ideally like to have 12 leds per string with each string having its own current source so no need for the equalising resistors (i think) nor do I need the additional circuit for the night light, just the basic circuit no4 would make me very happy.

I have 1000 leds on order from Hong Kong and they should be here this week, they are warm white in colour fv3.5v 25MA.

I don't like asking to be spoon fed info like this but I have gone as far as I can with this and before I pull out what little hair that I have left I thought I would ask for some help.
If all else fails I may have to go back to the voltage regulator and resistor plan.

Amanda's write up can be found here
part 1 http://fieldlines.com/board/index.php/topic,131213.0.html
Part 2 http://www.fieldlines.com/board/index.php/topic,131214.html
Part 3 http://www.fieldlines.com/board/index.php/topic,131215.html
Part 4 http://fieldlines.com/board/index.php/topic,131296.0.html
Part 5 http://fieldlines.com/board/index.php/topic,131217.0.html


This Amanda's circuit 4


And this is the layout I would like to use

Many thanks in advance

Russell
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Flux

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Re: Help please with Commanda`s led driver design
« Reply #1 on: January 27, 2011, 04:23:03 AM »
I haven't looked at this is detail but from the drawings you show I don't think transistor requirements are difficult.

Q1 will only see a few volts, being the drop across Rs and the turn on volts of the mosfet. The current handling is negligible with the 1m load. I would suspect something like a BC107 would be fine.

Again the mosfet shouldn't be critical, when running it will only see a few volts. It may be a good idea to choose something rated for 100v to deal with the start up condition and there are plenty of small TO220 mosfets that would do. From memory I think IRF 540 is in this region and is common and cheap. I don't know anything about 2N7000, if it is too low a voltage it may still work with a zener clamp across it.

I am sure others who have played with this circuit will be able to give definite advice.

Flux

commanda

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Re: Help please with Commanda`s led driver design
« Reply #2 on: January 27, 2011, 08:30:34 AM »
Sorry, I didn't see this earlier.

A quick check of the datasheet shows the 2N7000 good for 60 volts, so it should be fine.
The npn only ever sees the gate turn-on voltage of the fet, so really, it's voltage rating is not related to the supply voltage.
Any common NPN transistor will work fine.

The only rating you may need to watch is the dissipation of the 2N7000, which is 400mW.  Assuming your 48 volts is a battery, the peak voltage at equalisation is going to be approaching 60 volts. To work the numbers, we'll use worst case and say it is 60 volts.

12 leds in series at Vf = 3.5 volts gives a total of 42 volts. Take this away from 60 volts, gives us 18 volts across the fet. If you ran the leds at the full spec sheet current of 25mA, dissipation in the fet would be 18 * 25 = 450mW, exceeding the devices rating.
If you take the common-sense route, and run the leds at slightly less than max current, say 20mA, now your dissipation comes down to 360mW.
Considering this is absolute worst case, I'd say it was doable. Just be aware when you build it, that the fet should have free flow of air around it for the worst case situation. Even consider a small heatsink. A square inch or so of sheet aluminium.

Hope this helps.

Amanda

Hilltopgrange

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Re: Help please with Commanda`s led driver design
« Reply #3 on: January 27, 2011, 09:06:13 AM »
Hi flux and Amanda
                                   Thanks for the advice its a great help and exactly what I wanted! what a great resource this site is!

I will get these bits ordered and get going with it.

Many thanks again ;D

Russell
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ghurd

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Re: Help please with Commanda`s led driver design
« Reply #4 on: January 27, 2011, 10:08:54 AM »
Russell,
Might want to take it up to 14 LEDs per string.
More light for the same amps.
Less V on the 2N7000.

Amanda and Russell,
The Vf will be closer to 3.15V at 17ma, even on the HK stuff.
That will be a large percentage more voltage on the 2N7000.

Not sure if it is worth worrying about?  I usually worry too much.
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Hilltopgrange

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Re: Help please with Commanda`s led driver design
« Reply #5 on: January 27, 2011, 10:57:55 AM »
Hi Ghurd,
                I think I have found an alternative to the 2n7000 its bs170 same spec but 500ma http://www.fairchildsemi.com/ds/BS/BS170.pdf They are the same price but a bit more headroom so I think I will try them.

Many thanks
Russell
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ghurd

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Re: Help please with Commanda`s led driver design
« Reply #6 on: January 27, 2011, 11:20:11 AM »
I have some 2SK2782 I want to try.  Threshold is 0.8 to 2V.
Things are just a tad bigger than a TO-92 and have a metal slug built in to connect to a heat sink.
20A, 50A pulsed, 40W!   :o
http://www.datasheetcatalog.org/datasheet/toshiba/2981.pdf
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Hilltopgrange

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Re: Help please with Commanda`s led driver design
« Reply #7 on: January 27, 2011, 11:40:28 AM »
I think I was looking at them at on point! snag is I have looked at so my many now my brain is starting to melt.
All the bits have been ordered and should be here soon.
 Amanda mentioned in another post today about a little inverter circuit for a cfl, I'm going have to have a look at that one as well. It never ends lol.

I just noticed have been promoted to a senior member! does that mean I get a pay rise ;D its only took about 41/2 years!lol

Russell
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TomW

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Re: Help please with Commanda`s led driver design
« Reply #8 on: January 27, 2011, 11:46:58 AM »
I just noticed have been promoted to a senior member! does that mean I get a pay rise ;D its only took about 41/2 years!lol

Russell

Purely post number based.

You are entitled to a doubling of pay. 

2X0 is still zero.

Brings you in line with Kurt, JW and I on pay scale. "Goose Egg",  and the undying gratitude of members. ;D

Tom

Hilltopgrange

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Re: Help please with Commanda`s led driver design
« Reply #9 on: January 27, 2011, 12:08:55 PM »
At the rate I post it will take another 10 years to earn another star lol.

The knowledge available on this site never ceases to amaze me. You all do a sterling job!
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Rover

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Re: Help please with Commanda`s led driver design
« Reply #10 on: January 28, 2011, 07:27:59 AM »
...and I hought the senior thing was just for getting older.
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Bruce S

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Re: Help please with Commanda`s led driver design
« Reply #11 on: January 28, 2011, 09:35:55 AM »
I just noticed have been promoted to a senior member! does that mean I get a pay rise ;D its only took about 41/2 years!lol

Russell

You are entitled to a doubling of pay. 

Tom
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DamonHD

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Re: Help please with Commanda`s led driver design
« Reply #12 on: January 28, 2011, 10:57:24 AM »
No one sent me a Christmas Bonus.  Probably some rules about "not valid in former colonial powers".  Pah.  Or TomW's fingers in the till  AGAIN.  %-P

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