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Old 6th September 2008, 07:05 PM   #11
onform is offline onform  England
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thats good,

so the 1M resistor needs to go where? inbetween the transistors is that correct?

What value resistors do you think at R1 and R2 for 3.1vf??

this is stupid but the supply will not fry the first few led's will it.....it will increase V according to demand from the LED's..is that correct?? or am i getting it the wrong way round??
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Old 6th September 2008, 07:20 PM   #12
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That's the entire point of current source drive, lol that's what you asked for anyway.

Use ohm's law to work it out for different Vf
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Old 6th September 2008, 07:41 PM   #13
onform is offline onform  England
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Quote:
Originally posted by richie00boy
That's the entire point of current source drive, lol that's what you asked for anyway.

Use ohm's law to work it out for different Vf
I know i know...being new and still learning i'm constantly not sure if what i'm doing is correct...lol Re the resistors, no free lunch aye..lol

matt
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Old 6th September 2008, 08:08 PM   #14
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I'll run through it as you might get caught out with a couple of things.

We know the CCS is running at 5mA so that gives 2.5mA in the LED and 2.5mA in the resistor. We can see using the values I have above that the voltage across the resistor is 0.0025A * 560R = 1.4V, then there is the Vbe to take into account, 2.1 - 1.4 = 0.7 with the transistors shown.

So now with LEDs of Vf = 3.1V let's see what happens. Resistor voltage will be 3.1 - 0.7 = 2.4. We want 2.5mA in the resistor so that means we need 2.4V / 0.0025A = 960 ohms.

Vf is quoted at 2.1V (or 3.1V for your LEDs), but this figure does vary a bit with current and is usually stated at full current which you won't want to run at. So some tweaking may be necessary, but it's probably close enough for what you want.
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Old 6th September 2008, 08:39 PM   #15
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Dont want to sound snobby, But LED's mess up the sound making it grainy, I'd fit a switch to turn them all off when your listening to music........

But dont let me sway you!
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Old 6th September 2008, 10:07 PM   #16
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Quote:
Originally posted by richie00boy
Neat idea but it's going to need a supply voltage of 24x Vf yielding 74.4 volts even before the CCS headroom is taken into account. Also maybe the knob will be a bit stiff with two lots of switches on it?

Anyway I have a good little circuit for you here.

Click the image to open in full size.

R3 is the load i.e. your string of LEDs. As shown with LEDs having Vf=2.1 used for the CCS it's running at 5mA. If you use your blue LEDs having different Vf then you will need to change the 560 ohm resistors R1 and R2. The transistors shown will be fine for the application but small heatsinks are needed. The supply voltage should be 82-100 volts.
With good transistors used it will never start, so you need one more resistor to provide some initial current.
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Old 6th September 2008, 10:16 PM   #17
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Wavebourn,

onform told us that in post #11.

Rick
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Old 6th September 2008, 10:34 PM   #18
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Quote:
Originally posted by sawreyrw
Wavebourn,

onform told us that in post #11.


Oops... I did not read all posts carefully. Anyway, a reminder would not be odd.
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Old 7th September 2008, 09:34 AM   #19
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However I have built a few of these and never used the resistor and never had a problem. Also it always sims perfectly without

It would not hurt to include the resistor for keeping theorists happy though
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Old 7th September 2008, 02:46 PM   #20
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Default thats good..

Quote:
Originally posted by richie00boy
I'll run through it as you might get caught out with a couple of things. .
Thanks a lot that answers a few Q i was about to ask...lol just designing the pcb for the switch and supply at the mo,

Quote:
Originally posted by rocco gibralter
Dont want to sound snobby, But LED's mess up the sound making it grainy, I'd fit a switch to turn them all off when your listening to music........

But dont let me sway you!
This all operates outside of the amp cicuit and has it's own supply I will probably sheild the supplies from the main amp board anyway but thanks for the comment it will be interesting to see if it does affect anything. a simple swith can be addded later if it does.

Quote:
Originally posted by Wavebourn


With good transistors used it will never start, so you need one more resistor to provide some initial current.
I will probably try it without first but add in the provision for it when printing the boards..again we shall see...

thanks guys...
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