Homebrew PCBs

>
> The problem is that LEDs are really current driven devices. Go over the
> current and the LED either degrades quickly or pops due to heat. And that
> many LEDs in a sealed box will generate heat raising the ambient
> temperature.
>

Good point, I did not consider this at all. Traditionally, people think
LED's do not generate much heat at all, but it is still a conductor that has
some resistance and has current flowing, therefore heat will be generated.

>
> The normal choice with just resistors is to design for under the max
> rating of the LEDs. But the LEDs may very from device to device by as
> much as a 10th of a volt and as they get warmer that changes too and in
> the wrong directly.
>
> Say each UV light dropped 3.2V at 20 degrees C. If you put 3 in series
> that's 9.6V. If your supply is 12V then you need to drop 12.0-9.6V=
> 2.4V. Assuming you want 0.02A current you then need a 120R resistor.
> If they get warm and their voltage drops to 3.1V then you need
> 12.0-9.3=2.7V. To hold it at 20mA 135 Ohms.
>
> But with 75 Ohm 2.4V/75 = 32mA which may be above the device rating. And
> if the temperature drops the LED forward voltage to 3.1V the 75R resistor
> will increase the current to 35mA. That makes things worse and it
> thermally runs away and pops your LEDs.
>

I am blindly following an Instructable, so that may be my first problem. The
blindly following part, not necessarily the Instructable part. I am going to
rethink some things based on your notes here (thanks by the way!).

I think the box may work for a couple few weeks, but will most likely fail
as you suggested. I want a long term UV exposure box with consistent
results, so I am going to learn more about this based on what you've taught
me here.


>
> If the voltage isn't stable but can rise up to 14V then the same problem
> exists. 14-9.6V is 4.0V divided by 75 Ohms results in 53.3mA through
> the LEDs. Then you'd need a 200 Ohm resistor to hold the current to 20mA.
>
> There are lots of constant current LED driver circuits on the WEB. Or you
> can buy them premade. A simple version uses the LM317T as a constant
> current source. Configure it for 20mA and use a higher supply voltage
> like 36V or so. Then run 10 LEDs in series.
>
> I have a circuit that I designed for lighting up the cables on the Lions
> Gate Bridge in Vancouver. It has 32 LEDs in series and runs off 120VAC
> and there are 6 of those in each light fixture. Total power consumption
> is 15W per fixture. See my home page link below.
>
>
Again. thanks for the free education. I am going back to spice for a bit to
do some work and prove out the design. Basic electronics cannot be
overlooked!


Chris

> John Dammeyer
>
> Automation Artisans Inc.
> http://www.autoartisans.com
> Ph. 1 250 544 4950
>
> -----Original Message-----
> From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
> On Behalf Of Chris Kleeschulte
> Sent: Monday, September 26, 2011 3:50 AM
>
> To: Homebrew_PCBs@yahoogroups.com
> Subject: Re: [Homebrew_PCBs] Two Videos demonstrating the Kinsten UV
> Exposure Box
>
> Tom:
>
> Your questions lead to me to believe that I have no idea what I am doing
> with this UV led exposure box. My design is not using a driver chip..just
> a
> 75ohm resistor to drive 3 leds. This circuit will be in parallel with many
> other ones just like it. Would I need a driver chip? To secure the
> artwork,
> I was thinking that the glass plates would "sandwich" the artwork against
> the glass plates. Would there be a problem with this?
>
> On Sun, Sep 25, 2011 at 9:23 PM, Tom Biery <judsquare@...
> <mailto:judsquare%40yahoo.com> > wrote:
>
> > **
> >
> >
> > Are you driving the LEDs with a driver chip like a ZXLD1362?
> >
> >
> >
>
> [Non-text portions of this message have been removed]
>
> [Non-text portions of this message have been removed]
>
>  
>



-- 
Chris Kleeschulte
KK4DYJ


[Non-text portions of this message have been removed]