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.

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.

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.

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?
>
>
>

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