Re: Digital Current Control for LED array

On Fri, 23 Dec 2005 18:51:41 GMT, "John_H" <johnhandwork@xxxxxxxx>
wrote:

>
>"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
>news:e1doq1ddpdirk25oa8nc8g1dnvgt37edgk@xxxxxxxxxx
>
><snip>
>
>> Finally, once the array is trimmed up and all the LEDs are set to
>> the same brightness, the output of the power supply (remeber the
>> power supply?) can be lowered to dim the entire array and the
>> brightness of all the LEDs should track.
>
><snip>
>
>> John Fields
>> Professional Circuit Designer
>
>If the currents are supplied from a fixed voltage through resistors, when
>you change the power supply voltage the brigtnesses will not track unless
>you can guarantee an exact match of the LED voltages. I wouln't trust the
>match to be sufficient for small voltage drops in the resistors. If one
>chooses to use a large voltage drop across the resistors (low efficiency)
>then the match is very good until the brightness is reduced significantly
>and the LED voltages again become significant.
>
>It may not be cost-effective to use potentiometers rated at the necessary
>power levels to accommodate the large voltage drops.

---
Yes, that's why toward the end of the article I wrote:

"So, under those conditions it looks like what's needed is about a
15 ohm 15 watt adjustable power resistor."

What I was thinking was along the lines of an adjustable vitreous
enamelled wirewound reesistor like a an Ohmite Type 210 or a
Huntington AVT, with a Huntington 25 ohm 50 watt unit available from
Digi-Key for $4.50 qty 1.

>If the passive
>approach were used, a combination of fixed resistors for a set point along
>with more reasonable potentiometers might be warranted.

I agree, but I think that using a single adjustable fairly
high-power resistor instead of a rheostat and an assortment of fixed
resistors per LED would be even cheaper. Hard to beat $4.50 per
LED, no?

--
John Fields
Professional Circuit Designer
.

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