[sdiy] Switching leds on a constant current source

Richie Burnett rburnett at richieburnett.co.uk
Sun May 31 23:34:14 CEST 2015


Either use a make-before-break switch, to guarantee the current always has 
somewhere to go...

Or, connect a zener diode across the switched load, (e.g. from current 
source's output to ground if LEDs have a common cathode connection.) If you 
choose it's zener voltage to be higher than the forward drop of the highest 
LED, then it will only conduct when no LEDs are connected.  That way the 
current source will always be happy, and the supply current will be constant 
because there is always a route for the current to take, even if all LEDs 
are disconnected.

Hope this helps answer your question,

-Richie,


-----Original Message----- 
From: Rutger Vlek
Sent: Sunday, May 31, 2015 7:24 PM
To: synth DIY
Subject: [sdiy] Switching leds on a constant current source

Hi guys,

I'm working on a LED driver that switches between 16 different LEDs with a 
4067 analog switch. As I'm using a mix of different LED colors I'd like to 
stabilize the current thru the LEDs and the current draw from the supply by 
using a constant current source. From Douglas Self's small signal design 
book I obtained a nice certain, and I'm trying to fit the 4067. However I've 
been wondering if this switch will defeat part of the purpose of using a 
constant current source, as switching disturbs the path for the current to 
flow ( some 4067 data sheets  guarantee it's break-before-make action, while 
others don't). I've tried making a parallel path to the switch, so the 
current can flow somewhere when the switch is not closed to connect any LED, 
but this didn't work out too well (the resistance of this parallel path 
should be high enough for the LED path to draw most current away from it's 
when the switch allows the LED to light. On the other hand, a high 
resistance changes the load
  on the current source too much. Bleh!).

Any advice?

Rutger

ps. Sorry if this message arrives twice on the list. I had sent it a couple 
of days ago, and didn't see it occur on the list, nor any replies to it. So 
resending it to be sure....
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