[sdiy] Switching leds on a constant current source

Rutger Vlek rutgervlek at gmail.com
Mon Jun 1 19:26:05 CEST 2015

Hi guys,

Thanks for the input! Richie wrote exactly what I needed to know, the Zener trick works well in Spice and I assume it will in real world too.


On 31 mei 2015, at 23:34, Richie Burnett wrote:

> 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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