Danko Just for a moment, assume S1 (the parallel switch to D1) is the only LED he wants switched off. When you bypass D1 with S1 you change the circuit characteristics and increase the current through the rest of the LED's. Now for a moment assume he only wants D10 lit (S10 would be open). Without the 9 other voltage drops across those LED's the voltage through D10 and the associated current would be nearly a dead short to the power supply and would burn D10 out faster than you could blink.

I respect your opinions and views but I disagree with your approach. Particularly because the TS stated he might want to have all LED's out but one. I can assume there would be a circumstance where he wanted ALL the LED's off but the power supply still powered. That would be a dead short. Or at least through the one current limiting resistor (not shown in your diagram but assumed to be there) ALL the current would be going through that resistor and would get very hot if it was sufficient in size (wattage) to not burn out. But such a high wattage could potentially start a fire. This is why I disagree with your approach.

With the TS approach, each LED has its own resistor. So regardless of what state the others are in, the one that may be under consideration would be operating at the proper current. However, it occurs to me that perhaps you have a resistor at each LED and use the switch to either drop out both the resistor AND LED or just the LED only. In which case I see so many potential changes to current flow that I can't predict how the circuit would function. I suspect it would fail if the right set of switches were active/inactive. I could model it but I don't have that much time. I will model three LED's set up the way you suggest and see what numbers I come up with.

Look at his circuit, it has a current source.

 

Like Bob ^^^^ said. The series string is powered by a constant current source. Look at the power supply symbol.

Constant current sources maintain the desired current over a wide range by varying the output voltage. If there's nearly a dead short, the output drops to near zero to maintain the current. If 1 – 10 LEDs are in the string, the voltage will be whatever it needs to be to maintain the current. If the circuit is open, the constant current source will try its damnest to keep the current flowing by raising the output voltage as high as possible...which is limited to whatever its supply voltage is of course.


A word of warning: if you play with this on a current-limited lab supply, you need to be a bit careful. Say you set the current to 10mA with an LED connected. The current will be 10mA and the voltage across the LED will equal Vf (this is a good place to vary the temperature of the LED and watch Vf change).

If you disconnect the LED, the open circuit voltage will rise to whatever the voltage is set to on the power supply. When you reconnect the LED, if the maximum voltage is set higher than the LED will withstand, the LED will be damaged by the higher voltage present the instant the LED is connected (voice of experience).

 

Can your CC regulator handle that much heat?

Tony, I think you'll find that if you run the numbers on the rest of the scenarios up to 10 LEDs, the heat issue isn't as bad as you think.

Even at Danko's highest suggested input voltage of 60V, if all LEDs are switched out of circuit, the CC supply still only needs to dissipate 90mW (60V drop times 1.5mA.)

Given that most garden variety resistors and transistors are rated at least 100mW, and that 1/4W components are also still quite cheap and compact, I don't think 90mW dissipation is too much to worry about.

 

Here is a demo of what is described in post#62
I've shown two different simulations.

The first shows what happens with a CVCC supply and an LED string. The top trace shows the state of each switch.
Notice as each switch is closed/opened in sequence, the current remains the same. The supply voltage fluctuates to keep the current draw constant.


The next shows what happens when all switches are closed creating a short circuit. The top trace shows the state of each switch (they overlap so it appears as one trace).
Notice the current draw still remains the same but the supply voltage drops to almost 0.


If I were to disconnect the CVCC supply from the load, the supply voltage would zoom to 10v and current draw would drop to zero.

EDIT: Updated/Corrected images

 

Look at his circuit, it has a current source.

Did I use the wrong symbol for a current source?

 

Did I use the wrong symbol for a current source?

Lol! No, you drew it all up proper. I think some of the wording you used in post 59 kind of made it sound like you didn't understand how Danko's circuit would work, even though your schematics and calculations in post 60 clearly showed that you did understand how it would work. Once I saw post 60, I knew you had it.

P.S. This is why we love schematics and images so much. Despite our best efforts, we all use language a bit differently and it's easy to misunderstand one another when it's just words. Once you added your schematic all was crystal clear!