-- unless you're turning on LED's, Opto's or both in groups -- then you could gang things together.
You can get optos with different outputs -- do some web searches.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Hi,
What are you driving the LED's from? Computer, controller board, microcontroller? Any way you hack it, it will take 1 digital output per LED/OPTO; and this takes a lot of driver chips.
-- Luhan Monat (luhanis 'at' yahoo 'dot' com) "The future is not what it used to be..." http://members.cox.net/berniekm
LED's and optocouplers are somewhat amenable to pulse width modulation, assuming the switching frequency is sufficient to prevent the junctions from overheating. Most makers don't publish specifications for this mode of operation, because it's easy to damage the LED's if the modulation mechanism is not highly reliable. It only has to stick on once at, say, 5 Volts to turn that little 2 Volt junction into something much less useful. I'd look to incorporate some kind of fail-soft if I were contemplating such an arrangement.
On Tue, 22 Feb 2005 00:56:13 +0200, "valentin tihomirov" wroth:
Welcome to the REAL world.
Jim
We have to control hundreds of typical colour leds (a couple of mA) and optocouplers (1mA). The device is not portable; however, I would like 1 - not to waste energy on the ballast resistors and 2 - reduce amount of the resistors. Can you point to the guidelines on the issue? In addition, the optocouplers have open collector outputs; that is, the outputs have to be pulled up by more power and resistors.
Basically the "old" way with resistors or better: Constant current drivers are easily optimized: Lower the supply as much as you can... (To save the energy wasted over drivers/resistors)
Alternatively you go for something switched. Typically a current is built up in an inductor and discharged into the LEDs. The uC companies will have app-notes on this (Microchip for sure) and you can find dedicated LED switchers from National, Maxim, Linear, Semtec, Zetex etc. etc.
What's wrong with multiplexing? For indication on closerhand this will actually also save power: The eye is slow, so short high intensity burst will seem bright in terms of brighness/energy...
I'd go for this approach ;-)
/A
Keep the supply voltage low !
With a 2.5V supply, a typical led might have a Vf of 1.6V - the o/c transistor a Vce of 0.2 - 0.3 V, so only around 0.6 - 0.7 V is wasted across the current limit R.
The optocoupler *outputs* can be tied to whatever logic supply voltage you're using. No need to use especially low value pull-ups unless you're switching at very high speeds - in which case you should be using a dedicated coupler.
Do the math and see if it's worth using a simple switching regulator to supply that 2.5V if you're main system only uses 5V.
Graham
IMO, the packs violate the main design concept - divide and conquer.
Personally, I do not like useless wastes. As a noob, Im just asking about adopted practices. The power is 3.3v, diodes drop 1v..1.5v, the rest drops at the ballast. I suppose 50..100%. In one circuit there are 32 optocoupler diodes and source voltage is 30v; that is, 30 times more power will be burnt than required to activate optocoupler. So I think, the total reduction can be about 4 times.
Usually I just put a resistor in series with the LED and connect the low side to the output of an inverter. When the inverter output is low, the LED is on.
Occasionally, when the difference between the forward Voltage drop on the LED and the power supply voltage is small, I use some other approach (e.g., a simple transistor current source).
These both waste about the same amount of power.
If you really want to save power, you could use a switch-mode current supply to do the job. It is very hard for me to believe that this would be worthwhile in most cases, because switch mode supplies use inductors to store energy, and I just can't envision using an inductor for each LED.
Elsewhere in the thread, someone mentioned simple pulse-width modulation of the LED's. I don't know whether this would actually save power but it could make the circuitry more simple by eliminating the series resistors.
--Mac
Use resistor packs to reduce component count. Is the power saving really critical enough to worry about? How much do you want to save?
Ken
What do you mean by that/ You didn't mention anything like that before.
optocoupler
burnt
Maybe you should look at the design again and decide if you are approaching it correctly. If power usage and component count are so critical you should look from top down again.
Ken
Nothing special. The major design concept which reduces amount of information enabling creation of complex systems is hierarchy. The the partition incapsulates functionality into hierarchy modules. The purpose of the partition, is to break down a solid block of N elements (complexity = number of interconnections is about N^2) into loosely interconnected tight pieces. The resistor packs will increase connectivity between modules. In addition, my CAD does not support multple sections of a component spread over design modules enforcing the HW hierarcy.
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