| ??? 01/28/05 10:10 Read: times |
#85996 - Where is the current limiting from Responding to: ???'s previous message |
I was puzzled about the LEDs driven by +5V to +12V...
Erik Malund said: I have no Idea what, but something limits the voltage across the LED to Vf. With a LED 3 possibilities exist: Vf = broken LED. Payam Soltany said:
Once I turned on one of my displays while having a bug in the uC code. The time-slice given to each LED column was say half a second instead of say some milliseconds. Befor I managed to turn it off, some 30 or 40 columns each having 32 LEDs were barbecued! Here might be the answer. The power lines from multiplexed drivers to the columns are probably long enough to form a nice transmission line (or any not-only-resistive impedance) themselves. It takes time for the voltage pulse to rise on the column, and if you switch off fast enough, it might not go over some reasonable limit. Of course, this is a risky approach, no doubt. Btw. it depends on the construction of the LED diode whether it withstands 32xInominal current in a 1:32 duty cycle - it may. Commonly, LEDs are rated 8xInominal at 1:8 mux. Another question is, whether it pays off - there is a saturation in the "amount" of emitted power in form of light depending on the consumed current. There are laser diodes powered at 1:100 duty cycle by tens of amps in the pulse. However, I doubt most of commercially available LEDs do so. Jan Waclawek |
