LED voltage

[RGV250]

Hi,
I have an old matrix display I am working on, I think I have worked out the logic IC's.
I only got the display so do not have a clue on the power supply voltage, the logig is fed via a 7805 so that is easy but the LED matrix is supplied from the higher voltage side, there are several transistors/resistors for each row/column which I have no idea how they are connected. I recall reading somewhere that you can use a higher voltage if the "on time" is short. I have 12 & 24v supplies and was wondering if either will be safe. I did not pay for them so it will not be a monetary loss but do not want to destroy them unnecessarily.

Bob

[shantanu@india]

Bob,
Try to find out whether the a, b, c, d, e, f, g segments are shorted and connected to a common transistor via a resistance. There should be 7 segment drive transistors in that case. Also depending on common anode or common cathode type of displays p-n-p or n-p-n transistors would be driving the common supply point of each display unit. Generally the common anode/cathode points would not be current limited. Try with a 1 ms on time pulse.The easiest test is with a multimeter in diode check mode which injects just enough voltage for the segment to glow. Make sure you touch before the resistance.

[ken_k]

Hi Bob
LED's are normally current driven as the voltage across the LED will change with temperature. The current through the LED is normally set by a resistor so my guess is a 5V, 12V or 24V could be made to work by adjusting the resistor value, BTW some i.c. drivers have a controllable current source for setting the brightness. It is very normal to increase the LED current according to percentage "on" time duty. Most 30mA LED's will be fine at 100mA, I have pulsed seven segment displays at 1/2 amp when the on time is very short and the repetition rate low. One can work out the dissipation of the LED by first running on DC at its rated current and measuring the voltage drop across the LED to determine LED dissipation, and then when pulsing the LED use a scope to view the voltage drop across it, likewise scope across its current limiting resistor to determine its current while running, use this information with the percentage of "on" time to determine the actual power dissipated by the LED when pulse driven. I have driven many thousands of seven segment displays this way and cannot remember a failed display.

Bob do you have a partial schematic to show how the drive is arranged, my guess is there may be a high side drive that needs to be translated to suit the micro, this circuitry may have a voltage limit, it would be a pity to blow anything up.