LED clock Jumbo Digits

Question

This LED clock kit from EBAY uses 6X S8550 transistor to drive each of the small 0.5 inch LED display

need to use bigger digits 1.8 inch require 7.2 Volt DC to glow.

is it possible to connect 7.2 Volt to these transistors and drive the big LED segments ?

Sure I will keep the 5 volt regulator to supply the PIC chip with fixed 5 volt....

If this is possible, do I need to change the 7 resistors(each is 1 Kohm) going to the a-to-g segments ?

Former Member · Answer

If you increase the supply for the LEDs to 7.2V (well, greater than that, because you need to account for the voltage drop across the CE junction of the transistor as well as across the current-limiting resistor, so let's call it at least 8V), you are increasing the voltage at the emitter of the transistor. Normally, you would disable the PNP transistor with a HI logic level, which means the base and emitter of the transistor would be at the same potential, not allowing for current to flow from the emitter to the collector or vice-versa. When you increase the voltage at the emitter, even with a HI (5V) at the base, the PN junction will be forward-biased. With a HI logic level on all of the LED driving pins, there SHOULD not be enough potential difference to forward bias the segments of the display. This is definitely not ideal. Also, there will constantly be a current flowing (or attempting to flow) from the source, through the emitter-base (PN) junction of the transistors, back through the output pins on the micro-controller, even when the pin is at a HI logic level, attempting to turn "off" the transistor. I don't know what (if any) the potential for damage is here to your micro-controller in this configuration (pin configured as output/low impedence, with HI logic level at the output, with a higher potential attempting to flow back through the pin to the lower potential).

LED clock Jumbo Digits

Top Replies