Driving a LED with CMOS gate: sink and source

Sink/Sorce a Led

Hi
I'm a newbie in electronic.

I want to ask about specification needed to drive a led with cmos gate.

If I want to "sink" a led then I must look at CMOS "IOL" and Led "IF". (Corect ?)

If I want to "source" a led then I must look at CMOS "IOH" and LED ____ ?

Please give a hint.

Thank you

Aina Law

Re: Sink/Sorce a Led

Um, I think you need a bit of clarity with your terminology:

If the LED is EVER turned on (meaning light is coming from it), then the LED spec that you want to look at is the If (stands for forward current). For instance, if the forward current If of a specific LED is 30mA on the datasheet, then you should not EXCEED 30mA going through the diode (it is okay to use less current than 30mA, but not more).

If you are using a CMOS gate, it usually is an NMOS and a PMOS with their drains connected together. If you are SOURCING current from the CMOS gate, then this probably means that the PMOS transistor is on, the NMOS transistor is off, and current is flowing OUT of the output of the gate, and IN TO the anode (positive end) of the LED, and OUT of the cathode of the LED, to ground. This assumes that the LED is connected from the output of the gate to ground.

If you are SINKING current from the cmos gate, the NMOS gate transistor is on, the PMOS is off, and current is flowing from the LED INTO the cmos gate output. This would assume that the anode of the LED is connected to Vdd (your power supply), and the cathode of the LED is connected to the output of the cmos gate.

Re: Sink/Sorce a Led

LEDS ARE DIODES and like all diodes they need current direction properly set to operate. source means the device is providing the power and sinking is taking in the power. as far as 30 ma that is a question of power dissipation. you may put any current that you want evenamps BUT if you exceed the power dissipation it will blow. bottom line is not the current that destroy a device but ruther the HEAT

Re: Sink/Sorce a Led

Hi
You are right for sinking current you consider Iol and If of the led as for Io source you consider Ioh and also If of the led ( of course with a factor of safty ) but the output of the c mos series will not satisfy the required curent to light the led satisfactorily so use a buffer ( for example a high gain transistr with a resistance in base and a resistance in collector in series with the led)
Regards