NAND gate

[walid]

Hi
look at the figure shown below:
you noticed that the two nand gates have the same inputs, so they willl have the same outputs
the led is always on till all the i/ps are High
my question:
if the two nand gates have not the same inputs, and the o/p of one of them is high and the other is low, will the LED ON or OFF?
thanks

[audioguru]

The gates are probably Cmos which have a low output current if they have only 5V for their supply. So the gates are paralleled for double the output current to make enough current for the LED.

If one gate tries to make its output high and the other gate tries to make its output low then their output voltage will try to be at half the supply voltage but there might not be enough current to light the LED.

[walid]

thank you guru

but i think if one o/p high =5v
the other o/p low = 0 volt
so it will sink all the current and the LED OFF
what would u say

[audioguru]

Hi Walid,
Cmos gate outputs go to 5.0V or to 0V only if they don't supply any current. With the outputs connected together then one will pull up with its max current of about 2mA and the other will pull down with its max current of about 2mA so the output voltage will be about 2.5V with no current left to drive an LED. If the LED is a 2V red one then it might light very dimly.

If the gates are a high speed Cmos 74HCxx type, then the IC will burn out because then the currents would be about 50mA.