need help wif NAND gate

[rossu704]

i have a problem wif my logic circuit. suppose for logic gate the output voltage should be 5V rite for an input of 5V. but mine gave an output of oni 2.2V.
but d simulation in multisim shows d output voltage to be 5V. somehow wen i tested d circuit on breadboard the output voltage came out as oni 2.2V.


does anyone noe wat might caused d problem.

tenks

 

[audioguru]

You forgot to tell us if the NAND gate is a TTL 7400 or if it is a Cmos 4011.
If it is a Cmos then you forgot to tell us its supply voltage.

You also forgot to tell us its load resistance.

 

[rossu704]

nevermind..i have d solution now..its because of the led i've connected at the output..the led somehow dropped the output voltage to 2.2..

btw i did mention d input voltage is 5V.

 

[gramo]

The datasheet for the device probably gives a 'max' current rating, and perhaps the resistor (assuming you using a resistor - if not OMG) in series with the LED is not high enough.

 

[audioguru]

I can hardly remember an old TTL gate high output current but I think it can drive a few mA and still be above 3V.

The datasheet for a CD4011 Cmos Nand gate with a 5V supply shows that it can typically supply 4ma to a dead short to ground and will be at 2.2V with a load current of only 3.5mA. A very dim LED. A current-limiting resistor won't make any difference and is not needed.
Its output current is typically 10mA to 20mA with a 10V supply, but its voltage might not be high enough to produce a logic high.

 

[gramo]

A current-limiting resistor won't make any difference and is not needed.

Phew, I don’t use the devices as you can probably tell.. I was just imagining 5V with a single diode in series to earth.. bad juju

 

[audioguru]

Hi Gramo,
A CD4xxx with a 9V battery for its power supply can directly drive an LED without a current-limiting resistor. The Mosfets at the output of the Cmos device will limit the current to 10mA to 15mA safely. You can parallel a few inputs and outputs from a single IC to increase the current.