Output of logic gate as ground?

[musicboy]

Hi there.
I was wondering, say I have a simple logic gate (AND, for instance), and it's output is LOW, 0v, could this be used as ground?
Ie if I had an LED with its anode connected to +5v and its cathode connected to the output of a 5v logic gate - obviously if the output of the gate was +5v then the LED wouldn't light, but if the output was 0v, would it light?
Thanks in advance

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point

Yes and no. It depends what you're asking.

It is fine to connect an LED (with a series resistor for current limiting) between VCC and a gate output, and yes, when the gate output goes low, the LED will light. This is often done in practice.

In general though, you can't use a low output of a logic gate as ground because there is resistance between the output and ground - the ON-resistance of the active device (transistor or MOSFET) that is pulling the output low. So components that should be connected to the 0V rail of your circuit should be connected to the 0V rail, not to an output that is known to be low.

 

[BobK]

You must also look at the specs of the device to see how much current it can sink. Some are limited to a few milliamps and might not be able to drive an LED they way you are asking about,.

Bob

 

[KrisBlueNZ]

Good point Bob, thanks.

That is true whether you connect the LED+resistor from the output to 0V/VSS, or between VCC/VDD and the output.

Different logic families, and different I/O structures inside devices, have different characteristics, and this affects the maximum current they are specified to source or sink, and their internal resistance, which determines how much the output voltage will be pulled away from its ideal voltage by the load current.

For example, many Microhip PIC microcontrollers are specifically designed to be able to source or sink up to 30 mA on each I/O pin. That's more than most standard logic families are designed for. The older transistor-based logic (7400 and 74LS00 and a few others) have asymmetrical outputs that can pull low strongly, and sink significant current, but pull high only weakly. And with the CMOS CD4000 devices, the output resistance is greatly affected by the supply voltage.