How did 5 volts get to be the standard? (2025 Update)

The 5 volt standard goes WAY back to TTL logic As for the origins of 5V/TTL, that was a set of design tradeoffs. If you find a schematic of a TTL gate, you'll notice that it needs at least 3 diode drops internally, plus various resistor drops. What that doesn't tell you is the choice of reference currents (1.6 mA for a low input) which was in part determined by the current levels required to produce acceptable switching speeds. These current levels in turn set limits on the internal resistor values, and the voltages needed to feed them. You also need to factor in the state of semiconductor fab capability - the first TTL circuits were at the edge of what could be reliably produced. Imagine - 20 to 100 gates on a chip! That's (gulp) hundreds of transistors, with the masks all laid out by hand. All of this, including power dissipation limits, resulted in the standard TTL supply voltage spec of 4.75 to 5.25 volts. As it turned out, this was an adequately wide margin for practical systems, and the speed (10 -

To well before that with RTL and DTL in fact.

Yup 5v was what the original TTL ran on and those were the most common IC chips in the beginning. CMOS is more forgiving and will run on a pretty wide range of voltages. These days I suspect 5v was chosen because that is what you get from a USB port and they are everywhere. Before USB became the "go to" charging port 12v was getting pretty popular and it still is for a lot of stuff. It is pretty much #2 in that coaxial power plug you see on wall warts.

Available chips I think. Previously every company had their own logic and different voltages, and positive or negative logic. There was DEC logic, Collins logic, Zerox logic, etc. There was also Dynamic logic. A one was a pulse, no steady state signals except no pulse a zero. One dandy machine I worked on had Audio, RF, TTL, MECL, the SRE. We called it Sorry.

Greg