The familiar five volts standard from back in the TTL days always struck me as odd. Back when I was just a poor kid trying to cobble together my first circuits from the Forrest Mims Engineer’s Notebook, TTL was always a problem. That narrow 4.75 V to 5.25 V spec for Vcc was hard to hit, thanks to being too poor to buy or build a dedicated 5 V power supply. Yes, I could have wired up four 1.5 V dry cells and used a series diode to drop it down into range, but that was awkward and went through batteries pretty fast once you got past more than a few chips.
As a hobbyist, the five volt TTL standard always seemed a little capricious, but I strongly suspected there had to be a solid reason behind it. To get some insights into the engineering rationale, I did what anyone living in the future would do: I asked ChatGPT. My question was simple: “How did five volts become the standard voltage for TTL logic chips?” And while overall the answers were plausible, like every other time I use the chatbot, they left me wanting more.
Circular Logic
But, being able to tell your 1s and 0s apart really only sets a minimum for TTL’s supply rail. Why couldn’t it have been higher? It could have, and a higher Vcc, like the 10 V to 15 V used in emitter-coupled logic (ECL), might have improved the margins between logic levels and improved noise immunity. But higher voltage means more power, and power means heat, and heat is generally frowned upon in designs. So five volts must have seemed like a good compromise — enough wiggle room between logic levels, good noise immunity, but not too much power wasted.
I thought perhaps the original patent for TTL would shed some light on the rationale for five volts, but like most inventors, James Buie left things as broad and non-specific as possible in the patent. He refers only to “B+” and “B-” in the schematics and narrative, although he does calculate that the minimum for B+ would be 2.2 V. Later on, he states that “the absolute value of the supply voltage need be greater than the turn-on voltage of the coupling transistor and that of the output transistor,” and in the specific claims section, he refers to “a source of EMF” without specifying a magnitude. As far as I can see, nowhere in the patent does the five volt spec crop up.
Your Turn
October 2025