Ask HN: Why do some electronics use 6.2 volts DC?

Around 6.3V was a standard voltage for vacuum tube heater pins (the part that emitted free electrons via thermal emission inside the tube to be available for the electrical fields to accelerate).

My guess would be that historically 6.3V supplies made forvacumee tubes where commonly available when transistor based electronics where created so it made sense to utilize them. Works quite nicely for the typical 5V circuits as a "rough" input voltage to be feed through a LDO regulator, for example. And so it just stuck.

adammunich · 3 years ago

6.3V is the standard voltage of a farm tractor battery --these vacuum tube heaters were designed to use them.

12.6V is two farm tractor batteries (one car battery), which is why our computer industry uses 12V for motherboards (12 volts - 0.6V reverse protection diode).

Early computer power supplies used voltage regulators that were designed for car radios, originally.

madaxe_again · 3 years ago

That’s because you get ~2.1v out of a single charged lead acid cell, and an old tractor battery contains three. Your car battery has six.

So the real answer to “why” is because of the electrochemistry of lead.

Kinda similar to how a lot of our world is structured around the dimensions of two horses side by side.

bluGill · 3 years ago

Farm tractors and cars switched from 6 volt to 12 volt at about the same time, and for the same reasons. However the old 6 volt farm tractors are still around and used for framing, while 6 volt cars are rare collectors items (even though there were more of them).

tiedieconderoga · 3 years ago

But don't 12V lead-acid batteries have a wide voltage range between charged (~14.5V) and discharged (~10V)? I don't think they would provide exactly 12.6V very often, or for very long.

ddalex · 3 years ago

But is the SBR the width of two horses asses ?

ChrisRR · 3 years ago

Are you sure about that? Many computers used ±12V in the 80s/90s when LDOs weren't common at all

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ShadowBanThis01 · 3 years ago

Cool info, thanks!

eimrine · 3 years ago

That was my immediate thought also, but vacuum tubes I know commonly use 6.3V AC.

makomk · 3 years ago

The filaments will run off either AC or DC because they're basically just heaters. AC became the norm in mains-powered devices after electrification took off, but a lot of the early radios were powered by DC from batteries - initially this was seperate, possibly rechargable, low voltage battery for the filament supply and a disposable high-voltage battery, with the second battery being replaced by step-up devices using vibrators and transformers fairly early on.

jacquesm · 3 years ago

Not necessarily, they would run just fine of lead acid cells, 3 of which produce... 6.3V DC.

Could also be simple pragmatics or the economy of scale. If your business makes both 5V and 6.2V devices it could be a decent cost saving to make them both use 6.2V adapters, unless production number are low burning off that extra 1.2V in the 5V devices will generally be cheaper than separate power adapters for each and if nothing else it is one less inventory item to deal with.

brudgers · 3 years ago

Thanks. That makes sense.

The rhythm machine takes AC at 120 and 220 through two separate figure eight female jacks. It's from India.

Then there is a transformer that outputs ~6.2v from my 120v AC (don't know what it would be from 220). It would make sense if the transformer was sourced off a shelf at the cost of a voltage regulator which there appears to be.

ofalkaed · 3 years ago

That is an odd layout*, is the schematic online somewhere?

Since this is older gear, it is probably a 6.3V transformer, 6.3 and 12.6 volt transformers were some of the most common and cheapest voltages before the rise of cheap switcher supplies. 6.3 and 12.6 were the optimum voltages for powering the filaments of vacuum tubes and became standard voltages in transistor gear since they were cheap and available, even today transformers in these voltages are easy to find cheaply on the surplus market.

*Edit: Nevermind, I read that as you had 120 and 220VAC transformers followed by a 6.3VAC transformer, you have one transformer with 120/220 primaries and a 6.3VAC secondary. Not odd at all.

sitharus · 3 years ago

That's a pretty typical design for power supplies. Transformers output depends on the mains input. Where I am this is nominally 230V, but in reality ranges from 235-245V. So they often use a linear regulator to obtain the required voltage, but these need to drop from a higher voltage so they choose a range that would require the least dissipation.

Majromax · 3 years ago

6.2V AC? That's 9V peak-to-peak, so after rectification it would be 9VDC less diode drops.

lucas_membrane · 3 years ago

I had a little problem with my desktop computer (Intel I7-6700 CPU) a couple of months ago. It would start up and run OK, but eventually the file system would switch itself into the read-only state, causing it to lock up. After a few weeks of trying everything to fix it, I noticed that the slider switch on the power supply was in the 230 volt position, even though I'm sitting here in the USA and the thing has been plugged into the 115 volt mains for years. That it could just about run on half the voltage it required(?) makes me think that the rightmost digits on voltages are there for historical or marketing reasons only, like the old 101 mm cigarettes, the 81 mg aspirins, and the guitar amp that goes up to 11.

ofalkaed · 3 years ago

Those digits are fairly important to EEs.

Common switch mode power supplies are not picky about input voltages and generally have a wide range of operating voltages, having the switch on 230 likely just knocked 20 volts or so off the input voltage to keep the 230 below the max voltage, not half it. The reason this eventually started causing problems is probably because your wall voltage has dropped and/or components in the supply aging and drifting from spec meant it could no longer maintain good clean output which triggered a safe guard in the computer.

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tempest345 · 3 years ago

hw-guy · 3 years ago

Just a guess...maybe the 5V is supplied from a linear regulator with a maximum dropout of 1.2V, so the board is specified to require at least 6.2V from the wall wart.

JohnFen · 3 years ago

That sounds entirely plausible. Many of the hobby electronics projects I make require at least a 6V supply because of exactly this. I usually use a 9V wall wart, though, because those are very commonly available (so are easy to scrounge up).

arcticbull · 3 years ago

For what it's worth in hobby projects it's time to throw linear regulators into the dustbin. There's 7805 pin-compatible switching modules now!

Stuff like this. [1]

[1] https://www.digikey.com/en/products/detail/cui-inc/VXO7805-5...

Gibbon1 · 3 years ago

That's exactly it. Any less than the regulator won't regulate properly. More than that it'll dissipate too much heat.

regularfry · 3 years ago

Combine this with the sibling thread about lead acid battery voltages (2.1V per cell, 3 cells gives 6.3V), can we then say that this is the reason 5V circuitry is so common? 6.3V from battery ancestry plus a linear regulator?

kurthr · 3 years ago

Yep, modern LDOs can maintain 0.2-0.3V with good regulation, but older cheaper ones are a diode drop (~0.6V) or two.

Older cheaper voltage regulators are typically more like 2V drop and the optimum input voltage is somewhere around 7-9V for a 5V output. In practice a lot of devices used 9V power supplies because that was the lowest widely-available voltage that met the requirement. More modern LDO regulators would have plenty of headroom on plain 6V. It'd be kind of unusual for any electronic device to need the extra .2V in this particular case.

crote · 3 years ago

That doesn't really make sense, though. If you need 5V, why use a 6.2V wall wart and a LDO instead of just using a 5V wall wart in the first place?

RobotToaster · 3 years ago

That was my immediate thought too.

jschveibinz · 3 years ago

I think you nailed it.

It could also be that some of the circuit needs a higher voltage, just because some parts are 5V does not mean they all are.

lmpdev · 3 years ago

Possibly a linear regulator (does the device get warmer than you'd expect?)

Although the LM7805 would need ~7V minimum

RE those chips: almost all general purpose components have a usable voltage range. From LEDs to MCUs, most components can tolerate a few hundred millivolts from its ideal V_{f}, some even dozens of volts - especially solid state stuff like CMOS ICs

The only time I really recommend not f*cking with aftermarket PSUs is if it's primarily charging or powering: - lithium batteries - super or ultra capacitors

amelius · 3 years ago

Battery voltages go in multiples of about 1.5V typically. Perhaps that could be the reason?

pantalaimon · 3 years ago

Only Alkaline batteries.

b_t_s · 3 years ago

Alkaline batteries are still what runs most consumer battery powered gadgets. 4 alkaline batteries are nominally 6v(actually 5-6.6 depending on charge), so 6.2v seems perfect if you want to run a 4x battery device on a more permanent power source.

And carbon-zinc batteries too. And at 6V all of these architectures have a nice common multiple (3x2 or 4x1.5).

schappim · 3 years ago

It is mostly for voltage headroom/power dissipation along w/ the use of 6.2V Zener diodes(hear: voltage regulation). The use of 6.2V also gives you some margin for noise.