HN Explorer

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

51 points by brudgers ↗ HN
I don’t know very much about electronics.

I was poking around a new device and the power supply was supplying 6.18 volts to a board that appears to have 5v chips…the labels are rubbed off but one has “5v” written on it.

Then I looked in my box of wall warts and saw I have a 6.2 volt adapter. So I am curious why would an EE use 6.2 rather than 5 volts?

For context, the device is a tabla drum machine. The power supply is a bit janky. It uses line level AC. I would like to just power the board directly with a barrel plug and that seems within my ability.

72 comments

[ 3.0 ms ] story [ 128 ms ] thread
hw-guy
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
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
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...

stephen_g
Depends on what you're doing. For a lot of sensors and things you might want the power-supply ripple rejection (PSRR) you get through an LDO. I'm not doing hobbyist things, but in our designs we have a lot of places where we will have a switcher that generates an intermediate voltage to feed a bunch of ultra-low noise, high-PSRR LDOs for different components (isolates them from each other too). This is RF though, where that is especially important.
arcticbull
I just meant as a replacement for your main input power regulator - for the 7805 way too many people still use.

You are absolutely right that there is a role for LDOs in modern devices.

There's no reason for most hobbyists to be using super hot, super inefficient and super constrained high-dropout linear regulators from 1972 when there's a pin-compatible all-in-one switching module for roughly the same price. To pull more than a couple hundred mA at the lowest voltage differential you need a heatsink!

sidewndr46
Trying to use a switching regulator for everything becomes a complete nightmare when you have to debug a circuit. You scope something that should be a square wave or similar and it has the switching frequency of the regulator superimposed upon it everywhere.
crote
Most people don't even use input power regulators. 5V wall warts are dirt cheap due to USB charging and provide the exact voltage you need.

Why mess around with a 7805 when you already have a dozen 5V power supplies lying around the house?

arcticbull
Unless it's a USB power adapter - which is a good choice, btw - I'd be very wary about what's coming out of that 5V no-name wall wart. That power can look like ass and not be close to 5V. If you're doing that an LDO isn't a bad choice.
83
90% efficiency for $2.50ish. Pretty cool, thanks for making me aware of these.
blihp
IIRC, you can get them for ~$1 on aliexpress. They're handy little devices.
squarefoot
Linear regulators are still preferable when very clean supply and no spurious RF radiation are needed. Filters and screens can be easily added so that one gets both high efficiency and clean output, but they add to the cost and size.
Gibbon1
That's exactly it. Any less than the regulator won't regulate properly. More than that it'll dissipate too much heat.
regularfry
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?
Gibbon1
Yeah I think even earlier 6.3 volts was also the voltage used for the heating element on vacuum tubes. I think because very early radio's used 6V batteries for the coils. And a separate high voltage battery for the plates.

Would not surprise me if there are roots going back to telegraph.

kurthr
Yep, modern LDOs can maintain 0.2-0.3V with good regulation, but older cheaper ones are a diode drop (~0.6V) or two.
makomk
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.
kurthr
I think you just described the 7805?

https://www.tutorialspoint.com/what-is-voltage-regulator-780...

It's a great old school design, but there are somewhat newer capless designs that are about 2 drops like the LM2940.

There are a lot of other designs between a 0.2V current feedback pFET design, but not all of them have been so popular that their costs are close to packaging.

kurthr
That extra 0.2V is to go capless, which saves a few cents and some area on the PCB.
crote
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?
ofalkaed
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.
ofalkaed
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
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
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
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
6.2V AC? That's 9V peak-to-peak, so after rectification it would be 9VDC less diode drops.
lucas_membrane
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.
[deleted]
(comment deleted)
ofalkaed
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.

schappim
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.
amelius
Battery voltages go in multiples of about 1.5V typically. Perhaps that could be the reason?
pantalaimon
Only Alkaline batteries.
jacquesm
And carbon-zinc batteries too. And at 6V all of these architectures have a nice common multiple (3x2 or 4x1.5).
lmpdev
Can zinc carbon batteries just go and die?

I struggle to see how they're legal

They're one step away from landfill out of the manufacturer's doors

jacquesm
Yes, they're pretty bad. But they're cheap and that's why they still exist. The number of toys that I've repaired on account of those leaking - unprintable - s is larger than I care to remember. And yet people keep buying them. I'm pretty sure I've seen the same toys more than once.
b_t_s
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.
tempest345
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
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.

ddalex
But is the SBR the width of two horses asses ?
CoastalCoder
Horses don't "have" asses. They're both individual contributors.
[deleted]
(comment deleted)
dghlsakjg
That's why its pluralized
downvotetruth
The adjective form of nouns that refer to animals is usually singular when acting as adjectives (goose liver, dog food, rat race). So, it would be two horse asses. The preference for singular is likely to avoid confusion with the possessive form, which in this case would be missing an apostrophe or of for possession: two horses' asses or the asses of [those (specific / in context)] two horses.
madaxe_again
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.

psychphysic
It'd be very hitchhiker-esque if OP got the answer because 3x(1.69 - -0.36) ~ 6.3
foobarbecue
Also the electrochemistry of sulfuric acid (the other half-cell)
ChrisRR
Are you sure about that? Many computers used ±12V in the 80s/90s when LDOs weren't common at all
rrobukef
I think the cause and effect was switched. 12V was used because of 12.6V is two batteries, LDO's are used nowadays because 6V, 12V was standard.
bluGill
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).
namdnay
I think bikes are 6V, as well as stuff like quads, jet skis etc
sidewndr46
not even close. I'm not aware of a single vehicle still in production with a 6 volt battery system.

Many kick-start and pull-start engines do not have a specific voltage but may use an alternator wound with a number of different coils to produce different voltages.

stmpjmpr
Marine batteries are commonly 6v.
bluGill
Often used in series for more than 6 volts. At least in the applications i've seen, though i'm a long ways from a sea and lakes may be different.
sidewndr46
yes, because the underlying cell voltage is much lower than that. Many 6 volt batteries still exist because they are used in series to make the desired voltage. They are easier to move around and transport when they aren't build as all the cells in one unit.
K0balt
….As part of a bank of batteries producing a higher voltage. I haven’t seen any 6v equipment manufactured for any engine based equipment larger than a couple of KW for a very, very long time (mid 1960s). Lower voltage means higher amperage means more weight, more cost, more heat, more failures. Objectively, the world would be a significantly better place if we had fully transitioned to 24v or higher (up to 48v anyway) much sooner than later.

Interestingly, micro-miniaturization has reversed the trend of higher voltage = higher efficiency, at least for computing.

gary_0
Some people in the auto industry want to double the voltage again (for efficiency and to reduce the weight/complexity requirements for all the wiring, especially in EVs) but the 12 volt standard is pretty entrenched.

(Note that I'm talking about the voltage used for everything outside of the internal engine/li-ion electrical systems, which already use higher voltages as needed.)

madaxe_again
What’s old is new again. Military land rovers from as far back as the 1950s are all 24v.
[deleted]
(comment deleted)
eimrine
That was my immediate thought also, but vacuum tubes I know commonly use 6.3V AC.
jacquesm
Not necessarily, they would run just fine of lead acid cells, 3 of which produce... 6.3V DC.
makomk
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.
guerrilla
Definitely sounds like a linear regulator is in use.
[deleted]
(comment deleted)
lmpdev
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

Berniek
Well, 6.3V is an interesting value. The European standard voltage AC is 220V, many countries use 240V (or 230V) So we want 5V to power our whatever. Regulators need some overhead usually .6~1.2V so we need 6.2V as a minimum. BUT THAT IS DC. When you rectify AC (turn it into DC) you actually get PULSES of DC with peak of 1.414 time the AC value. To make it straight DC you need a capacitor. But as you draw current from the capacitor it can't charge up (it has a charge time constant caused by the impedance of the transformer and its capacitance) so you get ripple. The more current you take, the more ripple. So you put a regulator to keep the DC output below the ripple as much as you can. So now to do the maths. Using the SAME transformer on each of the supplies we get the peak output of 8.9V on 220V & 9.7V on 240V Now we need 5V so that gives us an overhead of 8.9-5 = 3.9V and 9.7-5=4.7V. That is plenty of overhead if we had DC but we have pulsed DC that is smoothed by a capacitor so as we draw current the capacitor can't stay charged fully so you end up with DC with ripple at the input to the regulator so that overhead needed by the regulator at the troughs of the ripple can be eroded. So you can drawer less current or give more initial overhead. The more overhead you start with the more heat the regulator has to deal with so you need it to operate with just the right amount of overhead. By the way the ripple component is like AC and its heating effects are actually reduced (the maths says its about .64 times the same value of DC). This is true for the non switching regulators, with those there is a new set of problems with the output ripple (which is usually very high frequency and easier to filter out even with small value capacitance). This may cause radio interference within the circuits but good design should eliminate that. The ideal overhead seems to be ~4V. The regulator power dissipation (it gets hot with bigger overhead) is a trade off. You can operate with less but you need a bigger transformer (to supply more charge current to the transformer or bigger capacitance and that means more cost). By the way so you have 110V or 120V AC. The 2 values are directly related to the 220 & 240V.

But why 6.3V? Well like a lot of traditional designs, the heater voltage of vacuum tubes was nominally 6.3V so the transformer design was already done. Also putting 6V battery with vacuum tubes is likely to shorten their life, (the DC equivalent of 6.3V used for heating is 4V), but the early batteries had a fairly high internal resistance so the voltage supplied to the heaters was usually much less.

A quick note is in order about values. Why pick individual values? History and experience tell us that certain values are efficient. Metric is very good for measuring distances, but not good for measuring bolts where the imperial system reigns supreme (a 1/2 inch long bolt has more useful applications than a 10 mil long bolt where a 15 mil bolt is too long). Same goes for fathoms. It is a much better measurement of depth because nearly all water bodies will have ripple or waves and waves of 6 feet make an error in depth of 1 in fathoms or an error of 6 in feet.

Turns out 6.3V seems to be efficient for both vacuum tubes and regulators!