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Ask HN: Why do some electronics use 6.2 volts DC?
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.
- amelius 4y agoBattery voltages go in multiples of about 1.5V typically. Perhaps that could be the reason?
- pantalaimon 4y agoOnly Alkaline batteries.
- jacquesm 4y agoAnd carbon-zinc batteries too. And at 6V all of these architectures have a nice common multiple (3x2 or 4x1.5).
- lmpdev 4y agoCan 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 4y agoYes, 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 4y agoAlkaline 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 4y agoAround 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 4y ago6.3V 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 4y agoBut is the SBR the width of two horses asses ?
- CoastalCoder 4y agoHorses don't "have" asses. They're both individual contributors.
- deleted 4y ago[deleted]
- dghlsakjg 4y agoThat's why its pluralized
- downvotetruth 4y agoThe 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.
- guerrilla 4y agoDefinitely sounds like a linear regulator is in use.
- deleted 4y ago[deleted]
- lmpdev 4y agoPossibly 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 4y agoWell, 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!
- hw-guy 4y agoJust 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.
- kurthr 4y agoYep, modern LDOs can maintain 0.2-0.3V with good regulation, but older cheaper ones are a diode drop (~0.6V) or two.
- makomk 4y agoOlder 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 4y agoI think you just described the 7805? https://www.tutorialspoint.com/what-is-voltage-regulator-7805-internal-block-diagram-and-applications 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 4y agoThat extra 0.2V is to go capless, which saves a few cents and some area on the PCB.
- RobotToaster 4y agoThat was my immediate thought too.
- ofalkaed 4y agoIt 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 4y agoCould 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.
- lucas_membrane 4y agoI 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 4y ago[deleted]
- ofalkaed 4y agoThose 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.
- brudgers 4y agoThanks. 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.
- schappim 4y agoIt 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.