Capacitors; Why 6.3V?

[ssashton]

10V, 25V, 50V 100V etc.. all seem sensible ratings for a log10 counting system.

What is the origin of the 6.3V and 63V rated parts? Seems a very weird number to choose!

[bob91343]

It's the geometric mean between 5V and 10 V with a little safety factor thrown in.  High enough for the paranoid to use on 5V buses.

[TimFox]

It is an amusing choice, since "6.2" is the closest E24 value.  6.3 V is a standard heater voltage for vacuum tubes, apparently derived from the three-cell lead-acid battery used in automobiles. 
The geometric mean of 5 V and 10 V is 7.07 V.

[Jay_Diddy_B]

Hi,

It is normal to design with 25% margin.

5V + 25% = 6.3V

48V + 25% = 63V

Jay_Diddy_B

[LaryPant]

48V + 25% = 63V

Good point, but that raises a new question, where does the 48 Volts come from?

A lot of standard voltages seem to have no apparent reason other than that they are appealing to the eye - the actual number.

For example, 12 V is pretty common, but why not 13 or 11? Probably just "tradition" because 12 V is for more pleasing to the eye than 11 V or 13 V. The same goes for 3,3 V.

Lary

(I do not know this for sure, but I recon that it is very plausible, please rectify me if I am wrong.)

[tkamiya]

12V is a nominal value for lead acid and alkaline batteries.  13.8V is the same during charging.
5V is a standard TTL power supply voltage.
24V is a large size battery's nominal voltage.
48V is a telecom standard which also used large banks of batteries for backup.

I never liked using 6.3V capacitors.  I usually use 10V or 15V or even 25V.

[T3sl4co1l]

It is an amusing choice, since "6.2" is the closest E24 value

You're on the right track -- off just a little on the multiple!

The Renard series has 1, 1.6, 2.5, 4 and 6.3, and splitting 40 into 35 (nearly an R20, bleh!) and 50 (an R10) is convenient for practical purposes.  80 (an R10) is also often seen.

This breaks a bit at higher voltages, where 160, 200, 220 (uncommon), 250, 350, 400, 450, 475 and 500 (and sometimes 550, 600 and 630) are seen, I think again for practical reasons, as these are approaching the physical limitations of electrolytic capacitors (or surpassing them; as far as I know, "630V" electrolytics are actually internally series connected).

Film capacitors pick up again from 400, 630 and 1000V, but oddball values are also commonly seen here, probably driven instead by standards -- ratings related to RMS, nominal-maximum, peak, etc.

Tim

[T3sl4co1l]

12V is a nominal value for lead acid and alkaline batteries.  13.8V is the same during charging.
48V is a telecom standard which also used large banks of batteries for backup.

Indeed, batteries aren't so common in telecom anymore, by direct application that is -- they're mostly using more conventional UPS architectures, I think? Maybe not? -- but "48V" power supplies are still called "rectifiers", even though it's been a long time since they merely rectified mains for the purpose of charging lead-acid batteries.

A lot of slow-moving fields have the same kind of... inertia of terminology.  HVDC power stations were originally built with mercury arc thyratrons; they're all (I think?) using SCR stacks now, but they still call the rooms valve houses.

Tim

[coppice]

It is an amusing choice, since "6.2" is the closest E24 value

You're on the right track -- off just a little on the multiple!

The Renard series has 1, 1.6, 2.5, 4 and 6.3, and splitting 40 into 35 (nearly an R20, bleh!) and 50 (an R10) is convenient for practical purposes.  80 (an R10) is also often seen.

This breaks a bit at higher voltages, where 160, 200, 220 (uncommon), 250, 350, 400, 450, 475 and 500 (and sometimes 550, 600 and 630) are seen, I think again for practical reasons, as these are approaching the physical limitations of electrolytic capacitors (or surpassing them; as far as I know, "630V" electrolytics are actually internally series connected).

Film capacitors pick up again from 400, 630 and 1000V, but oddball values are also commonly seen here, probably driven instead by standards -- ratings related to RMS, nominal-maximum, peak, etc.

Tim

But why has 6.3 been used for things like the commonest vacuum tube heater voltage? 6.3 comes up in a lot of non-obvious places in electronics.

[T3sl4co1l]

But why has 6.3 been used for things like the commonest vacuum tube heater voltage? 6.3 comes up in a lot of non-obvious places in electronics.

As mentioned, three lead-acid cells.  Heaters of 1.2V (single dry cell), 2-2.5V (single lead acid) and 6.3V (three) were very common.  Which started out DC of course, then became AC RMS filament transformers.  Happy coincidence that, measured in SI volts, it happens to be an R5.

Tim

[Circlotron]

Reminds me of when I was asking why you can get fuses rated at 3,15A.

[coppice]

But why has 6.3 been used for things like the commonest vacuum tube heater voltage? 6.3 comes up in a lot of non-obvious places in electronics.

As mentioned, three lead-acid cells.  Heaters of 1.2V (single dry cell), 2-2.5V (single lead acid) and 6.3V (three) were very common.  Which started out DC of course, then became AC RMS filament transformers.  Happy coincidence that, measured in SI volts, it happens to be an R5.

Tim

The only time I've ever seen 6.3V heater vacuum tubes used with lead acid cells was in car radios, where they put two tubes in series across the 12V lead acid battery. In other applications they usually used a single lead acid cell.

[TimFox]

The E series are like unto the R series, used for components that have 5%, 10%, etc. tolerances.  This includes the voltages on Zener diodes (e.g. 5.6, 6.2, 6.8 V) and normal resistors.
The E series are all based on multiples of 3 (E6, E12, E24, etc.), while the R series are based on multiples of 5.  One hears of the E series more often now, I believe, but both are standardized.

[T3sl4co1l]

The only time I've ever seen 6.3V heater vacuum tubes used with lead acid cells was in car radios, where they put two tubes in series across the 12V lead acid battery. In other applications they usually used a single lead acid cell.

Didn't have a 6.3V car (chassis positive?) when you were a kid I guess?

I don't know the history of those things, exactly when, or how common they were, I just know they were weird/scary enough to exist at some point...

Tim

[TimFox]

But why has 6.3 been used for things like the commonest vacuum tube heater voltage? 6.3 comes up in a lot of non-obvious places in electronics.

As mentioned, three lead-acid cells.  Heaters of 1.2V (single dry cell), 2-2.5V (single lead acid) and 6.3V (three) were very common.  Which started out DC of course, then became AC RMS filament transformers.  Happy coincidence that, measured in SI volts, it happens to be an R5.

Tim

The only time I've ever seen 6.3V heater vacuum tubes used with lead acid cells was in car radios, where they put two tubes in series across the 12V lead acid battery. In other applications they usually used a single lead acid cell.

When vacuum tubes were first installed in car radios, almost all passenger vehicles had "6 V" batteries.  The 12 V automotive systems became popular in the mid-50s, but the VW Beetle had 6 V until 1967 to 1972 (depending on market).  I remember when the American auto market changed to 12 V:  some naive people thought that a 12 V battery wire could be used to replace the wire in a 6 V car, not understanding that the current at 6 V would be roughly double that at 12 V (all other things remaining equal).  A special series of low-plate-voltage vacuum tubes became practicable with the 12 V supply, where an extra electrode near the cathode established a high space-charge-limited cathode current (with wasted power).  My mother's 1960 Chevrolet's radio had these tubes with a single germanium power transistor for the speaker.
For example, see the 12DK7 (somewhat normal tube):  http://tubedata.milbert.com/sheets/049/1/12DK7.pdf  The RCA data sheet says it can be operated directly from 6-cell batteries (both heater and plate).  A different tube, the 12DL8, operates with a space charge grid  http://tubedata.milbert.com/sheets/106/1/12DL8.pdf   with 40 mA of normal plate current and 75 mA of "wasted" grid current.

[CatalinaWOW]

You are not going back far enough.  Tube/valve heater voltages were established when radios first became popular - in the 1920s-1930s.  And in the USA at least they were popular in rural areas because it was a way to connect to the world.   But those same rural areas weren't electrified for another decade or two.  So battery sets were the norm.  I don't know the split between the 1.5 dry cells that were widely available because of telephone usage and lead acid cells, but those sources were the driver for the voltages.  My guess is that 6.3 didn't burn up on lead acid and worked well enough on a series string of four dry cells.

[Gyro]

The only time I've ever seen 6.3V heater vacuum tubes used with lead acid cells was in car radios, where they put two tubes in series across the 12V lead acid battery. In other applications they usually used a single lead acid cell.

Didn't have a 6.3V car (chassis positive?) when you were a kid I guess?

I don't know the history of those things, exactly when, or how common they were, I just know they were weird/scary enough to exist at some point...

Tim

12V cars followed the chassis positive protocol for a long while too - much to the benefit of the Germanium (PNP) transistor radios that followed the valve ones.

As I remember, the procedure for changing the polarity to negative chassis, for use with more modern radios and add-ons, was to simply turn the battery round (often involving longer leads) and then 'flash' the field winding terminal of the generator (not alternator) so that its remanent field caused it to output a positive charging voltage when the engine was started.

Irrc, the original reasoning for +Ve chassis was to improve the efficiency of the spark plugs by making the hot centre electrodes 'cathodes' and also to reduce corrosion on chassis connections. I'm not sure I believe the first one, as it would surely have been just as simple to wind the ignition coil secondary winding for either flyback polarity.

[TimFox]

A brief note, from a British perspective, of the heater voltage choices in the 1930s:
http://www.r-type.org/addtext/add014.htm

[james_s]

My recollection is that the early battery radios used dry cells for the plate voltage but the power hungry tube heaters typically ran off a 6V lead acid battery that could be borrowed from the car, farm truck, tractor, etc and then put back for recharging. All of those common voltages like 6V, 12V, 24V and 48V are multiples of the 2V nominal lead-acid cell voltage. A freshly charged battery would be a bit higher than that, sagging to a bit under by the time it is considered discharged, hence the need for some tolerance. Telecom and railroad signalling used lead acid battery banks up into relatively modern times, some may still to this day.

[TimFox]

The nominal cell voltage used to set those voltages was 2.1 V, hence 6.3 V.  Receiving tubes usually specified +/- 5 to 10% on the voltage, but that tolerance does not appear explicitly  in pre-war data sheets.   I do find tolerances on the current in 1950s data sheets.  The 6.3 V tube heaters were often wired in series with a “ballast” resistor across the 110 V line in AC-DC receivers:  the 1930s data sheets say to adjust for 300 mA in that operation.  The 12.6 V, 150 mA heaters came later.

[james_s]

I've heard of some early transformerless radios that had a resistive line cord, and others that used a special ballast tube, which of course was counted  as a tube in the marketing. I have a few AC/DC radios but mine have tubes like the 50C5 and 35W4, with 50 and 35 volt heaters respectively, specifically designed to operate in series with the rest of the tubes necessary to make a basic superhet radio. The classic AA5 design.

[TimFox]

The "All-American Five" sets used three 12 V, one 35 V, and one 50 V (0.15 A) heater for 110 V service.  I had one small short-wave receiver in my youth with fewer tubes that used a series resistor spread out along the line cord.
I also have a WWII merchant-marine AC-DC receiver (E H Scott) that used 0.3 A tubes.  To reduce LO radiation, the input RF amplifier was a 6K7 (with grid cap), while the IF amps were 6SK7 (single-ended).

[Circlotron]

Irrc, the original reasoning for +Ve chassis was to improve the efficiency of the spark plugs by making the hot centre electrodes 'cathodes' and also to reduce corrosion on chassis connections. I'm not sure I believe the first one, as it would surely have been just as simple to wind the ignition coil secondary winding for either flyback polarity.

An ignition coil for a positive ground vehicle has the primary and secondary in series and phased like an autotransformer. Negative is supplied to the free end of the primary and the other end of the primary that is also the start of the secondary is switched down to positive ground. When that end of the primary is released it jumps negative by several hundred volts, and seeing this switched terminal is also the start of the secondary winding, it adds to the negative-going secondary voltage in autotransformer fashion.

With a negative ground vehicle, the positive supply end of the primary and the start of the secondary are connected to the same terminal. The positive-going pulse when the grounded end of the primary is cut off does not add to the negative-going secondary voltage.

So there is no difference between pos and neg ground coil, just the way they operate is slightly different.

[CatalinaWOW]

Not sure I believe the corrosion prevention argument either.  The vehicle is unpowered most of the time, so corrosion performance should be dominated by electronegativity of contacting materials.  Even when powered corrosion should only be affected at bad joints.

[Gyro]

Nor me, I'm just relating folklore of the day. Maybe they just preferred their electrons to flow down hill.