16V Film Caps

[spacecommandant]

Hello all,

I've always understood that capacitor voltage rating should be double the pedal's voltage.
What are thoughts on using 16V film caps in a 9V pedal?

Thanks in advance...

[antonis]

What are thoughts on using 16V film caps in a 9V pedal?

That's because there aren't 9V capacitors..
(commercially available, at least..)

P.S.
Could you show us a 9V pedal exhibiting more than 9V somewhere??

[FSFX]

I've always understood that capacitor voltage rating should be double the pedal's voltage.

The voltage rating of a capacitor is the voltage that it is safe to run it at for its lifetime. All the major reputable manufacturers add a safety margin when specifying the 'working' voltage, so it is actually not true that you need to double the voltage unless you have some really poor fake capacitors. It is quite safe to run a 10v capacitor at 9 volts. The only reason for allowing an extra safety margin is to cope with any transient spikes that would exceed the normal voltage of your circuit or to cope with people putting 18 volts into your 9 volt pedal.

This document from Vishay explains all about film capacitors. Look for the bit about voltage rating.

https://www.vishay.com/docs/26033/gentechinfofilm.pdf

[ElectricDruid]

What are thoughts on using 16V film caps in a 9V pedal?

Yeah, 16V caps in a 9V pedal should be alright. If someone starts running it on 12V it's a bit close maybe, and there was an example on here recently of a commercial pedal running at 15V with 16V caps in it - and guess what?! It wasn't working!

A safety margin is a good idea, but "x2" might be excessive. I might go for "+50%" AKA "x1.5".

[spacecommandant]

Thanks, all!

[PRR]

The voltage rating of a capacitor is the voltage that it is safe to run it at for its lifetime. ....

Rated lifetime. On electrolytics that may be 2,000 hours.

https://media.digikey.com/pdf/Data%20Sheets/Panasonic%20Electronic%20Components/ECA-xxM%20Series,TypeA.pdf

Most recent whitepapers focus on temperature, because most of today's stuff is over-packed (from LED lamps to welders). Voltage matters but the trend is less clear. "Double" is a time-approved derating for any part which does not get much more expensive at the higher voltage (e-caps, rectifiers, to 200V).

Note that 2,000 hours is 83 days straight but 2 years in 20 hour a week gig-duty. You can sell gig stuff with 1 year warranty and not be killed by warranty claims. I have seen 35V-rated caps run at 36V for concerts over a decade.

Note that only a few e-caps in a pedal will run at the full 9V. Most will be half-rail at most, and maybe <1V.

[Rob Strand]

You really need to look-up the manufacturers info for cap lifetime.  It can get quite specific on cap type and manufacturer.

The lifetime for a cap is specified at the *rated temperature*.  So a 2000hr electrolytic is 2000hr at the rated temperature.  When running below the rated temperature the lifetime roughly doubles for every 10 deg C below the rated temp.  So 2000hr rated cap at 105deg C operating at 25degC is 80 deg C below rated so the lifetime is 2^8 = 256*2000hr = 512k hr = 58 years.   Times beyond say 15 to 20 years are not valid as there is more than one mode of failure and those modes of failure are likely to kick in after 15 to 20 years.

In a switch-mode power supply, the cap heats internally due it I^2R loss caused by high ripple currents through the cap's ESR and that makes the cap's effective temperature rise and the lifetime decreases.

Film caps start with a much longer base-line lifetime.

You can spend a lot of hours on this stuff - reliability analysis is a profession.

Some of the calcs are captured here,

https://www.mathscinotes.com/2016/02/estimating-the-lifetime-of-an-electrolytic-capacitor/

https://cds.cern.ch/record/2038610/files/arXiv:1607.01540.pdf

You can also see the dependency on voltage is about (Vrated / Vapplied)^ 3.5 but you might find this doesn't apply for all caps (you need to change the 3.5).  Then you might also find it only applies when Vapplied is near Vrated.
[For example this document uses a power of 8, which will make a big difference to the numbers,
https://www.cde.com/resources/technical-papers/filmAPPguide.pdf]

Notice also humidity comes into play.  The film in film caps can oxidize and separate from the leads, that might occur on  dodgy brand caps that are poorly sealed or use the wrong resins, or have pits in the resin - so many factors.

I think this was the video about film caps,

I have seen failed caps with oxidized foil!

You can read-up on component deratings in the MIL standards.   Deratings of 50% to 80% are pretty common, the idea is it greatly extends the life of the product.

[amptramp]

Derating is the percentage of a maximum design rating that a designer is allowed to use and NASA has a simple version here (much simpler than it used to be):

https://extapps.ksc.nasa.gov/Reliability/Documents/Preferred_Practices/1201.pdf

These values come from the analysis contained in MIL-HDBK-217 which you can download here:

http://everyspec.com/MIL-HDBK/MIL-HDBK-0200-0299/MIL-HDBK-217F_14591/

But this analysis is now regarded as approaching obsolescence for these reasons:

https://www.designnews.com/electronics-test/end-near-mil-hdbk-217-and-other-outdated-handbooks

Although a lot of respect is given to NASA derating, NAVSEA derating is more stringent, but I haven't seen it on the web.

[Mark Hammer]

Plenty of older pedals use 16v caps, without problem, because of WHERE they are used in the circuit.

What will the amplitude of an audio signal be?  Not more than 1v in most instances, so a 16v cap in series withthe audio signal should be just fine.

What will the Vref of a pedal run off a single-ended supply be?  Once the resistor divider does its business, any cap from the resistors' junction to ground will be dealing with half the supply voltage.  A 16v cap used to smooth the *entire* incoming 9v supply may end up off-spec.  But that same cap, used to smooth 4.5v will be just fine, thank you very much.

Once upon a time, there was a bigger difference in both price and size between caps at different voltage ratings.  But no longer, so 35v ratings in pedals are quite common these days.  Probably just as well, given how the zeitgeist - not to mention power brick capabilities - encourages people to try out their 9v pedals at higher supply voltages, whether it should be expected to yield audible benefit or not.

[GibsonGM]

Just because I want the flexibility Mark and others mentioned, to not worry very much when running below 30V, I tend to buy 35V units when I buy in bulk.  I've never suffered at all by using them.   YMMV

[anotherjim]

There are mechanical causes of failure...
I've noticed the lead-to-foil connections in box film caps are surprisingly fragile. Do not force them down tight to the board if the holes are not in alignment with the lead-out pitch. The sideways pressure can lever the lead away from the foil. I had a Marshall 900 series tube amp repair with an X-class box cap feeding the power tube bias with this cap failed. A look around the net showed this is not uncommon and the old cap can be difficult to remove and sometimes one lead is detached from the cap - exactly the same thing happened to me. Measuring with the vernier callipers showed the PCB pitch was a little over 1mm shorter than the cap pitch. The replacement cap was too tight also, so I had to bend doglegs in the leads with the cap sitting above the board.

[Rob Strand]

There are mechanical causes of failure...
I've noticed the lead-to-foil connections in box film caps are surprisingly fragile. Do not force them down tight to the board if the holes are not in alignment with the lead-out pitch. The sideways pressure can lever the lead away from the foil. I had a Marshall 900 series tube amp repair with an X-class box cap feeding the power tube bias with this cap failed. A look around the net showed this is not uncommon and the old cap can be difficult to remove and sometimes one lead is detached from the cap - exactly the same thing happened to me. Measuring with the vernier callipers showed the PCB pitch was a little over 1mm shorter than the cap pitch. The replacement cap was too tight also, so I had to bend doglegs in the leads with the cap sitting above the board.

Interesting.

There has to be a point where too much heat during soldering melts/distorts the film where the leads connect to the film.  Maybe not helped if the leads are splayed under stress on the PCB.  For me, heat isn't the only cause of failure either as I have two WIMA caps from ~1994 and one failed.  They have never been soldered.  I may have poked them into the breadboard a couple of times but they didn't get much use at all.  I tried to keep them pristine as examples of "good" polyester caps.

[FSFX]

Rated lifetime. On electrolytics that may be 2,000 hours.

You are mis-quoting that or mis-interpreting what the 2000 hours refers to. It is a test period for endurance testing, not a determination of rated lifetime. Those are two entirely different things.
Panasonic claim the typical lifetime of their aluminium electrolytic capacitors is 15 years under normal operating conditions and environments.
Operation at higher temperatures and in corrosive atmospheres will severely reduce the lifetime.
The 15 year time is based on the typical lifetime of the rubber seals which are the main causes of failure.
There is a Panasonic document (not the one you posted a link to) that has all of the lifetime information.

[Ben N]

10v makes me nervous. 16v? Nah, unless it's, say, a gain pedal that I might someday want to run on 18v. But the truth is that 16v & 25v are usually more or less the same size & cost, so I try to get 25v when I can.

[Rob Strand]

Plenty of older pedals use 16v caps, without problem, because of WHERE they are used in the circuit.

It's definitely a factor. 

When you increase the voltage on a cap the main thing that increases is the leakage.  If you have a 16V circuit which doesn't work with 16V caps the circuit clearly has problems with high leakage.  You might even find different brands of 25V caps also don't work.   It could also be that the circuit hasn't been switched on for a long time and the leakage has increased.

Back in the 70's circuits which used electrolytics and large resistors for timing long periods sometimes had problems.   Even a simple RC ripple filters used to produce DC from a PWM waveform could have measurement errors.   The more reliable circuits used tantalums.   Somewhere around the 1980 mark low leakage electrolytics were more available and that solved a lot of problems.

As for caps across power supplies, as a kid I remember pushing many a cap beyond its voltage rating without problems  .  I think I popped a 10V cap on 18V to 20V once.   All good fun but not recommended when designing products.

[niektb]

I work in the electronics sector where also reliabilty is an important design parameter (semicon) (I have to perform MTBF calculations and whatnot) and there we use some standard (forgot the exact number) which recommends a de-rating of 60%. This means that you have to multiply the rated voltage with 0.6 to get the upper bound of the working voltage. In this example that would be: 16V * 0.6 = 9.6V. Hey look, perfectly acceptable for 9V systems

Derating is the percentage of a maximum design rating that a designer is allowed to use and NASA has a simple version here (much simpler than it used to be):

https://extapps.ksc.nasa.gov/Reliability/Documents/Preferred_Practices/1201.pdf

These values come from the analysis contained in MIL-HDBK-217 which you can download here:

http://everyspec.com/MIL-HDBK/MIL-HDBK-0200-0299/MIL-HDBK-217F_14591/

But this analysis is now regarded as approaching obsolescence for these reasons:

https://www.designnews.com/electronics-test/end-near-mil-hdbk-217-and-other-outdated-handbooks

Although a lot of respect is given to NASA derating, NAVSEA derating is more stringent, but I haven't seen it on the web.

MIL is indeed being regarded as outdated, but reliability estimations in general are still very common. But the main reason for it is that it's a checklist where you can see if components are used near their boundaries (or atleast that's how I use it;))

[Phend]

 ^ Hum, in a few effects I have used a 12v A23 battery.
And have placed a 16v 47 uF electo cap between plus and ground.
Easy fix using 25v replacement.
After reading all this guess I will.

[antonis]

^ Hum, in a few effects I have used a 12v A23 battery.
And have placed a 16v 47 uF electo cap between plus and ground.
Easy fix using 25v replacement.

Batteries are considered the purest DC voltage sources..
(no need for reservoir capacitor either for smoothing purpose or for internal resistance bypass  ..)

[Phend]

 ^ Which brings up a question.
The ole EHX Muff Fuzz which is battery powered (no dc jack) has a 47uF electro cap from plus to ground.
What is this capacitor for ?
Was it an unnecessary component ?

[anotherjim]

I'm not sure if a 9v battery is necessarily a perfect bypass cap. Containing many small cells in series and we know what capacitors in series do to the total capacitance.
Then with opamps, consider what insufficient bypass might do for stability.