capacitor question

[trescat]

Would a capacitor rated at 200 V AC operate alright with a load of 240 V AC going through it ??, just wondering as I can get a hold of a 400VDC rated cap that has a 200 VAC rating on it and was wondering if it would be suitable for the quiteing the beast shielding project to eliminate electric shock potential, taking into account that there could be potentially 240 VAC going through it under Australian power conditions. Any advice is appreciated. Thanks

[erikh]

For about a nanosecond, sure. Then it will pop. For the QTB mod, the amount of voltage in the guitar signal is so small that the cap will be fine unless something goes wrong and 240V AC comes down the line, then that cap will do nothing but pop as soon as it is hit. If possible, get one rated higher than your supply voltage.

[trescat]

What happens when they pop ? do they go open circuit or closed circuit ?? Thanks

[erikh]

I believe it's open circuit, plus they smell real bad. As rule of thumb, use a cap that is higher than your amps supply voltage (240V). This will give you max protection.

[ChrisK]

The issue w/ an under-rated cap in failure isn't that they will "pop", but arc over internally and beome essentially a short, negating the capacitive (de)coupling/protection inherent in QTB. Some types of film caps are "self-healing", but I wouldn't bet anything (such as my life) on it.

The issue w/ most caps is that they are rated in Volts DC. For U.S. mains operation, a capacitor with a rating of at least 250 (or 400) VDC should be used since the peak voltage can come very close to 200 VDC (10% high line conditions X 1.414 = 186 Vpeak). This doesn't allow much, if any, headroom for line transients.

For 240 VAC mains operation, industrial practice is to use 600 VDC rated caps for the same reasons (10% high line conditions X 1.414 = 373 Vpeak). Again, headroom....

This may seem too conservative, but as a 30 year practicing electrical engineer w/ well over 100,000 products out in the field, safety is safety.

I've noticed that there isn't much of a market for extra-thin parachutes either.

It's only YOUR LIFE (and it ain't low voltage)......

[lbw]

Hi,

How does this circuit actually work? If say 240VAC is seen on the ground side, then the capacitor will charge up which will halt the flow of electricity to the grounded parts (e.g. bridge, pots, etc) temporarily for t=5RC.

What happens next when it wants to discharge? How does it actually protect the player?

Thanks.

[erikh]

Read this article (especially the last paragraph):


The purpose of the cap is to reduce the shock to below lethal levels, but as it says, DON'T stake your life on it. It's there to absorb as much of the current as it can, if it ever sees any that large, before it gets to you.

[lbw]

Hi,

I have read that before but it still doesn't explain exactly how it works. Once the capacitor gets charged, it's going to go (ideal) open-circuit for DC but for AC, all that current has gotta go somewhere!

Thanks.

[JohnH]

If you take the impedance of a cap as 1/(2Pi.f.C), then the impedance of a 0.33 MFD cap at 50 Hz is 9600 Ohms. So, the worst jolt youd get is similar to as if you were connected to the mains by a 9.6k resistor. Not pleasant but better than a direct connection!

In normal operation however, the cap is low enough impedance to ground most of the noise, which is a very weak signal. It is particuarly good at high frequency RF static/buzz, for which the cap has very low impedance

John

[jsip]

does anybody know where i can find this capacitor? i've been looking everywhere without any luck.
thanks.

[lbw]

johnh said:

If you take the impedance of a cap as 1/(2Pi.f.C), then the impedance of a 0.33 MFD cap at 50 Hz is 9600 Ohms. So, the worst jolt youd get is similar to as if you were connected to the mains by a 9.6k resistor. Not pleasant but better than a direct connection!

In normal operation however, the cap is low enough impedance to ground most of the noise, which is a very weak signal. It is particuarly good at high frequency RF static/buzz, for which the cap has very low impedance

John

Ahh, okay, so you're looking at it as Z = 1/jwC.

So Z = 1/[(0.33x10^-6)*100pi] = 9.6x10^3 ohms

At 240VAC (339V RMS) that will supply 35mA which isn't very pleasant but won't kill me. It will probably be less as my body will load it as well, but let's ignore that.

I can see that as the frequency increases, the impedance of the capacitor will decrease. Sort of like a high pass without the R (though there will be some R based on the resistance of the conductors and capacitor). This will then 'encourage' more current flow down this branch at higher frequencies. There will also be a -90 degrees phase shift, but that's not relevant.

How exactly does it protect at DC as isn't the circuit trying to protect from 'ground' being at a higher potential anyway?

Thanks very much for answering my question! Greatly appreciated.

[JohnH]

If your amp goes weird and feeds high voltage dc or ac into the guitar chord, it will arrive at the jack and then to the grounded parts inside the guitar. But to get to you, touching the strings and the bridge, it has to go through the capacitor. If it's AC, it's as discusssed above. If it's DC, the cap will fill up (giving a brief temporary jolt) and then let no more through, since it has very high resistance to DC.

John

[JohnH]

jsip said:

does anybody know where i can find this capacitor? i've been looking everywhere without any luck.
thanks.

Here is a question, only for real paid up electrical engineers to answer (ie, its a matter of safety, so no guesses please). If we are searching for a capacitor for protection, and cannot find one with a high enough voltage rating, is it safe to use two in series, on the basis that they will divide any applied voltage between them? - the final capacitance would presumably be halved by this.

[lbw]

Aside from the filtering, is there any reason why it wouldn't be better to use a small fuse (say >10mA) than the capacitor for protection?

[lbw]

johnh said:

jsip said:

does anybody know where i can find this capacitor? i've been looking everywhere without any luck.
thanks.

Here is a question, only for real paid up electrical engineers to answer (ie, its a matter of safety, so no guesses please). If we are searching for a capacitor for protection, and cannot find one with a high enough voltage rating, is it safe to use two in series, on the basis that they will divide any applied voltage between them? - the final capacitance would presumably be halved by this.

I'll close my eyes, put it in a black box, call it a two port network and say yes.

Jokes aside, assuming the transient response of the input is below the voltage tolerance of the combined capacitors, I don't see why (unless there's something non-ideal that I don't know about) this wouldn't work? It's fundementally just a voltage divider right? Though, I guess it depends what the instantaneous input is. If it's an impulse of 600V and you have two 300V capacitors, I don't think it would be pretty..

Am I missing some trick question?

[JohnH]

Lbw

Thanks - it was not a trick question, and I think caps in series for a higher volatge rating would work OK too, but thought some other peoples opinions would be good to get.

As for using a fuse for protection - I would be concerned that even if a fuses rating current is exceeded, it takes some time to blow, during which time youd get the full zap.

Of course, most of us dont have any of this protection!

John

[trescat]

You can find these capacitors at Solen -- www.solen.ca/
look for Solen Fast Capacitors -- PPE 630vdc: 0.1µF to 47µF