Electrical Engineers: Capacitor AC resistance Q

[liko]

I found an article explaining why resistance of a capacitor decreases as frequency increases and also as capacitance increases. It's interesting stuff, and "why" is a little counter-intuitive. In short, the equation for AC resistance at a given frequency and capacitance is:

 

X = 1/(2(pi)fC), where C is capacitance in farads and f is frequency in Hz. X is the resistance in ohms.

 

Setup for the question: My guitar's control cavity is shielded and all parts that are grounded for noise purposes are grounded seperately from those for "signal ground" from the tone, volume and pickups. I have a buffer cap for the shield grounds that, in the event of a catastrophic failure of the amp or a different-polarities wiring problem, should prevent a lethal shock through the strings. It's .22uF, rated for 630V. That, according to the equation, presents ~12kOhms resistance to 60Hz current, meaning I'd only get a 10mA shock. Certainly enough to feel at 120V, but not enough to injure or kill. The GuitarNuts website recommends a .33uf cap, which would give about 8k resistance, and result in about 15mA from 120V. Obviously as capacitance goes up, the AC protection increases, but DC protection (which relies on the cap charging and not the inherent resistance) goes down. For DC-only, a 1uf cap was recommended.

 

Now the question: Am I defeating the purpose of shielding my guitar with the buffer cap? the DC impedance of each of my pickups is only 6kOhms, and divide that in half for positions 2 and 4. That means that 60-cycle hum, which is far less voltage but the same frequency, would theoretically have an easier path to ground through the signal circuit rather than through the shield, once it made it to the pickups. The X-factor is whether the noise looks at the shield vs the pickups in that way, and "changes course" to take that path of least resistance? Or does EMI radiation not work that way?

[Web Surf]

Does anyone know of a guitarist that died from Electric shock ?

 

or even one that got shocked ?

 

A quick google brought up this ---

 

http://www.straightdope.com/mailbag/mguitarelectrocute.html

 

http://www.barnettimes.co.uk/news/localnews/display.var.842271.0.teacher_killed_by_electric_shock.php

 

Keith Relf of the Yardbirds : http://arts.enotes.com/contemporary-musicians/yardbirds-biography

 

 

 

 

ie about 5 occurances in 50 years ? and last occurances around 197x except the recent teacher story.

 

 

I suppose that with modern 3 prong electrical connectors etc, the chances for a shock are extremely rare

 

and Wireless makes shock even less probable.

 

 

 

 

If you touch 120Vac via a capacitor, the total circuit will be mains -> cap -> body resistance-> ground. Hence actual current through the body will be very very little ( maybe 1/1000ths of a Milli amp)

 

10mA through the body should be lethal as the heart and brain rely on picoamp signals to communicate. Shock buggers up the communications rather than fry up the organs.

[liko]

http://acapella.harmony-central.com/forums/showthread.php?t=1591764

 

Maybe not lethal, but getting shocked between the PA and your guitar is not unheard of even among HCers.

 

Anyway, no, I'm not really worried about dying from my equipment. I put the cap in there because it was in the GuitarNuts instructions, but now that I know a little more about what it might be doing as far as defeating the shielding, I was wondering if anyone knew if the extra resistance to 60Hz hum actually introduced more into the signal, since that's now an easier path to ground than the shield. Trying to get this beast quiet is a bigger priority to me ATM than making sure my amp doesn't kill me. If radiated EMI doesn't care as much as static or current electricity does about its path to ground, there's no problem and I'd like to have the extra insurance at a gig. If however the extra resistance is actually causing more noise, I want to replace the cap with a wire lead.

[ThomasD]

 

Now the question: Am I defeating the purpose of shielding my guitar with the buffer cap? the DC impedance of each of my pickups is only 6kOhms, and divide that in half for positions 2 and 4. That means that 60-cycle hum, which is far less voltage but the same frequency, would theoretically have an easier path to ground through the signal circuit rather than through the shield, once it made it to the pickups. The X-factor is whether the noise looks at the shield vs the pickups in that way, and "changes course" to take that path of least resistance? Or does EMI radiation not work that way?

 

 

The key point the part where you say 'once it made it to the pickups.'

 

Ideally the shielding should prevent most noise from ever reaching the pickups in the first place. What does make it to the pickups is already in the signal path anyway. That's why shielding can never fully eliminate noise. Well, unless you are willing to play enclosed in a Faraday cage...

 

Also, unless you have metal knob, or a metal pickguard the safety cap shoule be located between shield ground and the strings only. Since the strings are usually the sole electrical conduit to your body it is unecessary to send the rest of the shield noise through this capacitor.

[liko]

 

unless you have metal knob, or a metal pickguard the safety cap shoule be located between shield ground and the strings only. Since the strings are usually the sole electrical conduit to your body it is unecessary to send the rest of the shield noise through this capacitor.

 

 

I was thinking about that. I have plastic knobs, and the only other piece of metal I'd be touching that isn't grounded through the bridge (the jack plate) can't be isolated that way anyway, so at least I could move the cap to the trem ground. That would make the shield a practically zero-resistance path.

[petejt]

Keith Relf of the Yardbirds : http://arts.enotes.com/contemporary-musicians/yardbirds-biography

 

:cry:

[Mirando]

I f you want to prevent yourself from electric shock, you are much much much better off using a fast fuse!

The signal coming out of your guitar is the difference between the two cables. So yes you are messing with the signal. The low e-string is 82 Hz so if you are filtering at 60 hz you will be influenced.

Then you would only get electrocuted if you plugged your guitar into the output of the amp and it would have to be a high voltage amp because at twelve volt you will never be electrocuted (just hold the two terminals of a battery). In 1972 it was probably the 300 V part of a valve amp.

Speaking of chance, you're much more likely to be hit by a lightning strike....

[WRGKMC]

The proper place to put the cap is between the wire tant makes contact with the strings and the control plate. Every guitar gas w bare wire going to the bridge or tailpiece to allow the strings to be grounded then you touch them. the idea is to prevent heavy current to pass from from your strings to ground. The cap shouldnt be placed anywhere else or it will affect the sound. Running it off the jack to the ground bus will make the guitar sound thin and will introduce more hum.