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Post by gretschdude86 on Mar 14, 2006 3:56:00 GMT -5
I reviewed the very elaborate shielding procedure to a strat on the Guitarnuts site, and read that a 0.33 microfarad 400V capacitor is needed. I checked radioshack and all they have is a 0.01 microfarad 400V capacitor. Any idea if I should keep searching for the 0.33 or will the 0.01 do fine? I don't do any giggings with my squier strat which will be the patient in this procedure, and everything I use with it is fully grounded. Email me at Gretschdude86@yahoo.com with your response.
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Post by sumgai on Mar 14, 2006 14:24:44 GMT -5
Hey Gang,
What's the usual procedure when a member wants to be answered off-site like this? He asks us for help, then he wants a private answer. I'm tempted to not respond because it goes against the grain of what John A and RandomHero have aimed for all these years of selfless sacrifice, the sharing with one another to improve our chances of success.
OTOH, I do want to warn him that he's risking potentially life-threatining danger by using an incorrect part. Such a quandry. Anyone got any guidance on this?
sumgai
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Post by UnklMickey on Mar 14, 2006 17:50:18 GMT -5
Hey Gang, What's the usual procedure when a member wants to be answered off-site like this? He asks us for help, then he wants a private answer. I'm tempted to not respond because it goes against the grain of what John A and RandomHero have aimed for all these years of selfless sacrifice, the sharing with one another to improve our chances of success. OTOH, I do want to warn him that he's risking potentially life-threatining danger by using an incorrect part. Such a quandry. Anyone got any guidance on this? sumgai Sumgai, " i only knows " (lol) , the appropriate thing to do in this case would be to let him know that his D.C. shock protection (the purpose of this component) would be O.K. the same since the capacitor's voltage rating is the same. and his A.C. shock protection would actually increase, although this component wasn't intended to offer effective A.C. protection. even with a small value, a cap has limited protection for A.C. it would also be appropriate to tell him that he would have less coupling to his shielding and string ground by using such a small cap which would likely cause a decrease in his signal to noise ratio. it would be appropriate to do so in the public part of the forum. since his life would be in no greater danger if he were to use a cap of the value he quoted, there is no need to worry for his safety. since i never e-mail anyone from the forum unless i have previously established a dialogue with them here, i personally would not e-mail him. but that's just my personal e-mail protocol. if anyone wanted to e-mail him with the information, or to let him know the information was posted on the thread, that would not be inappropriate IMHO. unk p.s. it's worth noting that the cap in question is highly effective, in the extremely rare case, when the grid of the first tube of your tube amp shorts to the plate. but there are many other scenarios where it offers little or no protection. nonetheless, if it saves one life, it does have value. |
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Post by sumgai on Mar 14, 2006 18:28:47 GMT -5
unklmickey, By using a large value for that cap, the result is that the initial charge time (1T) is much longer than the proposed cap. It might not be long enough to completely escape a nasty shock, but it would give a better indication of 'something's wrong' to a quick-reflex person. The rise time of a 0.01 cap would be too quick for anybody, they'd get the AC flowing through them before the warning had time to register on their brain. Thinking like that, I'm half tempted to increase the value to 1mfd or more. But then again, I'm not gonna be testing it for 'out of bounds' conditions, so maybe even that value wouldn't work well enough as an early warning system.  sumgai |
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Post by sumgai on Mar 14, 2006 18:32:08 GMT -5
unklmickey, I'm like you, I'd rather share answers here, and not share my private email addy without good cause. Let's see if gretschdude86 reports back here, looking for an answer.  sumgai |
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Post by ChrisK on Mar 14, 2006 23:40:00 GMT -5
Yeah,
It's easy to forget the AC voltage divider inherent in capacitive circuits. There are at least three capacitors in the chain. The middle one is the aforementioned cap, the last one is what's (mis)happening in the amp/mains, and the first (lossy) one is you.
A larger capacitance infers a lower impedance and a slower rise time (charge accumulation time). A smaller capacitance will have a larger voltage developed across it for the same charge. I have mixed feelings about what should be done since both DC and AC faults are possible. If a DC fault is occurring, I want a large cap to slowly accumulate the charge. If an AC fault is occurring, I want as little capacitive coupling as possible between the cause and I.
The standard for U.L. safety is that no single component failure can allow a shock or fire hazard to occur. Single ain't enough for me. I have well over 100K product instances out in the field, and haven't toasted anyone or anything yet.
Since the life that I might save is my own, I use RF. It protects against all component failures, except for air failure (lightning). I also use a Ground Fault Interrupter on any AC circuit. (I don't play out, I get way too many requests when I sing...)
A legal paradigm exits in that if a company doesn't get all applicable/available safety listings for a product (agency evaluated, not just "designed to meet") and something untoward happens, the officers (at a minimum) can serve jail time for negligence. This could well be applied to the owner's of a venue.
RF should always be used, and keep yer lips 'n stuff off of the mic.
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Post by gretschdude86 on Mar 15, 2006 0:27:20 GMT -5
I'm no bad guy, folks. I just wanted to simplify getting information by having it straightly emailed instead of having to go through sites to get to this individual post. Dispite the hot headed post by one member, thank you for your advice.
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Post by sumgai on Mar 15, 2006 3:18:28 GMT -5
gretschdude86, Oh, hey, I wasn't being hotheaded, I was being curious, because as a newbie to the forum, I wanted to do things in the 'forum-approved' way. That's why I asked for guidance. It wasn't personal, believe me. That said, are you still gonna go with the Radio-Shabby 0.01 mfd unit, or can you wait until you find the proper part? sumgai |
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Post by sumgai on Mar 15, 2006 3:52:53 GMT -5
Chris, When the DC has finally charged the cap, the player is still gonna get a jolt. It may take longer to deliver, but that jolt ain't gonna be puny. We can only hope that there is sufficient time for reflexive muscle action to break the contact, because the body is gonna get only one shot at protecting itself. in re an AC episode..... uhhhhh, the logical endpoint of the 'smallest possible capacitance' argument is that you don't connect yourself at all to the circuit that may fail. Obviously, that won't cut the mustard. So, would you settle for a 1 pf cap, in those cases? I'm having trouble getting past the rise-time issue, and the fact that AC repeats periodically. I have this gut feeling that if you impede the rise time sufficiently, you effectively create an early warning system, and if the body is not sleeping at the switch, it should understand that something is drastically wrong well before a lethal dose of current has been delivered. (Actually, no one is that quick, it's the nature of the beast. But muscle contractions have been studied under adverse conditions, and there is a voltage level threshold that gives a body some measure of reflexive protection.) And it is current that is the bad guy here, not just voltage. I have many horror stories that I could relate, but this ain't the time or place for 'em. The thing is, when we create a voltage divider, then we've also just created a condition controlled by Kirchhoff's 1st law. So, that being the case, then we really don't gain anything by decreasing the capacitance of the 'protective' cap, do we? The current is the same at all points of a series circuit. QED. And it only takes about 1/20th of an amp (at the usual 120 volts) to start damaging the heart muscle. Much more than 1/10th of a amp, and we're in some serious trouble, muy pronto!  What can you offer to make me feel better here?  But can we make an RF system cost effective? And since I don't sing, I'm probably at low-risk for all this anyway. ;D Boys and Girls, this is why the first thing you learn in Basic Electricity class is to always keep one hand in your pocket! No, not the theory we're addressing, the fact that only you can prevent shocks to your own body. Learn it, heed it, breath, sleep, eat, dream and crap it, but for Gawd's sake, don't get caught flying over the work bench because you took your stupid hand out of your pocket while poking around in a live chassis. That's why we have pockets, for our safety! (no smilie, I'm being serious here.) sumgai |
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Post by stratomaster on Mar 15, 2006 10:27:25 GMT -5
a 0.33 microfarad 400V capacitor is needed. I checked radioshack and all they have is a 0.01 microfarad 400V capacitor. Radio Shack has a 1uf 250V capacitor. I used 3 (wired in series) to make a 0.333uf 750V capacitor. Is that an OK substitute? Thanks. |
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Post by UnklMickey on Mar 15, 2006 17:35:50 GMT -5
Stratomaster,
Welcome to GuitarNutZ.
sorry it took so long for you to get an answer to your question.
short answer:
yes, that will work. assuming they are the recommended type.
long answer:
i don't generally like to put caps in series, as often one of them will "see" the majority of the D.C. voltage across it, do the slight leakage that another one might have. in this case you have 3 in series, any 2 of which combined have more than the recommended voltage. so it's less of an issue in this app.
make sure the connections between caps are covered.
doing the 3 caps might be a bit bulky, but electrically it's okay.
i'll get to your other question tomorrow, if no one else helps you tonight.
unk
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Post by ChrisK on Mar 15, 2006 20:45:45 GMT -5
Walllll,
One of the tricks for using caps in series is to add parallel load balancing resistors (1 to 10 Meg Ohms), but......don't in this safety application.
Generally speaking, if both caps have a very high (20+Meg Ohm) resistance (low, low leakage), you'll be fairly OK. If one doesn't, you may have flash-over in the other cap. If both don't, "you die".
No one was being hot-headed. Posting and answering thru the site makes the thread query AND RESPONSES searchable by all that follow.
The problem w/ DC electrocution is that reflexive muscle control IS OVERRIDDEN (ya ain't got none). I watched a guy start to fry in the Army on 600 VDC. When we finally found an insulator (a baseball bat) to whack him with, it was almost too late (he did recover, mostly).
I bot an RF system for geetar for $50 at GC a ways back. It wern't the best by far, but it was much better than MY ears could tell.
Indeed per your QED! However, the smaller the cap, the larger the impedance, and the greater the voltage drop external to my body (if'n I'm a grounded dude). This impedance helps limit the current.
A divider of 1 pF and 10 pF will develop the same AC voltage division as 100 pF and 1,000 pF, but the former's impedance will be 100 times higher.
If I stick a 1 pF cap into an outlet, while I may be daft, I'll probably be ok. If I stick a 1 F cap in, I'll make the 5 o-clock show fer sure.
"I keep both hands in my pockets and have the tech's connect the probes."
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Post by sumgai on Mar 15, 2006 23:20:33 GMT -5
Chris,
I gotta keep this short and sweet, I've got a gig in a few moments, but I can't seem to stay away from this joint!
And in that same US Army, I saw a feller flip backwards over two workbenches, on a 1000 VDC transmitter final (a pair of 4x150D's, if I'm recalling that episode correctly). He lived, but he busted his arm, broke two ribs on the third benchtop as he hit it and fell to the floor, and had a fist-sized burn hole on the underside of his wrist. His fingers? About half of the right index and middle finger were still in the unit, causing a very noxious smell. He was got busted back to E3 for four months, for failing to observe normal safety precautions.
Seems logical to me, maybe I should breadboard this up and try it for myself.
Of course, you and I know it, but one of us better mention for the rest of the readers that all of these factoids are frequency dependent. At the standard power line frequency of 60Hz (in the USA), there's no practical difference between a 100pf cap or a 1 nf (1000pf) unit; the results of everything we've discussed won't be changed a lick.
That ain't working, that's the way you do it,
Let your techs do the work, get your readings for free!!
(Sorry, couldn't help it.)
Oh, look at the time, I've got to run. Ciao!
sumgai
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