[Squalish]

Quote:

Originally Posted by colonelkernel8 /img/forum/go_quote.gif This is a myth. Also, about caps, what's particularly dangerous is how quickly they discharge their energy as opposed to a battery or the like.

How so?

I'm sure I'm not the only one who's gotten a jolt of mains electricity in one hand (from one finger to another) and come away from it with little more than a bit of pain & a spasm. A finger to finger or wrist to finger circuit exposes a lot less current to the heart than hand to hand.

Caps discharge their energy as quickly as you let them - increase the resistance 10x, you've reduced the current 10x and prolonged the discharge by a factor of 10. To induce heart failure you need ~30ma for several milliseconds.

 

[AndrewFischer]

Quote:

Originally Posted by Squalish /img/forum/go_quote.gif How so? .

The internal resistance of a capacitor can be very low. A low impedance electrolytic could be .08ohms or even .03ohms.

I=E/R

I = 100V/.08ohms = 1250A



A short circuited alkaline AA battery will put out 10A maximum.

http://www.eveready.com/pdfs/ever_alkcylin_appman.pdf

http://www.panasonic.com/industrial/...eeu_fc_dne.pdf

 

[Uncle Erik]

High voltage doesn't seem all that dangerous any longer, especially after I got rid of my plate-throwing ex-fianceé.

The difference between a high voltage power supply and Crazy Rita is that the power supply's risks are predictable. To a degree, of course.

There have been a lot of great suggestions so far, but my approach is to look at the risks and work backwards from there. Electrocution is a biggie. So, working backwards, make sure you have the thing unplugged and discharged as much as possible. When it's hot, take care to use one hand, have someone else around, barricade the cat in the other room, and cover any and every risk you can foresee.

Fire is a possibility, too. Be sure to use correctly rated bits, keep a fire extinguisher handy, use good construction techniques, and so on.

You have to look at everything that can go wrong and then plan backwards to make sure those things don't happen or that you can handle them if they do.

 

[Logos]

This seems like really sound advice. Never stopped to think about it that way - to work backwards. It starts to sound a lot like a science experiment in a lab.

 

[ke4mcl]

electrical safety is something that unfortunately is learned not only by reading but also by experience, often times bad experiences. you'll undoubtedly have a few shocking experiences in your lifetime if your gonna be an electronic hobbyist.

i work on plenty of tube gear and even work on hot chassis antique radios and im not an oldtimer. here's the advice i can give you...

1) if its physically plugged in, consider it live. power switches can fail.
2) always wear rubber soled shoes
3) if you need to work on it while it's live, one hand in pocket or behind back always.
4) if it was just plugged in a second ago, treat the capacitors as if they are charged. they can retain a charge for long periods of time
5) caps can explode violently. try not to have them pointing at you when powering up uncovered gear. they can vent out the bottom also and spray their guts.
6) when powering questionable gear an extension cord is your friend. plug gear into end of cord faraway from you then go and plug other end of cord into wall.

 

[regal]

I learned by working with electrcians who work on mamoth capacitors and voltages everyday. After a few years you develop an understanding. The main thing is just don't ever touch anything until you've tested it with a meter, EVEN if its been unplugged for days. Its easier said than done. But once its engrained it becomes a habit like anything else. Thats why we always say start with LV stuff like the Millett Hybrid, SOHA, etc. Hopefully you won't ever touch anything on the LV amp before testing it but if you do it is less likely to kill you If you make a mistake and get shocked, Take a step back and ask yourself why you were so stupid, if this had been a real tube amp I'd be dead. The other thing is odds, electricity is a strange thing in that some people like my grandfather survived being struck by lightning, while a lady I read in the newspaper died after being shocked by a broken light bulb. Its russian roulette.

Think of your DMM as something you trust your life with, that's why I always say before you even think about a HV tube amp you must invest in a FLUKE. I have seen the $20 DMM's melt from high voltages, they are dangerous IMO.

 

[Logos]

Keep the great responses coming, guys.

Regal: Do you mean to imply that, even if unplugged for days, you would still recommend testing with a DMM before touching? This can be really hard to do with one hand behind your back. Some people have recommended alligator clips to use with only one hand, is there any you would recommend?

 

[mono]

While I am also insisting you check voltages before handling circuits, there is also the option to build bleeder resistors into your design, or just add them across the caps on existing designs. Don't assume they did their job, but be glad you spent that 4 cents on resistors... you never know whose hands a homemade project might end up in someday.

 

[xxbaker]

Quote:

Originally Posted by AndrewFischer /img/forum/go_quote.gif The internal resistance of a capacitor can be very low. A low impedance electrolytic could be .08ohms or even .03ohms. I=E/R I = 100V/.08ohms = 1250A A short circuited alkaline AA battery will put out 10A maximum. http://www.eveready.com/pdfs/ever_alkcylin_appman.pdf Electronic Components - Panasonic Industrial Company

That is true, but you're forgetting the other part of the circuit which is your body. While the resistance of the capacitor might be .08 ohms, that doesn't mean if you touch the two terminals you're going to get over 1K amps through your body. Factor in the body's resistance then recalculate. Most people's resistance is going to be at least 1000 ohms. This means your equation is now I <= 100V/1000 >= .1A which is still enough to stop your heart, but it's nothing like 1250A running through your heart.

 

[regal]

Quote:

Originally Posted by Logos /img/forum/go_quote.gif Keep the great responses coming, guys. Regal: Do you mean to imply that, even if unplugged for days, you would still recommend testing with a DMM before touching? This can be really hard to do with one hand behind your back. Some people have recommended alligator clips to use with only one hand, is there any you would recommend?

Yes capacitors can hold a charge for days. Use an alligator clip on the ground and use your one hand to probe with the positive lead to veryify that there is no voltage present. Then you can solder/desolder.

If I were building my first Bijou, I would just make absolutely certain that every component is right and in the right orientation as I solder them in. If you do that you don't even need to probe because the thing will work!. If it doesn't take a step back, learn how to safely discharge caps, learn to be very careful with the Fluke and you will be fine. I have shaky, shaky hands and I manage not to be shocked. It just takes practice, thats why we recommend the low voltage hybrids to start. BTW HV Hybrids are very difficult to build but sound out of this world. I don't want to discourage you. The currents we are using on headamps are fairly low, current kills not voltage. You can do it just practice practice and if your mind is sluggish (you know those type of days) stay away from the thing. The rewards are really great and you will find that the hardest part is actually the grunt work (case work.)

Edit: Thinking back the Bijou was actually the easiest amp I ever built, it is so simple if you get the layout right, I had to do no troublshooting at all, just don't fry the mofset. HV hybrids are another matter there are so many opportunities for frying components when you mix HV and LV devices.

 

[SiBurning]

What are the dynamic characteristics of voltage and current through various parts of the human body? What is the effect of skin moisture? Does your body's internal resistance change if you just ate something salty? Do the electro-dynamics change? Does the resistance remain the same when an electric current passes through the body or the skin membrane?

How does your body react to 400VDC vs 400vac? Will your reflexes inadvertently cause your hand to bash against something else in the circuit, perhaps even getting it stuck somewhere you hadn't expected? In high voltage, especially DC, will you ever be able to move at all, or will your body lock up hard and fast preventing you from moving at all?

What effect does frequency have on an electric field applied to a human body? Do the dynamics matter? Do you even need to complete a traditional electric circuit?

I wouldn't put too much faith in Ohm's law. It's a purely static analysis that ignores a whole lot of things. 12VDC may not hurt or impair you, but it can kill under a variety of bizarre circumstances, getting a small, unexpected shock being one of those.

 

[randytsuch]

Quote:

Originally Posted by mono /img/forum/go_quote.gif While I am also insisting you check voltages before handling circuits, there is also the option to build bleeder resistors into your design, or just add them across the caps on existing designs. Don't assume they did their job, but be glad you spent that 4 cents on resistors... you never know whose hands a homemade project might end up in someday.

I always install bleeder resistors in my tube projects, if they don't already have them. I normally use a high value, so you can use a relatively low wattage resistor. Means you might have to wait a little while for the voltage to bleed off, but it also does not draw very much power, and does not require a large 10 watt resistor.
Even with a bleeder installed, I ALWAYS check the voltage across the main filter caps, and make sure it's low or 0 before touching a tube circuit. If you don't have tubes installed, I have seen caps stay charged for days, at a high voltage.

Randy

 

[vhbaske]

Another danger you should be aware of in Electronics is that some rechargeable batteries explode and can burn beds or wood equipments. Another thing you should take care are the Lithium Ion batteries... I recommend to all who work with electrical equipment to learn to use a multimeter. I always test transformers with alligator clips and before plugging to the outlet, put the meter in a safe voltage reading. 200 VAC or more should be enough for the primary, but sometimes tube transformers could yield 600 volts or more, depending on the design of the amplifier or device.

 

[macm75]

Sorry if it was already mentioned but be sure your tools are also rated to perform the task at hand. Check ratings of probes, meters, etc.
For extra precaution you can also purchase high voltage rated gloves for those instances you need to get into powered equipment.
I used to get my hands "in there" and now I use clip leads for all measurements.
Power off -> clip leads -> turn on/measure -> power off -> etc. This method plus bleeders are good habits.
Lastly, for first time power up (my most hesitant moment of any amp build) I keep my head under the table.

 

[regal]

I also recommend safety glasses, you don't want to be sprayed with capacitor electrolyte.