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Lethal Voltages

I love reading this thread. Makes me remember when I was starting off in the field and was young (yes and foolish). You don't need to hear my war stories but, I did have an electronics instructor who had many sayings to remember things but, this one resonated through the years.

Voltage doesn't kill, voltage causes current.
Current kills!
:yikes:
 
Sorry about your mishap Mark and glad you're OK. It's probably good that these threads come up now and then, as long as they aren't at the expense of serious injury.

I can never get too many reminders about safety or tips on how to avoid accidents.

Timely thread too. We're about to start on Glen's little Bogen that came from you. :thmbsp:

John
 
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I think transformers can hold a jolt for a while also!

What do you guys or gals think about those penlight sized voltage detectors?

When I bought mine about 5-6 yrs ago they didn't have the range available so mine probably doesn't detect below 50V . You don't have to directly contact the high voltage and static electricity will set it off also, and just holding it close to a 110-120V outlet will set it off.

http://www.greenlee.com/catalog/Electric-Test-Instruments/VOLTAGE-CONTINUITY/Non-Contact-Detectors

Randy
 
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The only transformer that actually stores a charge is a flyback transformer. Power and output transformers however work on the principal of magnetic flux...and are unable to hold a charge.

Once you remove the power from a transformer; its essentially dead. Its the capictators in a power supply that hold a charge.

Sent from my SPH-M910 using Tapatalk 2
 
Hiya,

You can also fuse that cap discharging probe to prevent that melting plastic smell if you leave it attached like some of us (Me) do.

Frannie
 
Hiya,

The only transformer that actually stores a charge is a flyback transformer. Power and output transformers however work on the principal of magnetic flux...and are unable to hold a charge.
Click to expand...

I was under the impression and always have that its the CRT itself that holds the charge. Once the anode cap is removed from the CRT I have seen CRT's regain some charge back in the day. (I actually dropped one getting jolted back in the day) Maybe its some capacitance around the flyback circuitry you are seeing ???

Frannie
 
*Chuckle* Ah yes, there's nothing like being hit with 25-30 grand from a CRT, other than a microwave.
My experience was due to getting in a hurry and not discharging the tube. I just
thought "nah I'll just grab the rubber part and push it over without touching the anode" Slip -oops -zap.

Microwave ovens are much worse, less voltage but much more current.
 
Here's something I found on flybacks:

Unlike mains transformers and audio transformers, a LOPT is designed not just to transfer energy, but also to store it for a significant fraction of the switching period. This is achieved by winding the coils on a ferrite core with an air gap. The air gap increases the reluctance of the magnetic circuit and therefore its ability to store energy. The current does not flow simultaneously in primary and secondary.
Click to expand...

Its not storing a lot of juice; but it does in fact hold *some*. This is in contrast to a standard transformer which only has a flow on secondaries when current flows through the primary. In turn...the flyback itself probably doesn't hold a charge long enough to worry about; not including the caps.

The CRTs themselves, like I said; they have the ability to hold some charge. I've discharged them and had them still have slight charge days later.

But, largely; its the big caps around the flyback that hold the charge which can inturn go from 170 some volts to close to full bump through the flyback.

Sent from my SPH-M910 using Tapatalk 2
 
This is one of those "DUH" moments. Why I never thought of this before is beyond me.
I am going to build a half dozen jumper wires(alligator clips on each end) with a resistor permanently mounted in the middle. When ever I work on an amp now, the first thing I will do is jumper the caps to ground. I will leave these jumpers on until my work is finished. I think this will eliminate all possibility of another bite. Such a simple and yet extremely safe thing to do. Finally, I am going to put a ground buss bar on my bench - permanantly attached to earth ground. I will always hook the chassis to this before I begin any work.

DUH!!!!


Thanks for the feedback thus far!!!
 
I always use a DMM to measure the caps. If they have charge; I discharge them. Once they're discharged they're perfectly safe. I also dislike having jumpers in the way, especially since I'm usually replacing the caps anyway.

I worked on a HV plasma display in a pinball yesterday. +100 and -100VDC output...low amperage; still hurts. I simply just checked the caps before I started touching it. It has bleeder resistors so it wasn't a problem.

If anything; I get shocked by the 50vdc power supply on pinball drivers constantly...simply because the 15000mfd caps don't discharge and in the field; you don't have the luxury of a slow discharge.

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CRT's are basically a big capacitor if you look at how they are constructed. Two conductors seperated by an insulator. Generally the outside of the CRT is coated in some sort of conductive material to drain static, and the inside is of course hooked to the anode supply with the glass of the bottle between. If you're touching the outside of the CRT and then get across that anode connecton, WHAP.
 
Jcricket said:
I was trouble shooting my amp. I had the power off and unplugged. I had run one channel without tubes. I went in with the soldering iron to remove the transofrmer leads and WHAM, right through the soldering iron and through my had to the chassis. THat was an hour ago and my hand is still a bit numb.
I thought I had killed the charge on the b+.
I think I am going to name this amp Biter.
Click to expand...

DC voltage stored in a cap can be fatal. The ones in our amps are pretty small but, can cause big problems if the voltage path passes through your heart while seeking a grounding point. Your shock was minor and you see what the after affects are.

Years ago when Trane was developing the first all variable speed air conditioning system the outdoor unit contained three beer can sized DC capacitors. If you became the ground for one of these caps it resulted in death or serious injury. Trane abandoned the project and we are just now seeing the return of variable speed units thanks to modern VFD drives.

Use a bleeding resistor.
 
Have had a healthy respect for electricity ever since:

I took the power cord off of the slot car transformer (the one with AC wires wrapped onto screw downs), and then with the cord in the wall, touched the two ends together to see what they would do - when I woke up, the damn cord had melted into the carpet, the plug blew out of the socket, and the wall socket was pretty much melted slag. Lucky the house didn't catch on fire.

And in electricity class in high school, some bozo got the idea to charge some 75V big caps (probably 10K UF) and then try to short them out with a screwdriver - welded those stupid things along with a big boom. Teacher was laughing his head off while shooing them out of the lab.

Oh and when some idiot in lab stuck some copper wire into one side of the socket and laid it over the open side, and some kid unknowingly stuck his soldering iron plug into it. KAPOW! and another socket blown apart - luckily the kid jerked away so fast he didn't suffer any injury except falling off his chair.
 
Arc flash sucks. If you want to find some scary stuff, search youtube for arc flash explosions. It ain't no joke.
 
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