Question about Amperage

[R.G.]

Does having a fuse help at all with the actual safety of it? Wouldn't it blow before allowing a HUGE current to come across?

No. Fuses are not there to keep you from being electrocuted. They're there to prevent fires, and to force opening up the mains current for a subclass of the major faults that are possible.

Huge currents are not necessary. Currents through the human chest of between 10 and 20 ma at 50-60Hz can cause ventricular fibrillation - and that kills you unless you get shocked by the now-familiar chest-paddles setup, and quickly. Guitarists have died because they were connected to the "ground" in an amp that leaked much less AC than was needed to blow a fuse.

Just to be clear - I'm not running you or your skill level down, but your questions indicate no training or experience with AC mains power at all. I really think you ought to put off anything to do with AC wiring until you get the training and experience. I hate to see someone kill themselves because of things they don't know can bite them.

I would make the analogy of handling live rattlesnakes - in theory, it's easy; just hold them behind the head and don't let them bite you. No problem, except for some critical steps like how do you grab them behind the head without getting bitten, and how do you ever let them go? And what if they're big and heavy (and many of them are) and very, very strong (they all are) and very fast - how exactly do you deal with that? There are techniques that can handle the situation, but it's not something to learn over the internet. You need a live person with experience training you. Or a whole lot of luck.

Some comments:
1. How will you find someone with AC mains wiring experience in the specialty of wiring the insides of electronic equipment?
Electricians don't know that kind of thing - they know AC mains wiring to switches and such only. Probably your best bet is a ham radio operator over 50, for a number of reasons.

2. Electronic designers don't know this, in general. Most electronic designers, and perhaps the sub-set of EEs, have not much clue about this in their training, unless they happen to have had to design power supplies for formal safety certification. Pretty much the reason I know this stuff is that I did design power supplies for a living for several years - and had safety engineers check my work and tell me how very inept and deficient I was at it when I made mistakes.

3. Guys who build amps may or may not have a clue. They at least know not to touch the hot wires, but that may not translate into anything that would be recognized by a safety lab. I have seen immaculately hand-wired amps at guitar shows with several wiring safety defects in them that I could see through the open back panel.

4. Professional safety certification engineers and techs will NOT tell you how to make it safe; they will tell you a compendium of what is not safe, and even when the designs pass, passing does not mean "this is safe", it means "I can't find anything that is clearly unsafe, so I have to sign off on it."

[Tony Forestiere]

Does having a fuse help at all with the actual safety of it? Wouldn't it blow before allowing a HUGE current to come across?

Simple answer: Yes.

In all honesty, if you had to ask this question, I think you need to step away from the buffet. I got hit by 220 VAC working on my Mom's dryer. Luckily it threw me instead of grabbing me.

*edit* RG scooped me in a more erudite manner. Thanks, RG.

[acehobojoe]

As always thank you for the great advice RG, I owe you big time for all of your tips.
I'll find someone well qualified to teach me so that I can understand it well.

I stuck my finger in a lightbulb socket when I was young. That was enough.

I definitely want anything I build to be very sturdy and safe, and it's worth learning about to get it right.

Thanks!

[Jdansti]

Here's how a lot of people build power supplies safely. Instead of starting off with a raw transformer, use a manufactured power supply that puts out a higher voltage than the voltage you're shooting for and then build your power supply to regulate the voltage where you want it. The commercial supply can be regulated or unregulated. Unregulated supplies are cheaper.

For example, let's say you want a power supply that puts out 9V regulated and has a capacity of 500mA. Find a cheap supply that puts out 12-15 AC or DC with a capacity of 800-1000 mA. Build your supply with one of the standard DC jacks that we use on stompboxes and plug the output of the higher supply into it. If your store-bought supply puts out AC, just put a rectifier on the front end of your regulated circuit.

Doing it this way leaves all of the issues of dealing with mains power to the manufacturer. You only have to deal with the low voltage end. This is no different than what we do every time we build a stompbox and plug a power supply into it.

Something like this:



If you use this approach, you'll get a lot of help from the experts to help you complete your project.

[GibsonGM]

There's an old saying, Ace -

"It's volts that jolts, but it's mils that kills!"

As indicated, somewhere around 10-15mA at 30V can LITERALLY KILL YOU.  Wild, huh?  That is why about 30V starts to actually become HIGH VOLTAGE.   At least in the safety sense.   

A fuse can't be counted on to save you.  Might give your family something to bury instead of being charcoal after...   There ARE a few things that might, but they are not meant to be used as a 'backup' for learning and doing it right - they are for an actual FAULT, where something breaks, so they're not worth talking much about right now

The WHOLE concept is: don't EVER get shocked!  It's that simple.  Don't.    To do that, you have to go 1 step at a time.  I like John's idea of working with a slightly higher DC supply and regulating it down.   While still learning the principles.
Later you can try out a PS project with a transformer, done safely. 

[antonis]

Also, the regulator mentioned above is an adjustable regulator, but it doesn't offer 150mA, it is only at 100mA.
What would happen if I used this regulator that has .5A output? http://www.diystompboxes.com/smfforum/index.php?topic=111553.msg1027068#msg1027068
Would the current just go to its max, or would it cause some sort of problem?
EDIT: Actually, it should do just fine. it is rated for a MAX of 500mA, so with the transformer's rectified output, it should be producing the 150 or so mA as stated above. But should the current be regulated too?

I presume that you are a little confused about some basic functions...

When we deal with VOLTAGE regulators (or any other kind of constant voltage sources..) the field of interest about current capability is ONLY the maximum one (to satisfy the ratio of max_current/min_load..)

It doesn't matter if we use a source with current capability of 1kA or 1mA as long as our load requirements are less than 1mA...!!!!

[bluebunny]

Is it time for a sticky "MAINS SAFETY - how not to die (or kill people)" thread in this forum?  There are too many people stopping by these parts who I would really rather didn't die.

I can think of the perfect person to write it.   

[Brisance]

^^ I would conclude it with rule #1: If you have to ask, keep away from mains!

[acehobojoe]

Thanks! I think the power supply idea john had using a 12v supply or so, would be a good place to start. Try to regulate it. Then after that trying to rectify some sort of AC signal.
There's a power company near where I live. I'll ask them about training.

EDIT: here's an article  that seems pretty helpful
http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

[GibsonGM]

How about finding some online tutorials??  When I was *younger*, we would find these ARRL amatuer radio handbooks for various years...they had good sections in each one about basic electricity and electronics. 

Online, there are TONS of explanations and tutorials. Search "power supply tutorial" or "electrical safety".   Bet you'll find 100 links to look at in 5 seconds...

[R.G.]

The astute observer will notice that I always type in much the same warning about AC mains current, including that the internet is not where to get your training. While reading and watching stuff on the net is better than being naive about processes and procedures, it's not a good substitute for a human trainer.

A human trainer can watch you do test cases in a safe(ish) environment and make guesses about when you're about to do something dangerous and warn you ahead of time. A human trainer's comment that you killed the patient (or yourself!) is more effective than the network saying blindly "I knew you could do it" no matter what you do.

Yes, read and study stuff on the net - but find a human instructor to teach you and evaluate whether you got it or not.

In some cases, it really is life or death. Worse yet, something may work fine now, but electrocute you, or your children, years later.

[acehobojoe]

RG, can you train me in person?

[R.G.]

Actually, that would be fun. I wish my life had time for more stuff like that!

[bluebunny]

If you ever do find that elusive free time R.G., I'll be straight down to Heathrow and on a plane to Texas.  I'll be the one standing second in line behind Joe.     I'm sure you could fill a whole lecture-theatre.

[Mike Burgundy]

Mike just stuck his hand in the air and started making frantic "ooh, ooh, me, ooh" noises ;P

[acehobojoe]

One day, we'll have a barbecue and lecture. It will be great!

[PRR]

> AC is super dangerous

To clarify....

_AC_ isn't dangerous. Stick your guitar plug in your mouth and strum. You are eating AC. Very-very-very tiny AC does no harm.

The effects of AC and DC on the body are slightly different, but similar for any practical purpose.

The *DANGERS* from Wall Power that R.G. is warning about are:

HIGH VOLTAGE -- Voltages below 25 Volts are mostly harmless through the skin. Higher voltages are increasingly dangerous. (Yes, electrocution is often studied in current, but more volts cause more current to flow.)

HIGH POWER -- If you part-short a high-power stomp-supply you can get maybe 10 Watts of heat. Say "a candle". If you part-short a wall outlet you can get 2,400 Watts of heat. Say "powerful blow-torch". A wall-outlet starts fires hundreds of times better than your little warts.

[Brisance]

And to bust the "Only current kills myth", think of ohms law, you need voltage to overcome the body resistance. Short a car battery with your hands: nothing happens. And these are capable of hundreds of amps.

[acehobojoe]

https://youtube.com/watch?v=XDf2nhfxVzg

This guy, (don't do what he does) explains it quite well actually.

[R.G.]

And to bust the "Only current kills myth", think of ohms law, you need voltage to overcome the body resistance. Short a car battery with your hands: nothing happens. And these are capable of hundreds of amps.

Unfortunately, human body resistance is variable. The internal resistance is quite low, as the body fluids contain enough electrolytes to make the internal body very conductive. *Skin* is fairly resistive, so you're a conductive solution of fluids wrapped in an insulating container.

Mostly. *Dry* skin is resistive. It's resistance drops dramatically when it's wet or broken. Or if you're just upset. One of the "graphs" in a polygraph (lie detector) is one of galvanic skin response - skin resistance.

For heart stopping/starting, it really does seem to be current. The internal electrical paddles used to restart a heart directly during surgery when the chest is open are much, much smaller currents, and because they're used on wet body surfaces, the voltages are correspondingly even lower.

It's true that Ohm's law tells us how much current flows when a voltage is applied. The trap there is thinking that the resistance is a constant. It's highly variable for bags of liquid - like us.

When I was designing power supplies, electrical safety standards said that all internal capacitors had to be below 42V within some number of seconds after the power switch was off, 42V being the threshold where the august body of safety specialists thought it was safe. When I designed the power supply for the Workhorse amps, I found that the safety standards had been updated to say that anything between 9V and 42V AC or DC was hazardous depending on the current available. So yes, safety standards now say that car batteries are hazardous sources, for conditions of wet skin and metallic tools. Notice that "metallic tools" includes rings and bracelets.