Question About Safety Of Voltage Supply

You might be able to feel it. However, batteries will probably support even more current than a wall transformer. So, just pay attention when you are playing with it.

Regards, Bob Monsen

I don't mind if it hurts, just as long as it's not lethal, the radio is only using one tube, I've been told it doesn't use much current. I'll still be supplying the filament voltage via batteries.

In general that amount of voltage will not do anything. Even 120 is usually not lethal in most circumstances(I don't mean that you go around playing with it but I mean in most accidents when people get shocked).

At around 50V or more is when there is enough voltage to start to break through the skin. Your skin is essentially like a resistive coating. It has a high resistance but is thin. If the skin is wet or broken(With fluids) then its worse Dry skin has about 500kOhm resistance so its no big deal. (at

Wet skin is like 1kOhm so its 500 times more likely to get shocked if your wet. This means its about 100mA(@100V) and enough to kill if it goes through your heart.

Note though that the current does not flow ont he surface of your skin but goes into your fluids where there are electrolytes and its much less resistive(essentially your veins are like copper wires and your skin like a resistor).

This is why the 9V shocks your tonge. Its probably sending an aweful lot of current(probably 50mA is my guess on average) but does nothing because its only on your tonge. (although I imagine a 9V can kill if placed "across the heart".

The real issue is not shock. Its getting shocked in such a way that current passes through your heart. (although serious burns can occur otherwise but its usually rare)

The trick in working with "high voltage" is never to use two hands. The reason why most people who die from electric shock is because they use two hands and current will flow from one hand, through the blood into the heart then out the other hand. Use one hand and probably at most you'll get shocked. You can also wear some gloves if your still afraid. (even with one hand its possible the current will go through your heart but much less likely. If the other parts of your body are not being grounded in any way then it's usually ok)

Main's is much worse because ground is earth ground. If you are working on a power supply then you usually use a transformer and the only way to get shocked is for your body to make a circuit with the ground on the secondary side. This is pretty much impossible(except on your hand) if you keep one hand away. (although there are allways accidents)

I think as long as you are careful then it will be ok. Its about 30kV/cm for voltage to be able to "get you" through air. This means at 100V its only like 33um. (about the size of a human hair)

The main thing is to just think about what your doing and don't be clumsy. If you pay attention to what your doing and know the dangers and what is dangerous in what your trying to measure then usually you'll be ok. Then, in the rare chance that you get shocked usually it's not fatal. Its basically like driving a car.

I knew about the one hand in pocket technique. The radio has exposed fahnestock clips for the power, so I'll have to be really careful, applying power only when my hands are clear, and keeping the cats out of the room when operating the set. Would the set have hum powered from these switch mode power supplys? I have a 2200 MFD 50 volt cap I can use for a filter cap if needed.

Might be best to stay in aus.electronics Dave :-)

Jon,

Just FYI, I've measured the current in the 9V-to-tongue experiment at about 1 or 2 mA. YMMV.

Mark

I tried a dc supply, holding banana leads firmly in two hands, and it got uncomfortable around 90 volts or so.

Batteries would be no different.

John

Shocks are a total of the voltage, the current, the state of your body and even how you touch the voltage.

It's "safe" to touch much higher voltage, if by safe you mean "not fatal". On the other hand, when it happened to me a few times decades ago, I realized I didn't want it to happen again, especially not after I hurt my arm (though not-seriously) when my hand jumped off the high voltage and banged against a hard surface.

For someone else, that voltage might have been fatal. Or, if it had gone from one hand to the other, rather than from the finger to the side of my hand that was about what happened.

The 9v batteries in series can be as dangerous as a 45volt power supply (or as "safe"), the voltage is the same. The current of the power supply is likely more, but then few think about the potential danger of running 12v through their body at really high current.

"Safety" is also relative given the care you take. If you never touch the voltage, you will always be safe no matter what the voltage or current. But if you're careless, the safety starts disappearing.

Michael

But that open style that was common in the early days is dangerous. One famous Australian ham died a long time ago when he was doing something and his headphones brushed against some high voltage, and he was known for his techical writing so he should have known better.

Exposed wiring makes things a lot more dangerous than the voltage or curent of the power supply. It's just too easy to accidentally brush against something.

It isn't a joke, I have seen stories about people's cats dying when they brush against high voltage, and the only reason that can happen is because the high voltage isn't properly kept inside a box.

Michael

How would one go about running 12 volts through their body at really high current?

John

Use two needles stuck into your veins. You will probably get enough current to cause serious shock or even death if placed properly.

For the OP's circuit, with either batteries or wall-warts (especially with a big capacitor), it might be good to rig up a simple current limiter, for about 5-10 mA, and enclose the whole thing in an insulated box.

An LTspice schematic follows. It limits short circuit current to about 5 mA, but provides 44.6 VDC into a 10 kOhm load or higher. However, it draws a constant 93 uA from the battery. You can fiddle with the values, and maybe use a darlington (but you need to add an extra diode). It is also possible to build this with a PMOS transistor with negligible current draw with no load, but a little more power in the sense resistor while operating.

Paul

========================================================

Version 4 SHEET 1 880 680 WIRE -16 160 -80 160 WIRE 0 160 -16 160 WIRE 160 160 80 160 WIRE 368 160 256 160 WIRE 400 160 368 160 WIRE -80 240 -80 160 WIRE -16 240 -16 160 WIRE 0 240 -16 240 WIRE 112 240 64 240 WIRE 208 240 208 224 WIRE 208 240 176 240 WIRE 400 240 400 160 WIRE 208 272 208 240 WIRE -80 400 -80 320 WIRE 208 400 208 352 WIRE 208 400 -80 400 WIRE 400 400 400 320 WIRE 400 400 208 400 WIRE -80 432 -80 400 FLAG -80 432 0 FLAG 368 160 Vout SYMBOL Misc\\\\battery -80 224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR InstName V1 SYMATTR Value 45 SYMATTR SpiceLine Rser=1 SYMBOL pnp 256 224 M270 SYMATTR InstName Q1 SYMATTR Value 2N5401 SYMBOL res -16 176 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 47 SYMBOL res 192 256 R0 SYMATTR InstName R2 SYMATTR Value 470k SYMBOL res 416 336 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL diode 0 256 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 112 256 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value 1N4148 TEXT -114 506 Left 0 !.tran .01 startup

That's not normally a hazardous voltage (try not to touch when wet, though). The easiest solution is a 24VAC wallwart, feeding a voltage doubler rectifier.

Bad ASCII art follows:

The clamp diode across the top right is optional. For 300 mA output, capacitors should be at least 220 uF.

Oops. That should be 'diode across the top LEFT is optional.'

Thanks but I already have the two wall warts, and the parts to build a little box that allows me to connect them in series without cutting the DC plug off.

Or you could get an infection, and die from that. A 12-volt car battery can kill you, if dropped from a sufficient height.

Paranoia.

John

There is a real difference between AC and DC wrt current across the heart. DC, even in very high voltages, has the effect of clenching the muscles. So, a DC shock may stop your heart, but only when you are holding it. If you can let go,, you are probably going to be OK.

On the other hand, AC has the effect of putting the heart into fibrillation. This is often lethal, and actually used to be treated by passing a DC current (by charging big caps up to 1000V!) across the heart, which stops it and resets it. They now use a sine waveform (which is AC, actually, but is about 100Hz) that uses less energy, and thus causes less damage.

The upshot is that 50VDC isn't going to hurt you.

Regards, Bob Monsen

It doesn't really matter. If you get shocked through the heart by DC there is much more likely to be serious injury to the muscles and blood vessles than with ac. Also, if the DC is high enough you can actually create an electrolytic effect in the blood stream.

Would you rather get ran over with a tank or hit by a plane? Either way your dead so it does't matter much. (also 50VDC is not 50VAC rms so you can't compare them directly)

I would aggree that AC probably has a higher chance causing fibrillation than DC because of the way the heart is designed. But I would imagine that chances are if you get shocked with DC power or equal AC power across the heart that both with have approximately the same outcome. (although I really have no idea exactly how they are related. AC may be worse at low voltages than DC and maybe DC is worse at high)

And a fully discharged AAA cell can kill you if fired from a gun. Or if you eat enough of them. :)

Not at all. It's a good idea, at least, to know how to rig up a current limiter, especially when it can be built for less than the cost of one of the batteries. It could prolong their life in the case of an accidental short circuit or overload, and it would be very much advisable for Lithium batteries which can explode if short-circuited.

Here's the MOSFET version, using only four cheap parts:

Paul

============================================================

Version 4 SHEET 1 880 680 WIRE -16 160 -80 160 WIRE 0 160 -16 160 WIRE 160 160 80 160 WIRE 368 160 256 160 WIRE 400 160 368 160 WIRE -80 240 -80 160 WIRE -16 240 -16 160 WIRE 48 240 -16 240 WIRE 176 240 176 208 WIRE 176 240 112 240 WIRE 208 240 176 240 WIRE 400 240 400 160 WIRE 208 272 208 240 WIRE -80 400 -80 320 WIRE 208 400 208 352 WIRE 208 400 -80 400 WIRE 400 400 400 320 WIRE 400 400 208 400 WIRE -80 432 -80 400 FLAG -80 432 0 FLAG 368 160 Vout SYMBOL Misc\\\\battery -80 224 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR InstName V1 SYMATTR Value 45 SYMATTR SpiceLine Rser=1 SYMBOL res -16 176 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 470 SYMBOL res 192 256 R0 SYMATTR InstName R2 SYMATTR Value 22Meg SYMBOL res 416 336 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL pmos 256 208 M270 SYMATTR InstName M1 SYMATTR Value IRF9Z24S_L SYMBOL zener 112 256 M270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value BZX84C6V2L SYMATTR Description Diode SYMATTR Type diode TEXT -114 506 Left 0 !.tran .01 startup

SNIP

Here's a link to a somewhat technical analysis of low voltage hazards. It is probably an overkill answer to the OP's question, but it is readable and provides a scientific basis for its conclusions.

Chuck

----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----

----= East and West-Coast Server Farms - Total Privacy via Encryption =----