I search the net, I pour through my books - I still can’t find what I need, so I turn to the dopers. Just how do the little step down converters change the 120 ac voltage to, say, 6 v dc? I think I’m ok with the stepping down of voltage, via proportionally fewer windings around a coil, but how does the ac get converted to dc? Someone recently was talking about how scary it was seeing a kid playing with a converter, plugged into the wall, that had its exit wire split, and the kid was using the wires to light a small bulb. I saw no problem with it, but I couldn’t explain how the voltage had been made safe to play with. I’m right aren’t I?
Magic. Actually, most of them just use a transformer to step down the voltage, followed by:
A rectifier circuit (full wave bridge rectifiers are common)
A regulator circuit (Optional. Usually a 78XX.)
A filtering circuit (usually one or two electrolytic caps)
This describes a linear supply. Switching supplies are a little more complex.
One or more diodes.
Usually not a problem. The voltage those things produce is usually less than 20 VDC. And it’s isolated from the mains via the transformer.
It is generally as safe (or more so due to the inherent current limitations of most small power supplies) than a battery of the same voltage rating.
The six volt example would be pretty safe. This would be especially true with a small adapter like what might charge a cell phone, etc.
How they work (typically, there are many variations):
- Transformer steps down the voltage like you said.
- output of transformer feeds four diodes arranged as a full wave bridge. (searching on full wave bridge should get you diagrams and explanation)
- output of diodes charge a capacitor.
- capacitor providers the output.
Alternate way of look at 3 and 4 (I actually prefer this way, but is often confusing to non electronics people): output of diodes provider the output with a capacitor setting in parallel to the load.
Purpose of the capacitor in either case is to smooth the output voltage.
Amount of ripple in the DC output is proportial to the load current and inversely proportional to the capacitance. This is all assuming a simple unregulated power supply.
A simple diagram of a full bridge rectifier with a capacitor. The input AC is usually stepped down via a transformer. The output is usually further regulated using a transistor to smooth it.
It’s not that the wallwart’s power is DC that makes it sort of* safe. It’s that it’s only 6 volts and probably .6 Amps or less.
It is not really at all safe. If the kid shorts the wires together parts inside the wallwart will fail and then all sorts of unpredictable (and possibly serious) stuff happens. Letting your kid play like that is called “Going for a Darwin Award by proxy.”
For a simplified version with purty pitchers there’s a nifty article at www.howstuffworks.com called “Inside a Power Cube Transformer.”
Generally this is not true. Not to say there are not some exceptions out there.
The output current is generally limited by the output impedance of the secondary windings on the tiny transformer to a couple hundred milli amps or less. Shorting them out typically won’t even make them particularly warm.
I was never led to believe that 0.6 Amps was anything but lethal.
Fortunately, a 6VDC source isn’t likely to be designed to provide that much current. My training had me considering any DC source of 30V or higher to be capable of delivering a lethal 100mA through the heart. Deriving from this assumption a typical body resistance of approximately 300 ohms, 6VDC would smack you with about 20 mA.
Of course, power supplies designed to deliver current at a specified level regardless of the voltage, will call for a different set of assumptions. But this doesn’t usually describe the situation with the AC-to-DC adapters commonly made available with household electronic devices.
Typical body resistance is more on the order of 300k ohms for dry unbroken skin.
It is difficult to get hand to hand resistance measurements much under 400k ohms with dry hands.
Folks, the maximum current capability of the wall wart is usually of little significance when it comes to safety. 0.6 A or 6 A – it doesn’t matter a whole lot. As an example, you can place both hands across the terminals of a 12V car battery and you probably won’t even feel it. Last I checked, a car battery can put out a whopping 600 A if it had to…
A wall wart is modeled as a voltage source. The current through your body is NOT determined by the “maximum current capability” of the wall wart. It is instead determined by two things: a) the output voltage of the wall wart, and b) your skin resistance. The ratio of the wall wart’s output voltage to body’s skin resistance is usually low enough that it does not constitute a danger…
Correct. “Constant current supplies” are normally only found in labs. (We have one in our lab.) Caution must be taken with a constant current supply that has a compliance voltage over 30 V.
[sub] For various reasons, I’ve been putting off reading soyouwannaknowelictricaltypestuff book I have. My shame is great.[/sub]
Now, The Master adresses what would happen if you held the hot wire while sitting in a bathtub. I’m curious as to what would happen if you were on an insulated platform(say, dry wood and jelly jars). Would you be safe or merely die in a different way.
NOTE-I have absolutely no intention of trying this at home. None, zip, zero. Until I know a good deal more about electricity, I will confine my tinkering to battery powered idiocy. So you don’t have to worry about my hearing ‘Yes, if you…’ and then screwing things up and cooking myself.
If there is nowhere for the current to flow then you won’t notice a thing, Doc.
IIRC, electric shocks you get from walking over a carpet are from your body having a potential of at least hundreds of volts. I want to say it is over a thousand but I also don’t want to do a search 
I now realize my first post was a somewhat incomplete in answering the OP, so I’ll try again:
It is fairly easy to convert AC to DC. This is the task of a rectifier circuit. A rectifier circuit is usually pretty simple, as it typically consists of one or more one-way current devices. As the name implies, these devices allow current to easily flow in one direction while blocking current that’s trying to flow in the opposite direction. (But they’re not perfect; for passive “one-way current devices,” there’s always a little bit of voltage drop under forward bias, and a little bit of current leakage under reverse bias.)
One of these devices will provide half-wave rectification. When arranged as a bridge, four will provide full-wave rectification. If using a transformer w/ center tap on the secondary windings, two will provide full-wave rectification. There are various other arrangements, but these three probably constitute 98% of what’s out there.
So what are these “one-way current devices?” In the old days vacuum tubes were commonly used to rectify AC to DC in power supplies, and germanium crystals (“cat whiskers”) were used for signal-level rectification. Today, the solid-state diode is now used almost exclusively for this task.
erislover is correct, i.e. as long as you’re very well isolated from earth ground, you shouldn’t feel anything.
But this is not an absolute statement. While rare, it is still possible to get shocked even if you’re perfectly isolated from earth ground. Two possibilities come to mind:
- You touch the hot and neutral wires.
- There is a lot of capacitance between your body and earth ground. I = C * dv/dt, thus lots of capacitance could mean a significant amount of current. Unusual but possible.
Let me make this completely clear.
An typical undamaged wallwart produces fairly harmless current. That is what I said. I in no way said or implied that it was anything but.
OTOH, a damaged wallwart, caused by shorting the leads which a little kid can easily do, is something else. It can get very hot very quickly (my personal experience as well of others). Enough to cause a fire. They should be protected by a fuse or fusible resistor but nowadays electronics are made so cheap that you can’t rely on them having any built-in safeties. I have found many old but undamaged wallwarts getting quite hot on loads near their limits.
In addition to the heat problem, once it is damaged, there is no prediction as to what behavior it will have. Even under mild heat, the shellac insulation of the transformer can break down and you can get 120V AC coming out the other end. I’ve seen it happen with transformers inside consumer electronics often enough that I check for continuity between the AC cord and transformer output on any PS I am starting to debug. (Again, the cheap way things are made nowadays…)
The kid can easily cause the wallwart to fail. Failed electronics should be assumed to be serious hazards.
I thought all wall warts incorporated a fuse on the output stage. Would assume the manufacturer’s lawyers (or the importer’s lawyers) would insist on it due to liability concerns. But maybe not…
At any rate, no argument here. A wall wart’s output should have over-current protection.