Basic Training for working with 120V AC

I'm planning to build a digitally-controlled light dimmer circuit. I'm not trained in electrical engineering, and I don't want to do something that an electrician would immediately recognize as dangerous and stupid.

Please suggest a reasonable home set up for prototyping circuits involving 120V AC power.

• Relevant E&R questions: un, deux. Commented Jan 7, 2011 at 11:04
• My tip - Don't be satisfied with what you've read on the internet, consult an expert in person Commented Jan 7, 2011 at 11:23

As far as lab equipment goes, a 1:1 safety transformer (AC mains : AC mains) is worth a lot. They aren't cheap, but I would not want to work without one. Mine is home-built and uses two 250 W transformers back-to-back.

The trick of the 1:1 transformer is this: Current from its secondary winding can only go back to the other end of this exact winding. As long as you touch any circuit connected to the 1:1 transformer with one hand only, you are safe because the current from your finger can not go anywhere. You are a bit like a bird on a wire. A regular wall outlet is referenced to earth, just like you are when standing on the floor: In a fault, current from the live pin of the outlet runs through you, the floor and to earth, which is equal to the other end (neutral) of the wall outlet.

Keep in mind that current always needs a loop to flow in: Any energy that comes from the transformer can go back to this transformer only, and can not go anywhere else, especially not to the ground via your body.

• Can you elaborate a bit on the role of the 1:1 transformer? Naively, I would expect electricity that has been transformed to the same voltage to be exactly as deadly as electricity that has not. Commented Jan 7, 2011 at 15:48
• It's the same voltage between live and neutral as unisolated but not between live and earth. Commented Jan 7, 2011 at 16:33
• This is commonly called an isolation transformer and many also include a circuit breaker. They make it so that there is no circuit between the isolated side and the actual Earth ground, which keeps large amounts of current from flowing through you or your equipment into the ground. Current can still flow between the two lines on the isolated side, and that is what the circuit breaker is for. Commented Jan 7, 2011 at 19:52
• You can get similar (but not as good) protection from a GFCI outlet. It measures current leaving and entering the outlet. If they don't sum to zero it shuts the power off. I think I would trust the isolation transformer more. Or both. Commented Jan 7, 2011 at 20:40
• @ Ranieri: Done. Answer is now longer. Commented Jan 7, 2011 at 20:53

You don't have to probe a live circuit. Get test leads with insulated clips on them. Clip onto the circuit with power off. Turn it on, stand back, and look at the reading. Never use an oscilloscope on a non-isolated circuit. I would also suggest some kind of pilot light in the circuit so you know when the power is on. (Just unplugging is not enough if you have large capacitors that store high voltage, but I don't think you will for a light dimmer.)

• In the extreme example of this, I know people who work on +10kV power electronics. The Device Under Test is hooked up, the oscilloscopes are set to trigger properly, everyone leaves and closes the metal fence around the equipment, everyone leaves the lab floor, gets into the control room, and THEN energizes the circuit.
– W5VO
Commented Jan 7, 2011 at 5:53

If you're not comfortable with it, don't do it. I wouldn't mess with AC unless you're totally confident in what your doing.

If you must, use the one hand rule, and keep one hand in your pocket while working with live AC. Also use insulated tools.

• Wear shoes with rubber soles. Commented Jan 7, 2011 at 8:26
• Related, do not use ESD-safe equipment anywhere near energized circuits; especially clothing and boot-straps. Commented Jan 7, 2011 at 19:41

Apart from what @bt2 said

1. Don't be alone, so someone can call 911.
2. Show them where the off switch is.
3. Don't wear a ring, watch, necklace.
• +1 for don't work alone. This is true when messing with any electronics that could possibly hurt. In one video I saw of a plasma generator, the device actually had two buttons required to activate and they were far enough apart that it required two people. I recommend you take the same design even when just prototyping anything with high voltage(including capacitors) Commented Mar 27, 2011 at 14:52

Since it would be impossible to turn you into an electrician in a single answer, I'll recommend some reading.

For the US, home wiring is governed by the NEC (National Electrical Code). It's a huge, detailed, expensive publication. For a DIYer, I strongly recommend purchasing the most recent edition of Wiring Simplified. It's probably available at your local (or big-box) hardware stores (I've seen it at Ace Hardware and Home Depot), or here on Amazon, should be around \$10. I use it when planning any electrical job at my house, parents' cottage, or workshop.

From the Amazon description:

Revised and completely updated for the 2008 National Electric Code, this small manual continues its 75-year history of demonstrating how to install safe, convenient, and economical wiring. Encouraging readers to tackle jobs small and large, the guide covers everything from repairing a table lamp to wiring a whole house. After introducing the basics—standards, codes, safety practices, and an overview of how electricity is measured and delivered—chapters show how to design a layout for lights, switches, and receptacles; run a line from a utility pole; install wires, circuits, and grounds; and handle special projects such as replacing old wiring and wiring detached garages and accessory buildings.

There is a section on safety, and the book recommends good practices, but it's not a replacement for training. It's a good reference for knowing what you're working with, though, and common-sense rules (like the one-hand rule, disconnecting the breaker, and wearing rubber-soled shoes) together with this book will make you a much better electrician.

• My original question mentions that I'd like to know what constitutes "training". Do I have to go through a 2-year college program? or are there organizations which offer in-person basic training by people who know what they're doing? Commented Jan 23, 2011 at 0:20

The inimitable Dan of Dan's Data has an entertaining article on avoiding electrocution. The follow up mail on the subject contains further tidbits.

• Haha.... very funny! +1 Commented Jan 7, 2011 at 11:14

I recommend going to your local hardware store and buying a light switch with a neon tube in the switch. Also buy the metal junction box to install the switch in and a faceplate. Run your AC input into one side of the switch, and the other side to whatever circuit you're playing with. This is a good way to be able to see at a glance whether power's on, and to have an easy cut off. At work, I have this installed on a wooden board with the output wires going to a pair of terminal blocks. This makes it easier to change what's connected. Whatever you do, make it perfectly clear which switch position is 'on' and which is 'off' for safety.

If you make it plug in and not hardwired you can prototype anything. If you want to sell it you will need to have your device meet ul or some other governing body that oversees your perticular type of electrical device.

• I think you've answered a different question to that asked at the top of the page. Welcome to EE.SE. Commented Mar 24, 2023 at 23:04

If you take a regular rotary dimmer switch (Potentiometer/variable resistor/rheostat) and hook that up with a stepper motor you could get away with not knowing a whole lot more than how to control a stepper, how to keep up with the stepper's position, and how to wire up the dimmer switch (very easy). Oh, and you've got to power the whole switch control setup somehow.

If you want to do more than that, like use solid state switches or pretty much anything else living on a board you've designed, you've got more issues, and even your power supply may become a lot more complicated since you don't want your control circuits to become a possible path for shorting out the system. High voltage can cause arching at some distance normally, and with residential AC you've also got to worry about extremely high current from lightning.

If you want this system to be energy efficient then you've got a lot more problems ahead of you. The easiest way to dim the lights is by introducing resistance, but is wasteful, and if you want the lights to run off AC it's difficult to get around easily.

You might want to have a look at the NEC (National Electrical Code) if you are in the US.

I'm not an EE or electrician, btw.

• -1: sorry, but the question was about main safty training, not about dimming lights. Commented Jan 23, 2011 at 8:08
• @BG100: My answer told a novel way to electrically isolate the high voltage and/or AC circuit from the control circuit, which is the safest way I know if to do this type of design. Commented Jan 30, 2011 at 6:04