help with small electrical fire.

OK, so the theory is to turn on and fade a household bulb with a MOSFET. Step one was to convert 120VAC to 120VDC RMS. I’m pretty sure that’s were things went horribly wrong. It all happened so fast, my poor little Duemilanove lit up like a Christmas tree =(. I’m not too heartbroken, the FTDI chip was messed up anyways. I had to program it with an AVR Pocket Programmer, fried that too :0, that pissed me off. I’m glad it didn’t kill my desktop. Here’s a basic schematic, where’d i go wrong?

lights.bmp (645 KB)

No capacitors, so you turned 120V AC into 170V peak. That FET only had a Vdss of 200V - too close for comfit. However if you are in the US then that would give you actually 340V so that exceeds the breakdown voltage of the FET.

That FET requires a Vgs of 10V so it wasn't going to work anyway.

You had no isolation transformer that means you connect direct to the mains. This is illegal in most countries, for the very reason you have discovered.

The ground was not connected so the whole thing was floating.

i knew it was 170V peak, can you explain how you got 340V? 120VAC neutral is connected with ground in the breaker box.

where'd i go wrong?

If you have to ask, you probably shouldn't have been using raw household AC voltage. ;)

I'm am glad you are still alive to report the story, and give us yet another example why begineers should not play with 120/220vac in their projects.

An isolation transformer with series fuse would be a minimum requirement while one is testing out a design. It might not have saved the arduino, but prevented the fire. ;)

Lefty

My understanding of US mains wiring is not from direct knowledge but only from some one (electronic engineer) who worked in the states for some years. He told me that although you had only 120V on one connector the phase was such that across live an neutral you got the full 240V. So that is what I was applying in that reply.

From this web site:- http://en.wikipedia.org/wiki/Electrical_wiring_in_North_America

It says:-

Leg as in 'hot leg' refers to one of multiple hot conductors in an electrical system. The most common service in the U.S., single split-phase, 240 V, features a neutral and two hot legs, 240 V to each other, and 120 V each to the neutral

You have three wires coming into the house, line, another line, and neutral. Actually 4, the ground is connected to the copper water line and a copper plated steel rod buried 8 feet into the ground. Between either line and neutral you have 120VAC RMS. Between one line and the other line you have 240VAC RMS because they are 180 degrees out of phase of one another.

The fuse is a given, but i skipped it for now cause it was a prototype. D'OH!!!

This was my first high voltage project i plugged in. Glad it only cost me $45. I've gotten hit with 120VAC plenty of times, it's not that bad. When the arduino fried it did make that cool 60Hz sound, so much better than that lame 50Hz sound. ;)

The fuse is a given, but i skipped it

It would not have saved you, fuses are not fast enough to prevent damage to semiconductors.

arduino fried it did make that cool 60Hz sound

I would have thought with full wave rectification that should be 120Hz anyway 100Hz is so much more retro. :stuck_out_tongue:

:P lol

back to the design, can the FETs handle the voltage or was there a short? the only path it could take was through the gate.

I think you blew them up by not turning them (the FETs) on hard. This caused rapid over dissipation in the FET body which caused it to short. You then had all that voltage flapping round the arduino.

Do things in stages. Get an isolation transformer. Wire the bridge rectifier and capacitors. Measure the voltage you have. Insert the FETs and the load and tie the gates down to ground. Test again. Insert opto isolators between the FETs and the arduino. Test again.

Going out on a limb, if it fried, it obviously can't handle it. Opto isolators is a good idea. When in doubt, make sure no path exists.

is this an isolation transformer? http://www.diytrade.com/china/4/products/8063742/ET_Common_Mode_Inductor_for_Line_Filter.html or do i need this? http://www.newark.com/triad-magnetics/f-29u/power-transformer/dp/28M9251

@JetIgniter2K

I am sorry for your Arduino and I happy that you are OK. To control an ordinary AC light is very interesting. If I was planning to do that, I will use a TRIAC in combo with a opto-coupler TRIAC type. A TRIAC is design for AC operation. A TRIAC work like SCR. Here a link. http://www.ask.com/wiki/TRIAC

You have three wires coming into the house, line, another line, and neutral. Actually 4, the ground is connected to the copper water line and a copper plated steel rod buried 8 feet into the ground. Between either line and neutral you have 120VAC RMS. Between one line and the other line you have 240VAC RMS because they are 180 degrees out of phase of one another.

Correct. Same system in Canada. The two country follow the same standard. 2 Live ( Black or Red wires ) 1 neutral ( white wire ) and 1 Ground ( green or bare wire ) . I did change a fuse box and re-wired to a breaker box at my old address. Measure 120 VAC between : Live - Neutral , Live -ground. 0 V : Neutral - Ground ( They are connected together at the box ) 240 V : Between Live 1 and Live 2

I've gotten hit with 120VAC plenty of times, it's not that bad.

However you do realize that many have died from 120vac shocks? So were you just lucky or just smarter then those that have been killed by such voltages? Do know know the current level required to stop your heart?

Lefty

Insert opto isolators between the FETs and the arduino.

That of course would then require a DC voltage source suitable for the gate drive, derived from the 120ac circuit.

PWM on full wave rectified AC will not work due to the beating between the two frequencies. You need to smooth the voltage before applying PWM to it.

retrolefty:

I've gotten hit with 120VAC plenty of times, it's not that bad.

However you do realize that many have died from 120vac shocks? So were you just lucky or just smarter then those that have been killed by such voltages? Do know know the current level required to stop your heart?

Lefty

Just to be complete it takes 0.5 mA to start causing pain and several mA can send your heart fibrilating. Voltages over 40V I think are rated as potentially dangerous, certainly mains voltages can and do kill. Even if it doesn't kill a shock can cause you to injure yourself in other ways (spasm, fall over, fall into machinery, burn yourself with a soldering iron, do a backflip (yes, this can happen)). Other issues with mains is that accidental shorts cause small explosions that can injure your skin or eyes with flying molten metal. Ditto with failing semiconductors - mains supplies can give many hundreds of amps before the fuse trips, causing the 'smoke' to be released supersonically.

Don't mess with mains!

I just happen to have an article about TRAIC SCR. It from "Electronics Experimenters Handbook" January 1989, page 120 "Working with Triacs and Scr's" by Ray Marston. It the use of TRIAC and SCR connect with AC main. and how it interact with a digital circuit. And It show how a dimmer switch work. Very interesting, I will use such a circuit and control a simple light bulb. I wil use precaution and proper contruction techniques. The parts will be select with proper or higher rating.

Best if you post your schematic on this forum for comments before you build it.

So were you just lucky or just smarter then those that have been killed by such voltages?

He's probably just a poor conductor.

Don