Post by thread:[EE]: Safety of 12V dc to 110v ac using D cells

Hi All, I saw a kind of 12v dc to 110v ac inverters selling in electrical stores that allow people to drive their laptops in a car. I want to use it to drive a small device that uses 110v and consumes power less than 3 Watts. But instead of using the car battery, I want to use 8 pieces of D cells as the dc source. The reason is, the 110v ac generated this way would be very safe even when someone accidentally touches it? It is hard to imagine 8 D cells could kill someone, right? Could anybody give some comments to my argument? Thanks in advance! John

> dc source. The reason is, the 110v ac generated this way > would be very safe > even when someone accidentally touches it? It is hard to > imagine 8 D cells > could kill someone, right? Could anybody give some comments > to my argument? It ain't the volts, its the amps... Doesn't take very much AC current to stop a heart or throw it off rhythm if applied correctly. Rob

> The reason is, the 110v ac generated this way > would be very safe > even when someone accidentally touches it? Probably, but not for the reason you think. Touching one pole is mostly harmless if you are not connected also to the other pole, so an isolated 110V is much more safe than a ground-referenced 110v. > It is hard to imagine 8 D cells > could kill someone, right? No problem at all. With the right circuitry and connected correctly I'm sure 8 D cells can kill. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

> I saw a kind of 12v dc to 110v ac inverters selling in electrical stores > that allow people to drive their laptops in a car. I want to use it to drive > a small device that uses 110v and consumes power less than 3 Watts. But > instead of using the car battery, I want to use 8 pieces of D cells as the > dc source. The reason is, the 110v ac generated this way would be very safe > even when someone accidentally touches it? It is hard to imagine 8 D cells > could kill someone, right? Could anybody give some comments to my argument? Batteries absolutely, positively can kill someone. I'll trot out the old story: I once saw someone connect the end terminals of 60-odd 9V batteries through the palms of his hands (not intentionally, of course). The entry and exit points were third degree burns. Of course, that was 360+ alkaline cells, instead of 8, but the key problem is still there: with the output from that inverter, or the wall socket, or your car's cigarette lighter, the current is limited somehow. A fuse, a polyfuse, a breaker, whatever, but limited. Batteries have a much looser limit, namely the internal resistance of the cell. So while your wall breaker will trip at 10 or 20 amps, a battery will happily carry on sourcing current way past that (up to hundreds of amps, for some types), if only for a brief time. In other words, the current limitation on this system won't come from the fact that it's running on D-cells. It'll come in the form of fusing or a circuit breaker on the output (although the input is no doubt fused as well). In other words, the output from that inverter is neither safer nor more dangerous than the wall outlets in your house. Mike H.

John Waters wrote: > Hi All, > > I saw a kind of 12v dc to 110v ac inverters selling in electrical > stores that allow people to drive their laptops in a car. I want to > use it to drive a small device that uses 110v and consumes power less > than 3 Watts. But instead of using the car battery, I want to use 8 > pieces of D cells as the dc source. The reason is, the 110v ac > generated this way would be very safe even when someone accidentally > touches it? It is hard to imagine 8 D cells could kill someone, right? > Could anybody give some comments to my argument? > > Thanks in advance! > > John > > The very first item I designed as a young engineer was a custom detonator for oil exploration (sesmic) explosions in the Guatemalan jungle. It converted 6 D cells (9V) to 110V DC in order to set off downhole explosions using a transistor-driven oscillator, a transformer, an SCR , a heavy cap, and an early optoisolator. While blasting caps are "guaranteed" to explode at 6V, we wired two in parallel and applied 110 VDC because we'd had failures at 12V (normal blasting voltages) due to heavy moisture. Even though we used a charge bleeder (a heavy resistor to bleed off the charge) , the 110VDC was easily capable of killing anyone touching it. --Bob -- Note: To protect our network, attachments must be sent to spam_OUTattachTakeThisOuTengineer.cotse.net . 1-520-777-7606 USA/Canada http://beam.to/azengineer

> It ain't the volts, its the amps... It's really the right combination / right circumstances. Even though a single D-cell can deliver a few amps, it hasn't enough potential http://en.wikipedia.org/wiki/Electric_shock http://www.stanselectric.com/shock.html You have people hit by lightning (sometimes several times, poor buggers) who avoid death because the path through the body missed the heart. Enough to blow them out of their shoes though But grab a live appliance with the wrong hand ....... The situation with an inverter powered by 8 batteries is similar to that of an isolating transformer. You probably wouldn't even feel one touching one output terminal (assuming the whole thing really is floating and there's no ground reference) but one in each hand is not to be recommended

> -----Original Message----- > From: .....piclist-bouncesKILLspam@spam@mit.edu > [piclist-bouncesKILLspammit.edu] On Behalf Of Jinx > Sent: Monday, November 14, 2005 4:46 PM > To: Microcontroller discussion list - Public. > Subject: Re: [EE]: Safety of 12V dc to 110v ac using D cells > > > > It ain't the volts, its the amps... > > It's really the right combination / right circumstances. Even > though a single D-cell can deliver a few amps, it hasn't > enough potential Yes, I was being a bit facetious. See if you can get your hands on IEC 60479-1 which relates to the effects of current passing through the human body. I only have excepted portions of it. An official copy is around $150 but you might find somebody with a copy you can borrow, read and return. A good library might have one, especially a university library. IEC 61140 has information relating to rules of protection. Again, I don't have a full copy handy, just excerpts. Rob