The danger isn't only because it's AC and has potentially a ton of current should the circuit be shorted... the danger is that when using AC there are a ton of grounding opportunities in most/any room of a building, meaning more opportunities to accidentally short the already dangerous circuit. Think the metal water faucet, it's a ground, the metal screw on the plastic light switch cover could work (if the paint is scratched a bit), etc, etc...
Adding a transformer decouples the circuit electrically via a magnetic coupling. This means all the unnoticed ground connections in a room aren't dangerous if you are touching it along with a hot wire from your shortable circuit.
From wikipedia 'Electric Shock':
"
A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. At around 10 milliamperes, AC current passing through the arm of a 68 kg (150 lb) human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object.[2] This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.
The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest; more than 30 mA[3] of AC (rms, 60 Hz) or 300 – 500 mA of DC can cause fibrillation
"
On Wed, Jul 3, 2013 at 3:05 PM, Avery louie <inactive.e@gmail.com> wrote:
The question is how long it takes before failing, and how many amps to gets to before failure. Someone with more knowledge should weigh in on this, but 1A is probably dangerous at 120v. It is tricky to say, because it depends on how you come in contact with it.
Ohms law says that V=IR. You load is the resistance of your gel, which at 120v draws less than 1A. The problem is if you get wet and touch it, and have a lower resistance, and draw more amps. If the chip takes too long to burn out, you are toast.
According to the datasheet the fuse time is about 8.5 ms with a maximum draw of 50A. Ouch. It also looks like it can survive 10 cycles (1/6 sec) at 25A.
Its the amps that kill you, which is why low voltages are generally considered 'safe' - it is hard to draw much current with only 5v of potential.
It is probably fine, but definitely build at your own risk!
--A
To view this discussion on the web visit https://groups.google.com/d/msgid/diybio/CAL4KOmigMyZzknUVBUouj2gLJ9%2BU%3D0SnJm5LL%3DqnWLB%2BnrgXBQ%40mail.gmail.com.On Jul 3, 2013 4:56 PM, "Josiah Zayner" <josiah.zayner@gmail.com> wrote:--Cool other people have also thought of this so I know that it should be really usable.
The 120V-> 220V converter is a good idea! Connect that to a higher voltage bridge rectifier and a voltage divider output like 175V run your gels in 20 minutes for like $15 or $20.
Is this still unsafe even with the 1A load limit?
I mean obviously it is unsafe not being enclosed.On Wed, Jul 3, 2013 at 3:45 PM, Patrik D'haeseleer <patrikd@gmail.com> wrote:
Here's an older article that does exactly that: dimmer + fuse + rectifier = 120VDC variable power supply for $20:
http://www.science-projects.com/PowerSupply.htm
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As you mentioned, this is horribly unsafe, but it does work. One upgrade would be to add an isolation transformer. Or better: get a 110->220V travel transformer (or a 220->110V one, and reverse it), so you get isolation from the mains *and* voltages up to 22VDC.
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