Re: [DIYbio] A simple and cheap >100V DIY electrophoresis power supply

1) Fuses don't change anything here, they are to protect your wiring,
not yourself.
2) Current limit makes no sense regarding safety here. The current
that runs through the gel is much higher than the current that would
ever run through a person who's grabbed the electrodes, as the
resistance of the gel is much lower. A commercial power supply won't
shut down if you grab both electrodes as well.

Let me explain the common sense behind the regulations that require a
power supply to have the output decoupled from the input. Suppose you
have a 12v power supply. It's totally safe to grab the electrodes with
bare hands, so most people won't see it as a problem. Until they grab
an electrode and some grounded object, at which point they connect
themselves between the phase wire and the ground. Boom.

In case of a 100+v power supply, you should not grab the electrodes in
the first place, so the fact that you should not grab both an
electrode and a grounded object does not add that much risk.

I think the parallel between anti-GMO and this case is correct, people
tend to overestimate unfamiliar risks.

And one thing that that's related to safety that I've missed. The
switch should disconnect _both_ wires.

On Fri, Jul 5, 2013 at 2:54 AM, Jonathan Cline <jcline@ieee.org> wrote:
> On 7/4/13 4:10 PM, Alexey Zaytsev wrote:
>> They take the theoretical worst case scenario and run it as
>> something that's sure to happen.
>
> You can do anything you like, supposedly. If you've been thru any lab
> safety class then you know there's many things which are recommended not
> to do. Does it mean they'll all occur - no - they are best practices.
> I wouldn't use a bunsen burner in a small closet without any ventilation
> for example yet you might insist that you want to - whatever - that
> doesn't make it a good practice or allow any justification that "oh,
> it's okay! I do it all the time!". I wouldn't add agar to water and put
> in a microwave unattended to boil for several mins and then remove
> immediately. You know why, right? Yet this discussion has begun to
> split hairs, about what is and isn't safe, even if all the information
> is freely available with a little looking around. There are a lot of
> things which are common sense after basic education of what not to do.
> Should we have a long discussion about thousands of them or suggest
> people to do a little bit of basic homework first. I suggest the
> latter. Especially if the former has already failed to yield much other
> than stubborn answer-backs. Do we want some newbie to get hurt and
> their parents call the regulators and then talk of regulation starts up
> again? Um, no way. Should bad ideas be kept on a web page for others
> to similarly try bad practices? I suggest not. You mention GMO
> scares, well, that just shows how paranoid the public and/or regulators
> can be related to anything mildly scientific looking, yet still you'd
> like to stick to less-great practices which might add safety risk? Not
> a great idea.
>
>
> On 7/4/13 4:42 PM, Alexey Zaytsev wrote:
>> Ok, I've actually looked at the original article.
>> Don't ever build anything this way.
> How many votes will go this way, I wonder. ;-D LOL!
>> But add a switch and a fuse, build
>> it a lot less flimsy, put it in a plastic case,
> 2 fuses and a PTC at least and it's still not a good idea as I suggested
> before. Putting it in a case is still not that safe since the buffer
> on the table is always a part of the electric circuit.
>
>> and it's not all that
>> much unsafer then a commercial power supply.
>>
> Guaranteed that the commercial supply will have current limiting. Or it
> couldn't be sold commercially, period. A Zener is a simple method of
> current limiting but may not be cheap, a feedback circuit is better. At
> which point, building the circuit homebrew to save time and money is
> questionable, although it still has great educational value.
>
> ## Jonathan Cline
> ## jcline@ieee.org
> ## Mobile: +1-805-617-0223
> ########################
>
>
>
>
>
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