Understanding Ohm's Law is key to understanding electronics in general, and this topic in particular. I assume you know it well, but some others on the list might not, so a short review might be in order.
The current in a circuit is the voltage divided by the resistance.
If I wanted 10 ma to go through my body, from one thumb to the other, I would first need to know what the resistance was between my thumbs. So I take my multimeter, set it to Ohms, lick my thumbs, and hold the leads. Depending on how hard I press on the leads, I get between 100,000 and 150,000 ohms. Try this yourself to get the value for your body.
Now we have the resistance, and we know the current we want, so we solve for the voltage needed to provide that current. 10 milliamperes times 100,000 ohms is 1,000 volts.
That's for filtered DC.
With AC or pulsating DC, we have the problem of capacitance at the junction between the skin and the electrode. The dry skin acts like a dielectric, leaving the electrode and the salty water under the skin as the plates of the capacitor. This dramatically lowers the resistance of the body, into the single k ohms range. This is why grabbing a 120 volts AC in your hand makes it impossible to let go -- 15 milliamperes or more are causing the muscles in your hand and arm to contract involuntarily.
Given that an 8 pack of 9 volt batteries costs about $20 and can be clipped together in series to provide 76.8 volts in a DIY-friendly, no soldering required fashion, we might look into how well that works for electrophoresis. I's still add a current-limiting resistor in series though. That 8 pack will last about 50 hours at the 10 ma rate, or 25 hours at a 20 ma rate.
You could spring for rechargeable batteries if that became an issue.
In my country, you won't get into legal trouble telling kids how much fun it is to French kiss light sockets. But that doesn't make it a good idea. If you add an isolation transformer, a current-limiting resistor, perhaps a filtering capacitor, and recommend that the device only be plugged into a GFCI socket, you might not get angry parents threatening wrongful death lawsuits. Making it difficult to touch any of the metal parts (like using a plastic drawer to slide the gel into contact with the electrodes that are protected by a plastic cover) might prevent you from suddenly throwing the device through a window when you brush against it accidentally.
You built a nice device, and it works. But when a middle school kid builds the same device for his science fair project, will he be as careful as you were? At least mention the risks, and ways to mitigate them.
On Thu, Jul 4, 2013 at 1:42 PM, Josiah Zayner <josiah.zayner@gmail.com> wrote:
The electrophoresis power supply in my lab runs at hundreds of milliamps so I feel comfortable using at least that much.
I don't think this is about what can kill you because my laptop power supply outputs ~ the same amount of wattage 90W.
I Googled "death by laptop power supply" and couldn't find anything.
I assume that sticking my right finger one plug and my left in the other plug of this power supply would not be a good idea. But otherwise how can one really hurt themselves with this? Isn't what people are saying the same for anything? for a wall socket and any power supply and many types of electronics?
I don't know if I would suggest people use EtBr at home but thousands or hundreds of thousands of professional Scientists have used it consistently for tens of years and there is not a cancer epidemic amongst us.
Gel green is $200 for 4L at 3x. For 50mL gels that's 250 gels. or $0.8 per gel
Ethidium bromide is $55 for 10mL 1% bottle. I usually add 2uL to a 50mL gel. That's 5000 gels at $0.01 a gel
That's almost 100 fold cheaper. I would use it at home if I could afford a nice UV-illuminator. Instead I use methylene blue which is super cheap but also super awful resolution-wise.
There was a recent discussion in a stem cell thread saying that there should be no working with mammalian cell culture because it could be hazardous.
What are DIYBio people allowed to do these days?
I thought this was a cool little hack that costs ideally a couple bucks, I tested it thoroughly and think it would help alot of people out, it has helped me out but I guess people will only die from using it.
I guess I will try harder and hopefully no one will suggest my next hack or experiment will kill someone. :)
And Jonathan, no I will not take that blog post off the internet, that really cracked me up though. I mean on a scale of 0 to 9 this is probably a 1 or 2. I mean have you ever browsed the instructables website? heh
Thanks again everyone for the feedback though. I think I learned that it is good to keep other people's safety in mind.On Thu, Jul 4, 2013 at 1:22 PM, Simon Quellen Field <sfield@scitoys.com> wrote:
To view this discussion on the web visit https://groups.google.com/d/msgid/diybio/CAA0yOM64Xx%2Bvghp06VON95WD6Ns6QvX8jNV7pdYwqbjHwbovHw%40mail.gmail.com.Let's look at what is needed for electrophoresis, and what is reasonably safe.Current is what moves things through the gel. The more current, the faster things move.Current is also what heats the gel. Too much current, and the gel falls apart, but even before that the results may be compromised.Current is also what kills people."http://hypertextbook.com/facts/2000/JackHsu.shtml"It seems that 60 milliamperes is an upper limit for safety, but 15 milliamperes might be enough that you lose the ability to let go of the electrode. 10 milliamperes is painful.I have seen mentions of common electrophoresis currents in the range of 20ma to 41ma. I have also seen higher currents used but presumably we want the lowest that is effective so we can remain as safe as possible while getting the job done.Electric fences pulse the current, so that if you touch the fence, it hurts, but you can then let go. This seems like a good safety measure to add to a home-brew power supply if we are using more than 15 ma.It is not hard to use transformers to isolate the power supply from ground. That is good practice. Add a ground fault circuit interrupter (your lab should have one anyway, like kitchens and bathrooms do).If you have a voltage of 220 volts, a 15 k ohm resistor will ensure you don't go above 15 milliamps. For 120 volts, use 8k ohms. This is the maximum short circuit current before the power goes through the gel -- i.e. what you would feel if you grabbed both electrodes. This means our amateur power supply would limit the size of the gel we could run, and increase the time it took for the run, in the name of safety.Measure the current going through your gel when you do a successful run. Then calculate the resistance you need in order to cut that in half (Ohm's Law), and do another run, probably for twice the amount of time. Continue reducing the current until it is below 15 ma, or until you no longer get a good run, or until the time it takes to do a run is out of the question. That will give us some data to use in deciding whether the idea of a safe electrophoresis power supply is possible( it may not be). There are no safe shotguns or chainsaws, but some people still keep them in their homes.-----On Thu, Jul 4, 2013 at 9:53 AM, Josiah Zayner <josiah.zayner@gmail.com> wrote:
I still don't understand why it would then still be a bad idea. Maybe you could email me off list to discuss more in depth.
Josiah.Zayner [at] gmailTo view this discussion on the web visit https://groups.google.com/d/msgid/diybio/CAEUkM4tjvfBtWh%2BFHOCENZZsbXO7dLn8H2SbwfiotL0v0%2BESpA%40mail.gmail.com.On Thu, Jul 4, 2013 at 11:48 AM, Jonathan Cline <jcline@ieee.org> wrote:
No you won't be fine. You need to add 2 fuses and PTC at a minimum. And even then it is about as good an idea as playing with EtBr in your kitchen, i.e. not a good idea at all.On 7/4/13 9:33 AM, Josiah Zayner wrote:So from what I am understanding from what everyone is saying (Thanks for the comments) is that if I cover the circuit in a box to allow a ground and add in inductors to isolate it from the AC it should be fine?
Most all power supplies have the final DC output at > 100VDC and a couple hundred milliamps so that should not be an issue only with this device.
On Thu, Jul 4, 2013 at 11:06 AM, Jonathan Cline <jncline@gmail.com> wrote:
Do not build the suggested circuit which runs directly from AC wall jack. The article should be taken off your blog in my opinion similarly to suggesting that people should use EtBr at home (which also should just not be done).
Search the archives for my description of a simple current-limited power supply for electrophoresis at 220V. The description is as follows. Total cost about $22-$25 from components bought online, no electronics knowledge required, some soldering is required. Uses a 120V AC to 12V DC cigarette lighter adapter, then a 60W, 12VDC to 220V AC automobile inverter (EU version, for 220VAC), an EU wall plug (to plug into the EU automobile inverter), a bridge rectifier, and a filtering capacitor. That yields 220VDC, current limited to 1A.
Don't forget that AC voltage is listed as RMS so the actual peaks are higher (1.4x).
It would be possible to put all this together on a custom circuit board, overall though not really worth the effort since the inverter uses a switching power supply internally which would require some tuning to get right. It would be a good project for an undergrad EE as a class project though, for ex. in a Power Systems class.
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