Re: The open-source Pacer PCR

Um, maybe not.

A nine volt battery can easily put out 10 amps or more into a short circuit.

That probably exceeds the current sourcing of any of your lab power supplies

used to run gels.

Doubling the width of the gel will cut the resistance in half, that part is true.

That will double the current.

But so will doubling the voltage (Ohm's Law applies to gels if you keep the

voltage and current fairly low).

Clipping nine volt batteries together is a cheap and easy way to get high

voltage. I bought a few hundred cheap heavy duty nine volts and clipped

them together in series when I was playing around with homemade Geiger

counters and ionization chambers. At 69 cents each when bought in hundreds,

it's hard to find a more stable high voltage source. No filtering needed, and

for most applications that need 1,000 volts, the current draw is so small that

the battery life is basically the shelf life.

200 volts is just 20 or so batteries (at low current they are 10 volts). They clip

together nicely in series.

I am not an expert at running gels, so I defer to Cathal's experience.

But if a 5 cm run does not get you the resolution you need, you can simply use

twice as many batteries in series to get double the voltage. But instead of

making the gel narrower, why not make it shallower?

Suppose you have a salt solution of 0.25 moles per liter.

It has a resistivity of about 8 ohms centimeter moles per liter.

So a 10 cm long run that is 10 cm wide and 1 cm deep will have a

resistance of about 32 ohms.

At 100 volts, that is about 3 amperes, which, as Cathal would be quick to

point out, is probably a lot to expect from a dozen nine volts in parallel.

But at a millimeter deep, it is only 300 milliamperes, which is easy to do

with batteries. And if we then narrow it (as Cathal suggested) to 5 cm, we

are only drawing 150 milliamperes, and life gets even simpler.

And, of course, if your solution has less than 0.25 moles per liter of salt,

then the resistance is even higher, and the current requirements are even lower.

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On Mon, Dec 12, 2011 at 1:21 AM, Cathal Garvey <cathalgarvey@gmail.com> wrote:

The width of the gel, then, corresponds to current draw, so if you're
using 9V batteries (which have sweet F-all current to give), you're
better off with narrow gel rigs.