Re: Tell me about your lab

On Nov 8, 11:31 pm, Patrik <patr...@gmail.com> wrote:
> On Nov 7, 10:47 am, Cathal Garvey <cathalgar...@gmail.com> wrote:
>
> > Rescued Bio-Rad Power Supply (Attn hardware hackers: Open Source PSU needed)
>
> I've been wanting to explore how well this one works - sure seems very
> simple:
>
> http://www.science-projects.com/PowerSupply.htm
>
> You'll get a PWM output, but that may not matter for gel
> electrophoresis. Nothink that a hefty capacitor couldn't solve anyway.

Power supplies for gel electrophoresis have previously been
discussed at length. Several suggestions & recommendations
are listed in the FAQ.

The circuit in the link above will perform OK since the
rectifier is stated as "<0.2 amp". That is low to OK. Smaller power
consumption for average electrophoresis boxes is in the range
40W - 60W as previously discussed on this group (do a search)
and larger supplies seem to be about 200W (apparently overkill).
The circuit yields full wave rectified output, not PWM output
(although
PWM has different meanings, typically it has square edges). For
electrophoresis, rectified output is fine. Full wave rectified
output
means the sine wave has the negative portion flipped to the positive.
Likely with that circuit the fuse will blow when the dimmer is
accidentally turned to max or when the leads are accidentally
shorted. It is not a very safe circuit.
Previously I discussed using a 120VAC to 12VDC converter +
12VDC-to-220VAC inverter as input to a circuit like this which would
limit current and total power (i.e. sting yet not endanger safety).

The DNA in a gel does not care if the voltage is pulsed or unfiltered,
only that the potential exists to move it. No capacitors are needed
on the output.
Some papers exist on pulsed electrophoresis with the idea that it
runs gels slightly faster yet this effect seems debatable.

Running higher voltages than 200V does not seem to be common
for gel boxes. It is used in capillary electrophoresis (discussed
here previously). Running the gel faster will result in melting the
gel due to heat generated from the power dissipated, which is
also why alternative buffers are used (discussed here
previously) and fans are used on some gel boxes.


BTW Typical coax cable has fixed impedance of 50 ohms so it is
highly unlikely that you'll be able to store power in it regardless
of
length.


## Jonathan Cline
## jcline@ieee.org
## Mobile: +1-805-617-0223
########################

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