Current for gel electrophoresis - Fried my DNA I think (Feb/21/2008 )
Realised today that the power pack (Major Science) I'm using has a current selector and was set to 700. That may be the reason why most of my gels were failures. The inconsistencies may be this current. Does anyone use power packs that comes with the Volt, Time, and Current buttin? If yes, what is the current used?
we set our power supply for DNA gels between 70-80V for small gels.
Yes, I'm using 6V/cm for my 1% gel to get a PCR product of about 1.4KB. The problem seems to be in the current which is set at 700 ampere.
Yes, the current setting is much to high. (I^2)R heating would be alarming. And any salt contamination would cause the gel to run funny. How about dropping it to 240mA (maximum)?
But very cool power pack though.
Are you using the correct power supply???...This could sound stupid, but usually there is 2 kinds of power supply in a lab. one is use for the agarose electrophoresis and the V limits is 300 or less and the one that is use for the acrylmide gel that use more than 300V both looks pretty similar so check first whats the limit of the power supply. Other thing is that some power supply have a low limit/high limit key.
just because your maximum current was set to 700mA doesn't mean that you came anywhere close to the maximum. it is unlikely that you reached it at voltages that low.
as pernesblue said, you may want to (remember to) set the limit lower so that you can avoid excessive heating.
did you happen to notice the actual current reading during the run?
Yes, checked it already (http://www.major-sci.com.tw/pviewitem2.asp?sn=634&area=39&cat=166). Its for electrophoresis and not PAGE. So, 240mA will do just fine? All this while I thought its the voltage that's the factor.
A maxima of 200mA to 240mA should be fine. I get very nice gels (although they run a little slow) with a maxima of 100mA
The standard gel is run at a set constant voltage with a maximum current setting. This maximum current setting prevents excessively high currents from running through the gel causing overheating(agarose melting) and the gel to run strangely.
As a gel runs, its temperature increases. Increased temperature reduces the gel's resistence. So at a set voltage, reduced resistence causes an increase in current flow (V = IR). If you set a maximum current setting, the machine will work to prevent the current running through the gel from exceeding this current setting. The powerpack does so by reducing the power to the gel (P=VI). Less power at set voltage means less current. And less current means the less heating as heating is (I^2)R. The gel cools down, resistence increases and power goes up again to maintain the set voltage.
So as mdfenko has mentioned, while having a 700mA current maximum doesn't mean the gel comes anywhere close to it, it does mean the current running through your gel has a lot of head space to climb.
Most simple power packs have an in built maximum current setting around 200 to 240mA. However your machines is more advance. Bigger gels can handle higher currents. So if you have a very big gel you can increase the current maximum, allowing the gel to run faster.
Voltage is the factor for running gels, but you need a cap on the current to prevent the gel from overheating and even melting.
Got it at last. Thanks a lot people!