Rather simple ? regarding gel electrophoresis

[Curt Ashendel]

On 29 Nov 1994 21:58:36 GMT, 
J.Chang  <jhc at minerva.cis.yale.edu> wrote:

>just out of curiosity, i've always wondered why, when i set it for
>the same voltage, different power machines read different values
>for the mA.  when i set the machine at 100V, for example, one machine
>might read 70mA while another reads 90mA.  
>
>these machines are the same brand and model, so i don't think it could
>be because of different machine specifications.
>
>also, does this discrepancy in mA affect results (resolution, etc)?
>i mean, do some of these machines produce better results than others?

This is rather common, even with the same model and make.  We even have one 
power supply with meters  (non-digital) that changes the current and 
voltage when switching from the 1X meter range to the 10X range.

I beleive it is a consequence of the current measuring and the current 
limiting circuit, which has its own internal resistance that varies among 
power supplies.  I checked our with a volt meter, and the voltage is 
accurate, so I assume that the current indication is erroneous (due to that 
fact that the gel's resistance cannot change when it is moved from one PS 
to another).  Since current has absolutely no effect on electrophoresis (at 
any one instant), this only affects your gels with respect to how fast they 
run and how much heat they generate.  If you want reproducible conditions, 
use current control, but always set your current using the initial voltage, 
since the same types of gels will have identical (or nearly 
identical) resistances.

Curt Ashendel
Purdue University
West Lafayette, IN
ashendel at aclcb.purdue.edu