NOTE: Utilizing Automatic Crossover
Certain electrophoretic techniques require the careful adjustment of operating limits
to utilize a function of this power supply known as automatic crossover. Automatic
crossover is a method used when the experiment requires two or more modes of
operation during the course of the experiment.
The most common example of a technique which requires this function is semi-dry
electrophoretic transfers of proteins or nucleic acids. Semi-dry transfer chambers
consist of two closely spaced parallel plates which serve as electrodes. Positioned
between these plates is a sandwich consisting of buffer – saturated filter paper
sheets on the outside and the gel and charged membrane on the inside. Typically,
the transfer process is most efficiently accomplished by applying a constant current
flow between the two plates. As the transfer progresses, the buffer in the filter
paper begins to break down. This leads to an increase in overall resistance
between the two plates.Since the power supply is trying to maintain constant cur-
rent, it increases the voltage output to compensate for the increased resistance. Left
unchecked, the increasing voltage would eventually reach a potential great
enough to arc over between the plates, resulting in damage to the chamber and
experiment failure.
Automatic crossover can be used to prevent this unfortunate incident from
occurring. Set an operating limit for voltage at a value below the arc – over
threshold (for example, a limit that is 10% above the starting voltage). As the
voltage increases during the experment, it will eventually reach the operating
limit for voltage and “automatically crossover” to constant voltage operation.
At this point, the current will start to drop as the buffer breaks down, but the
experment and chamber will be unharmed.
9
6.
Gently press the D.C. Output On
key (24). The Red D.C. Output
On light (23) should illuminate
and the D.C. output as indicated
by the three displays (3, 5, 7)
should rapidly increase until the
operating voltage limit is
reached. At this point the Yellow
At Limit light (volts) (2) should be
the only “at limit” light illuminat-
ed.
7.
Once the power supply has
reached the operating voltage
limit, note the actual values
displayed for milliamps and
watts.
8.
Add 10 watts and 15 milliamps
to the actual values noted
above.
9.
Rotate the Milliamps Adjustment
Knob (6). The power supply will
automatically switch to the
Display Set Mode and the
Yellow Set Display light (18) will
be illuminated. Adjust the
Milliamps Adjustment Knob
downward until the new limit
value has been achieved.
Repeat this process with the
Watts Adjustment Knob (9) .
Once set to the new operating
limits, the power supply will auto-
matically return to the Actual
Display Mode and the Yellow
Actual Display light (17) will be
illuminated.
6, 18,
9, 17
24, 23,
3, 5,
7, 2
On
Off
DC Output
Milliamps
Watts
8
NOTE:
It may be necessary to readjust the watt or milliamp operating limit during
the course of the separation process to ensure that the entire procedure is performed
at a constant voltage. This readjustment, if necessary, is required to compensate for
large changes in resistance which occur during certain types of electrophoresis.
THERMO EC HIGH VOLTAGE POWER SUPPLIES
THERMO EC HIGH VOLTAGE POWER SUPPLIES
