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PA-4000 User Manual Ch3: Rev.1-1/05
3.1.5
Pre-Pulse Load Estimator
The load is what the pulse voltage “sees” and it is a function of the material being
electroporated. In a cuvette, this is a function of the cuvette plate spacing, the cuvette plate
area and the conductivity of the aqueous solution. In tissue, it is a function of the electrode
spacing and the conductivity of the tissue. The user may not know these. In order to estimate
the load so an estimate of pulse amplitude can be provided, the PA-4000 uses a pre-pulse
generated by the computer before the high voltage is turned on and the protocol is started. This
pre pulse is 2.2 µs in duration and 2 volts in amplitude. The pulse is placed across the
electrodes and the resulting current measured. Since the voltage is known, the resistance is
calculated. This is done by the microprocessor and presented at the bottom of log report, as a
resistance in ohms and a conductance in siemens, see Figure 3-1.
The pre-pulse data is also used by the microprocessor to detect an output short or
excessive conductivity of the ionic solution. If the load estimate is less than 10 ohms the high
voltage will not be enabled and an “Output Shorted” message will appear.
3.1.6
Pulse Droop
As explained in Section 2.2.3, rectangular wave electroporators also use storage or
reservoir capacitors. When the high voltage is turned on this capacitor is charged to an initial
voltage by the internal power supply. A pulse is generated when the capacitor is momentarily
connected to the load and electrons in the capacitor are drained off like water running out of a
reservoir. After a set time, the pulse width, the capacitor is disconnected from the load. Long
pulses allow more electrons to run out and the voltage decreases (reservoir level drops) just as
in an exponential discharge pulser. In rectangular wave electroporators the maximum pulse
width is usually defined at the point that the pulse voltage at the end of the pulse is 95% of the
initial voltage level. This voltage drop is called droop and is determined by the size of the
internal reservoir capacitor and the load resistance. Droop is calculated by:
Droop (%) =
pulse width (sec onds )
C ∗ R load
When using highly ionic loads, the electrons are depleted faster. Caution is required is setting
pulse widths in these situations.
Typical pulse widths yielding 5% droop or less in the PA-4000 are:
3-6
Load
ohms
10
Low Range
4 to 400 volts
High Range
5 to 1100 volts
420 µs
150 µs
20
300 µs
40
840 µs
1.68 ms
100
400
4.20 ms
20.0 ms
600 µs
1.50 ms
6.00 ms
Cyto Pulse Sciences, Inc. P. O. Box 609, Columbia, MD 21045 410-787-1890