HEKA Elektronik EPC 10 USB, Hardware Manual, Page: 45 / 81

6. Operating Modes
The
EPC 10 USB
is fundamentally an instrument for measuring small electrical currents. It uses a current-
to-voltage (I-V) converter circuit to convert the currents to an analog voltage, which is then made available at
the current monitor outputs for display or recording. At the same time that pipette currents are being recorded,
the potential must be specified, and the various operating modes of the
EPC 10 USB
correspond mainly to the
different ways of controlling that potential.
6.1 Voltage Clamp Mode
This is the basic patch clamp mode, and is implemented by the circuitry shown in the figure below. The pipette
potential is derived from the signal applied to
Stim-DA , with a variable offset added from the V-membrane control.
The sum of these two sources is displayed and monitored as the V-mon signal. Before being applied to the pipette
a further variable offset is added from V
0
(which includes the value of LJ, see chapter 7 on page 43).
6.2 Current Clamp Mode
The current clamp mode can be used to measure the resting potential or spontaneous action potentials in a
whole cell recording. The measured membrane potential will be shown on the
V-mon display in the acquisition
software. The same signal is available at the Voltage Monitor output at the front panel of the
EPC 10 USB
.
For stimulation, a commanded current can be injected while the pipette potential is measured. The commanded
current is determined by the sum of the voltages from the ”External Stim.Input CC” and the
I-membrane control
in the software.
Figure 6.2: Current Clamp mode
In current clamp mode, the input of the probe acts as an high-impedance voltage follower circuit. The feedback
resistor is used for stimulation in current clamp mode. This can be done by applying a defined voltage to the
feedback resistor.
Figure 6.1: Voltage Clamp mode