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Control Description
The OC-725C is comprised of three functional channels: A high impedance voltage
sensing channel with capacity compensation and input offset to measure membrane
potential, a current sensing channel with bath clamp to clamp the bath and measure the
membrane current, and a high voltage amplifier to deliver the clamping current.
The complete voltage clamp system consists of the OC-725C, the voltage recording
probe with electrode holder, the current sensing bath probe with silver wire electrodes,
and the current cable with electrode holder.
Front Panel
The instrument front panel is divided into six control blocks titled voltage electrode
[V
m
], bath electrodes
[I
m
], clamp, commands, and current electrode
. The instrument
rear panel has BNC connectors for gain telegraph output
and V
e
x10 output, an alarm
on/off
switch, and binding posts for circuit
and
chassis ground.
Voltage Electrode
The
voltage electrode
control block contains the voltage probe connector,
the
voltage meter, the V
m
offset control, the electrode test
and
buzz pushbuttons, and
the V
m
output bnc. Controls for negative
capacity compensation
are also located in
this block.
The
voltage meter
reports the membrane
voltage (V
m
) with a full scale range of
±199.9 mV.
The
voltage probe
connector
is a 7-pin
DIN connector for attachment of the
voltage probe to the instrument.
The V
m
offset
control is a 10-turn
potentiometer providing up to ±200 mV at
the
voltage probe input for offset of membrane junction potentials.
The V
m
electrode test control is used to determine the internal resistance of the voltage
probe. When the pushbutton is depressed, a constant 10 nA current is passed through
the voltage electrode producing a voltage drop of 10 mV/M
Ω
of probe resistance. The
measured potential is displayed by the meter or reported at the V
m
x10 output BNC
(at 100 mV/M
Ω
).
The V
m
buzz push button facilitates penetration of the voltage electrode by producing a
1 kHz square wave at the pipette tip.
The V
m
x10
output BNC reports the membrane voltage in mV multiplied by 10.
negative capacity compensation (-C) has been added to the OC-725C allowing for its
use as an electrometer in intracellular measurements. Input capacitance up to 45 pF can
be neutralized using the two associated controls. A lit LED indicates the active status of
this circuit.
