Gamry Instruments Interface 1000, Operator'S Manual

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Appendix F: Heat in Interface 1000 MultEchem Systems
Appendix F - 2
Potentiostat/Galvanostat Power Model
A potentiostat requires a somewhat more sophisticated power-dissipation model. A simple potentiostat power
model can be found in Figure E-1. The electrochemical cell has a voltage Vcell that plays a role in the
instrument’s power dissipation.
The variable resistors take different values depending on the potentiostat/galvanostat’s operating conditions.
The upper portion of the model describes a fairly constant power draw in the Interface 1000 circuitry. This
power comes from the normal operation of the instrument: running its microprocessor, powering its
operational amplifiers and D/A and A/D converters, etc. There is some variability in this power depending on
signal levels in the analog circuitry, probably from 6 to 10 W, but we can assume 8 W.
The lower portion of the figure shows the power effects that come from the Interface 1000 power amplifier
polarizing the electrochemical cell. V+ and V− are 24 V power supplies generated within the Interface 1000.
R1 and R2 model the power MOSFET transistors that control current flow through the cell.
When the cell current and cell voltage are both small, R1 and R2 have approximately equal values: about 240
Ω. This creates a steady-state current of about 100 mA through R1 and R2, resulting in “quiescent” power
dissipation of a little less than 5 W.
The values of R1 and R2 change when current flows into or out of the cell. When +1 A flows into a zero-
voltage cell, R1 is around 24 Ω and R2 becomes very large. In this condition, power-amplifier dissipation is 24
W and the total device power is about 32 W.
The Interface 1000’s total power dissipation with the cell polarized is approximated by:
2
+
1
+
3
=
2 2
+
R
V Vcell
R
Vcell V
R
Vin
P
With large cell currents, the current from one leg of the circuit dominates power-amplifier dissipation and total
device power becomes:
( )
Icell Vcell V
R
Vin
P +
3
=
+
Or
( )
Icell V Vcell
R
Vin
P +
3
=
Note that Vcell can be either positive or negative.
Power dissipation within the Interface 1000 raises the temperature within the unit and on the exterior of the
unit. In rare cases, the temperature increase can create an over-temperature event, upon which the unit shuts
off the cell current and enters a protective latch-up state. You can only recover from this state by turning off the
power on the Interface 1000, waiting for the device to cool, and turning the power back on.
Discharging a Battery
The highest power application for a potentiostat is discharging a battery stack (or a stack of other energy storage
devices such as electrical double-layer capacitors or fuel cells). Unlike single cells, stacks can have voltages that
stress the power-dissipation capabilities of a potentiostat. In the calculations that follow, 8 W of load-
independent power dissipation is assumed.
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−
−
−
−
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