Gamry Interface 5000, Руководство оператора

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Appendix F: Heat in Interface 5000 Multichannel Systems--Discharging a Battery
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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 5000 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 8.5 to 9.5 W, but we may assume 9 W.
The lower portion of the figure shows the power effects that come from the Interface 5000 power amplifier
polarizing the electrochemical cell. This model assumes no power reduction as described below. V+ and V
– are
8.5 V power supplies generated within the Interface 5000. 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 42.5

. This creates a steady-state current of about 200 mA through R1 and R2, resulting in “quiescent” power
dissipation of a little less than 3.5 W.
The values of R1 and R2 change when current flows into or out of the cell. When +5 A flows into a zero-
voltage cell, R1 is around 1.7
 and R2 becomes very large. In this condition, power-amplifier dissipation is
42.5 W and the total device power is about 51.5 W.
The Interface 5000’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.
The Interface 5000 contains a special power-reduction mode that limits power dissipation and temperature rise
during discharge of energy-storage devices. The power-supply voltage on the MOSFET discharging the cell is
lowered to approximately 2.3 V. In the discussion that follows, power and temperature are calculated both with
the full power-supply voltage and the reduced voltage. Power dissipation within the Interface 5000 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 5000, 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 or other energy-storage devices (such as
an electrical double-layer capacitor or a fuel cell). In the calculations that follow, 9 W of load-independent
power dissipation is assumed.
Let’s look at the power in an extreme high-current battery discharge:
•
With a single battery with a voltage of 6.5 V, the maximum power dissipation in the instrument assuming a
− 8.6 V power supply is 15.1 V × 5 A + 9 W= 84.5 W.
• With the same battery and reduced power-supply voltage of − 2.3 V, the instrument’s power dissipation is
8.8 V × 5 A + 9 W = 53 W.