Section 4: Theory of operation
2606B System SourceMeter® Instrument Reference Manual
4-2
2606B-901-01 Rev. B / May 2018
Overheating protection
Proper ventilation is required to keep the 2606B from overheating. Even with proper ventilation, the
2606B can overheat if the ambient temperature is too high. The 2606B has an over-temperature
protection circuit that turns the output off if the instrument overheats. When the over-temperature
protection circuit turns the output off, a message that indicates this condition is displayed. You cannot
turn the output on until the instrument cools down.
Power equations to avoid overheating
To avoid overheating, do not operate any channel on the instrument in a manner that forces the
instrument to exceed the allowable power dissipation, which is computed using the general power
equation, shown below. Factors such as the ambient temperature, voltage and current of the
instrument channel, and operating ranges affect the instrument temperature. Exceeding the allowable
de-rated power dissipation may cause the temperature protection mechanism to engage.
You do not have t
o be concerned about overheating if the ambient temperature is ≤ 23 °C and you
are not using the 3 A range. However, if the temperature is above this level or the 3 A range is in use,
the instrument may overheat if operated in a manner that exceeds the allowable de-rated power
dissipation for the ambient temperature.
The left side of the general power equation describes how much power an instrument channel is
generating. The right side describes the cooling capacity of the instrument (which is de-rated at
temperatures above 23 °C). This equation incorporates all of the factors that can influence the power
dissipated by the instrument. It becomes possible for the instrument to overheat when the power
produced is not less than the power dissipated by the cooling system. The larger the left side is
compared to the right side, the faster the instrument will overheat.
Where:
V
OA
The instrument output amplifier voltage. This constant is 18 for the 100 mV, 1 V, and 6 V ranges.
It is 40 for the 20 V range.
V
P
The voltage level the instrument is attempting to force while at the pulse level.
When operating in quadrants I or III (sourcing power), the sign of this voltage must be positive
when used in the power equations.
I
P
The current flowing through the instrument channel while at the pulse level.
V
B
The voltage level the instrument is attempting to force while at the bias level.
When operating in quadrants I or III (sourcing power), the sign of this voltage must be positive
when used in the power equations.
I
B
The current flowing through the instrument channel while at the bias level.
T
AMB
The ambient temperature of the instrument operating environment.
P
DER
= T
AMB
- 23
This factor represents the number of watts the instrument is derated when operating in
environments above 23 °C. The maximum output power of each instrument channel is reduced by
1 W for each degree C above 23 °C.
P
DER
is 0 when the ambient temperature is below 23 °C.
P
OC
= Power dissipated by other channels in the unit.
This is the sum of the left side of the power equation for each of the other three channels in the
unit.
Duty cycle
The value from 0 to 1 that represents the amount of time spent at pulse level versus bias level:
For DC operation where the off state is 0 V or 0 A, Duty Cycle = 1.
I
P
and V
P
The voltage and current levels when the instrument is on.
