Section 7: Theory of operation Model 2601B-PULSE System SourceMeter Instrument Reference Manual
7-2 2601B-PULSE-901-01A April 2020
Limit principles
A limit acts as a clamp. If the output reaches the limit value, the 2601B-PULSE SMU attempts to
prevent the output from exceeding that value. This action switches the source from a voltage source
to a current source (or from a current source to a voltage source) when a limit is reached.
As an example, assume the following:
• 2601B-PULSE instrument: V
SRC
= 10 V; I
LIMIT
= 10 mA
• Device-under-test (DUT) resistance: 10 Ω
With a source voltage of 10 V and a DUT resistance of 10 Ω, the current through the DUT should be
10 V / 10 Ω = 1 A. However, because the limit is set to 10 mA, the current will not exceed that value,
and the voltage across the resistance is limited to 100 mV. In effect, the 10 V voltage source is
transformed into a 10 mA current source.
The 2601B-PULSE SMU output does not exceed the compliance limit, except for the compliance limit
conditions described in Source-measure capabilities (on page 4-1).
Power equations
Proper ventilation is required to keep the 2601B-PULSE from overheating. Even with proper
ventilation, the instrument can overheat if the ambient temperature is too high or the 2601B-PULSE is
operated in sink mode for long periods. The instrument has an overtemperature protection circuit that
turns the output off if the instrument overheats. When the overtemperature protection circuit turns the
output off, a message indicating this condition is displayed. You cannot turn the output on until the
instrument cools down.
Duty cycle with the pulser disabled
When the pulser is disabled, overheating can occur if the instrument exceeds the maximum duty
cycle. Factors such as ambient temperature, quadrant of operation, and high-power pulse levels
affect the maximum duty cycle. Exceeding the calculated maximum duty cycle may cause the
temperature-protection mechanism to engage. When this happens, an error message displays and
the instrument output is disabled until the internal temperature of the instrument is reduced to an
acceptable level.
You do not have to be concerned about overheating if the following are true:
• The instrument is used as a power source and not a power sink.
• The ambient temperature is ≤ 30 °C.
• Pulsing is not occurring in the extended operating area (EOA).
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