Source Limits - Keithley 2450 Reference Manual

Section 2: General operation

Source limits

The source limits (also known as compliance) prevent the instrument from sourcing a voltage or
current over a set value. This helps prevent damage to the device under test (DUT).
The values that can be set for the limits must be below the setting for the overvoltage protection limit.
This limit can also be restricted by the measurement range. If a specific measurement range is set,
the limit must be more than 0.1 percent of the measurement range. If not, an event is generated and
the limit is automatically changed to an appropriate value for the selected range. If you set the
measurement range to be automatically selected, the measurement range does not affect the limit.
If you attempt to change the source limit to a value that is not appropriate for the selected source
range, the source limit is not changed and a warning is generated. You must change the source
range before you can select the new limit.
The lowest allowable limit is based on the load and the source value. For example, if you are sourcing
1 V to a 1 kohm resistor, the lowest allowable current limit is 1 mA (1 V/1 kohm = 1 mA). Setting a
limit lower than 1 mA limits the source.
For example, assume the following conditions:
•
Current limit 10 mA
•
Instrument sources a voltage of 10 V
•
DUT resistance of 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 10 mA, 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.
In steady-state conditions, the set limit restricts the instrument output unless there are fast transient
load conditions.
If the source output exceeds the source limit:
•
On the Home screen, LIMIT is displayed to the right of the source voltage.
•
The Source value changes to yellow.
•
The Limit value on all swipe screens changes to yellow.
The source is clamped at the maximum limit value. For example, if the measurement limit is set to 1 V
and the measurement range is 2 V, the output voltage is clamped at 1 V.
For additional details on using limits, see
Setting source limits
Using the front panel:
1. Press FUNCTION and select the source and measurement combination.
2. On the Home page, select the button next to Limit.
3. Set the value.
4. Select OK.
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Model 2450 Interactive SourceMeter® Instrument Reference Manual
Operating boundaries (on page 4-4).
2450-901-01 Rev. B/September 2013