SMU — Source Measure Unit¶

Auto-generated reference

For the definitive command list extracted directly from source code, see REPL Command Reference.

Controls source measure units (SMUs) that can both source a precise voltage and simultaneously measure current (or vice versa).

Multiple SMUs are named smu1, smu2, etc.
Address a specific SMU directly: smu1 set 5.0 — or use use smu1 then smu set 5.0

NI PXIe-4139

±60 V / 3 A four-quadrant SMU (20 W source, 12 W sink). Uses the nidcpower Python package (not VISA/SCPI). Install with: pip install nidcpower. Output range: −60 V to +60 V, −3 A to +3 A DC (10 A pulse).

PXIe boot order

The PXIe chassis must be powered on before the host PC boots. PXIe devices are enumerated during BIOS/POST — if the chassis powers on after the PC, the instrument will not appear in scan or list. Power cycle the PC with the chassis already on to fix this. See Troubleshooting for details.

smu on / smu off¶

Enable or disable the SMU output.

smu on     # enable output
smu off    # disable output

Safety limits set with upper_limit / lower_limit are enforced before enabling.

smu set¶

Set output voltage and optional current limit.

smu set <voltage> [current_limit]

Parameter	Required	Values	Description
`voltage`	required	−60.0 – 60.0 (V)	Target output voltage.
`current_limit`	optional	0.0 – 3.0 (A)	Current compliance limit (always positive). If omitted, the existing limit is kept.

smu set 5.0          # set 5 V, keep existing current limit
smu set 5.0 0.01     # set 5 V with 10 mA current limit
smu set -12.0 0.1    # negative voltage for four-quadrant operation

Warning

Setting a voltage without a current limit uses whatever limit was set previously (default: 10 mA). Always specify the limit when powering an unknown DUT. DC source power is capped at 20 W and sink power at 12 W by the hardware.

How to sink current

Use smu sink <current> for sinking and smu source <current> for sourcing. The SMU acts as a programmable electronic load when sinking. By the standard 4-quadrant SMU convention, negative current = sink from DUT (current flows into the SMU) and positive current = source into DUT. smu sink rejects non-negative input with a ValueError; smu source rejects non-positive input. See smu sink / smu source below.

smu meas¶

Take a single measurement and print the result.

smu meas [v|i|vi]

Mode	Alias	Description
(no arg) or `vi`	`both`, `all`	Atomic V+I+compliance read in one call (default)
`v`	`volt`, `voltage`	Measure actual output voltage only
`i`	`curr`, `current`	Measure actual output current only

smu meas       # prints e.g.  V=5.000012V  I=0.009987A
smu meas vi    # same as above; appends [COMPLIANCE] if current-limited
smu meas v     # prints e.g.  5.000012V
smu meas i     # prints e.g.  0.009987A

Tip

smu meas / smu meas vi performs an atomic measurement that reads voltage, current, and compliance state in a single driver call — use this instead of separate v and i calls when you need a consistent snapshot.

smu set_mode¶

Switch the SMU between voltage-source and current-source modes.

smu set_mode voltage <v> [current_limit]
smu set_mode current <i> [voltage_limit]

Parameter	Required	Values	Description
`voltage` / `current`	required	mode keyword	Select sourcing mode. Alias: `v` / `i`.
second arg	required	float	Setpoint in volts (voltage mode) or amps (current mode, −3.0 to 3.0).
third arg	optional	float	Compliance limit for the opposing quantity (always positive).

smu set_mode voltage 3.3 0.1    # source 3.3 V with 100 mA current limit
smu set_mode current -0.010 3.0 # sink 10 mA from DUT (electronic load), 3 V compliance
smu set_mode current 0.5 5.0    # source 0.5 A into DUT, 5 V compliance

Current mode polarity

In current mode, negative current = sink (draw current from the DUT, acting as a load) and positive current = source (push current into the DUT). This is the standard four-quadrant SMU convention. smu set_mode current accepts either sign. Use the dedicated smu sink / smu source commands below when you want strict sign enforcement.

Safety limits (upper_limit / lower_limit) are enforced before the mode switch is applied.

smu sink / smu source¶

Configure current mode with strict sign enforcement.

smu sink   <current_A> [voltage_limit_V]   # current must be negative
smu source <current_A> [voltage_limit_V]   # current must be positive

Both commands route to set_current_mode after validating the sign. They follow the standard 4-quadrant SMU convention: negative current = sink (current flows into the SMU from the DUT, SMU acts as a load) and positive current = source (current flows out of the SMU into the DUT).

Command	Accepts	Rejects
`smu sink <i>`	`i < 0` (e.g. `-0.05`)	`i >= 0`; raises `ValueError("Sink current must be negative ...")`
`smu source <i>`	`i > 0` (e.g. `0.05`)	`i <= 0`; raises `ValueError("Source current must be positive ...")`

smu sink -0.010 3.0    # sink 10 mA from DUT, 3 V compliance
smu sink 0.010         # ERROR: Sink current must be negative
smu source 0.5 5.0     # source 0.5 A into DUT, 5 V compliance
smu source -0.5        # ERROR: Source current must be positive

When validation fails, the SMU's mode and setpoint are left unchanged.

Recording measurements

Use assignment syntax to measure and store the result to the measurement log in one step:

<label> = smu meas <v|i> [unit=<str>]

Parameter	Required	Values	Description
`label`	required	string, no spaces	Name for this entry in the log.
`v\\|i`	required	`v`, `i`	What to measure.
`unit=`	optional	string	Unit shown in `log print`. Defaults to `V` or `A`.

smu_vout = smu meas v unit=V
smu_iout = smu meas i unit=A
calc power smu_vout * smu_iout unit=W

See Log & Calc for full details.

smu compliance¶

Query whether the SMU is currently in compliance (i.e. the output is being current- or voltage-limited by the instrument's protection circuit).

smu compliance

Prints Not in compliance when the DUT is drawing within limits, or a warning banner IN COMPLIANCE - output is current-limited when the limit has been hit. Use smu meas vi to see the compliance flag alongside a measurement.

smu source_delay¶

Get or set the settling delay the SMU inserts between applying a new source value and taking a measurement.

smu source_delay           # read current delay
smu source_delay <seconds> # set delay (0 – 167 s)

smu source_delay           # prints e.g.  source_delay = 0.0020 s
smu source_delay 0.05      # set 50 ms settle time
smu source_delay 0         # disable delay

smu avg¶

Get or set the number of samples averaged per measurement.

smu avg        # read current value
smu avg <N>    # set to N samples (integer ≥ 1)

smu avg        # prints e.g.  samples_to_average = 1
smu avg 10     # average 10 readings per measurement call
smu avg 1      # back to single-sample mode

Higher values reduce noise but increase measurement time proportionally.

smu temp¶

Read the internal temperature sensor of the instrument.

smu temp

Prints the temperature in degrees Celsius, e.g. 34.5 degrees C. Useful for confirming the unit has warmed up to a stable operating temperature before precision measurements. Expected range for a powered-on PXIe-4139 is approximately 20 – 60 °C.

smu get¶

Show the current voltage setpoint, current limit, and output state.

smu get

Prints: Setpoint: <V>V @ <A>A, Output: ON|OFF

smu state¶

smu state <on|off|safe|reset>

Value	Effect
`on`	Enable output
`off`	Disable output
`safe`	Disable output
`reset`	Disconnect and re-initialize the session

Safety limits¶

upper_limit and lower_limit work with SMUs using the same voltage and current parameters as PSUs:

upper_limit smu voltage 5.5     # SMU output ≤ 5.5 V
upper_limit smu current 0.1     # current limit ≤ 100 mA
lower_limit smu voltage -0.3    # SMU output ≥ −0.3 V

See Safety Limits for the full reference.

Typical workflow¶

upper_limit smu1 voltage 6.0
upper_limit smu1 current 0.05

smu1 set 5.0 0.02     # 5 V, 20 mA limit
smu1 on
sleep 0.5

vout = smu1 meas v unit=V
iout = smu1 meas i unit=A
calc power vout * iout unit=W

log print
smu1 off

Tips & Gotchas¶

Quirks specific to the supported SMU model.

NI PXIe-4139¶

PXIe boot order matters. The PXIe chassis must be powered on before the host PC boots. If the PC boots first, the PXIe card will not be detected. Power cycle the chassis and reboot.
4-quadrant operation. Can source and sink both voltage and current. Be careful with polarity.
Compliance limits. Always set current compliance when sourcing voltage, and voltage compliance when sourcing current. The toolkit enforces this.
nidcpower required. The NI-DCPower Python driver must be installed.