# Does setting a junction's instance IC change its other parameters?

Check the model-card critical current and effective instance area. Instance IC can select a scale factor rather than changing only one isolated value.

## Compatibility

2.9.0 release target; review-build/source evidence

Published guidance with release-applicability limits. Confirm package applicability before following this guide.

## Start with an explicit model

Specify the model-card critical current explicitly when reproducing a circuit. Historical releases have used different defaults, so copying an old explanation's default into a current calculation can change the model.

The maintained reference documents effective area scaling: critical current and capacitance scale with area, while normal resistance scales inversely. An instance critical-current specification can determine that effective scale relative to the model card.

## Check the intended physics

Record model and instance settings together, including area, IC, RN, capacitance and RTYPE where applicable. Do not assume that specifying an instance IC leaves all other effective device parameters unchanged.

Validate a small junction circuit before interpreting differences in a larger network. Detailed RTYPE/subgap and temperature behaviour still need the matching model reference.

## Sources and verification details

Recorded inputs/builds only. Complete walkthrough and final-package acceptance remain separate.



- support:SUP-022: Historical junction scaling explanation
- josim-pro:docs/manual_source/sections/syntax.tex: Josephson junction parameters and area scaling
- owner:technical-review-20261102: JoSIM-Pro 2.9 chosen as the public starting path; release availability must be checked before publication

- The owner accepted the modelling explanation on 2 November 2026. Applicable release/platform and unrecorded walkthrough checks remain pending.

## Related guides

- [A junction model is missing or behaves like the wrong model](/knowledge-base/josim-model-syntax)
- [RN, R0 and temperature give unexpected junction resistance](/knowledge-base/josim-junction-resistance)