I am trying to measure the saturation voltage of a bipolar transistor (SS8050), using a Keithley source-meter, the measurement does not measure the voltage between base and emitter correctly, with a two-wire connection the measurement is correct (figure 1), the wiring diagram is the same as in (figure 2). Am I doing something wrong? 
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- \$\begingroup\$ Welcome! Looks like you are dropping 96.9 mV in your cable at 800 mA? \$\endgroup\$winny– winny2025-04-01 11:25:16 +00:00Commented Apr 1 at 11:25
- \$\begingroup\$ Thanks for the reply. The wires on the terminals are 1.5 mm^2 length ~70cm, could they give such a big voltage drop ? maybe it's a matter of soldering the wires.... It's not the measurement circuit ? \$\endgroup\$wewq ewqewq– wewq ewqewq2025-04-01 11:40:21 +00:00Commented Apr 1 at 11:40
- 2\$\begingroup\$ The copper alone would have about 30 mV of drop. Contact resistance can easily make up the rest. There is a good reason why 4-wire measurements is the gold standard. \$\endgroup\$winny– winny2025-04-01 11:49:15 +00:00Commented Apr 1 at 11:49
- \$\begingroup\$ @wewqewqewq High level injection effects and ohmic resistance get conflated into measurements made at 800 mA. Not knowing the exact details of your procedure, I also worry about heating effects making results time-dependent and difficult to control. Can you discuss the details of your methods? (These measurements should be set to take place in region II, only, and not region I or III.) \$\endgroup\$periblepsis– periblepsis2025-04-01 23:27:32 +00:00Commented Apr 1 at 23:27
- \$\begingroup\$ @winny Yes, 0.8 Amps is quite a large enough current to cause such a voltage drop. Thanks for helping me understand this. \$\endgroup\$wewq ewqewq– wewq ewqewq2025-04-02 06:29:38 +00:00Commented Apr 2 at 6:29
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