I am planning to use HVIL for an electric vehicle. When I searched for it, I found a diagram in Tesla's HVIL Diagnostic Guide. They use a current source to check whether the interlock loop is open or not.

Is there any reason to specifically need a current source to check this, or can I check the interlock loop using just a voltage source?

Tesla interlock loop diagram


1 Answer 1


The intent is that the interlock works reliably irrespective of the number of elements in the loop. Specifying a current source automatically adapts the voltage to the number of elements, whereas a voltage source would have to be tuned to the number of elements (effectively becoming a current source!).

You can also use the actual voltage to check the number of elements in the loop, and ensure there isn't a short.

  • \$\begingroup\$ Can't you do the reverse by having a voltage source and checking the actual current? \$\endgroup\$
    – user253751
    Feb 24 at 19:17
  • 2
    \$\begingroup\$ You could, but with a constant current source you get equal voltage steps for each unit on the path, whilst for a voltage source, the steps get smaller and smaller for each unit you add. If you have circuitry in each unit to check the loop state, the constant current approach gives a stable threshold irrespective of the number of units. \$\endgroup\$
    – colintd
    Feb 24 at 19:55

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