Can I replace the 62 Ohm resistor in this schematic by an inductor ? Would it be any better and how to size it ?

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  • \$\begingroup\$ 13.8 nominal? So on my car that can be 15.3... \$\endgroup\$
    – Solar Mike
    Apr 25, 2021 at 18:09
  • \$\begingroup\$ Why do you want to replace it with an inductor? The resistor will drop 3.1 V at 50 mA which will help reduce dissipation in the voltage regulator. D1 won't conduct until 26 V. \$\endgroup\$
    – Transistor
    Apr 25, 2021 at 18:13
  • \$\begingroup\$ @Transistor Becuse this guy uses no resistor at all: electronics.stackexchange.com/a/178039/35737 ... I don't know, is that a better approach ? \$\endgroup\$
    – kellogs
    Apr 25, 2021 at 19:34
  • \$\begingroup\$ S/he has made a \$ \pi \$ filter. The inductor works with its neigbouring capacitors to filter out noise. I'd go with @TimWescott's answer. \$\endgroup\$
    – Transistor
    Apr 25, 2021 at 19:37
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    \$\begingroup\$ I don't know. I'd say the design intent is to present a high impedance to very sharp edged current spikes. \$\endgroup\$
    – Transistor
    Apr 25, 2021 at 19:43

1 Answer 1


The key word in the specification of R1 is "fusible". It's not there for filtering, or at least it's mostly not for filtering. It's there to limit the current to D1; D1 is there to limit the voltage to U1. In the event that the input voltage transient is so bad that D1 can't hold up, R1 should be sized to burn up before the whole board catches on fire.

So -- no, and inductor would be contrary to the purpose. If you used an inductor there, then there wouldn't be any current limit into D1 -- either D1 would fail, or if you did use an inductor that would burn up at D1's current rating, your assembly wouldn't stand up to typical automotive over-voltage events.

  • \$\begingroup\$ How about this design - electronics.stackexchange.com/a/178039/35737 - does it provide no real safeguarding against the automotive -100V from alternator field decay, 300V inductive load switch transients, 125V load dump transients and 75V from ignition with battery disconnected? \$\endgroup\$
    – kellogs
    Apr 27, 2021 at 8:38

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