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I'm in need of help understanding a relay schematic from an automotive relay that has broken on my Triumph. I can find replacement relays of the same model number, but I was curious as to why the relay is wired differently to most other relays I have come across.

It is a Hella 4RA 940 010-71. It has a resistor in parallel with the coil, but then has a diode in series with the coil as well. See the below pictures.

Photos of relay switch

Schematic of relay switch

I understand the use of the resistor, but the diode confuses me. If it is low side switched, then when switched off, surely a huge positive voltage spike will appear at the negative side of the coil, which the diode won't block, as it is forward biased.

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    \$\begingroup\$ Maybe it's a SSR? Proper datasheets don't seem to be something that Hella takes pride in... Could be some custom part perhaps. \$\endgroup\$
    – Lundin
    Commented Jan 3, 2023 at 11:27
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    \$\begingroup\$ Maybe the Diode is there to prevent accidental "reverse-swiching" as it is somewhat safety related. E.G battery connected in reversed or something. \$\endgroup\$ Commented Jan 3, 2023 at 11:46
  • \$\begingroup\$ It may be something like @ElectronicsStudent suggests for the reasoning behind the diode. The resistor takes the place of a freewheeling diode and alows the relay to open faster, extending the life. The spike will not be huge it will be icoil * R at the maximum. \$\endgroup\$ Commented Jan 3, 2023 at 17:20
  • \$\begingroup\$ Funny thing, I can't say I've ever seen a relay with that series diode... until today when I'm working on something and there one is. Different manufacturer, EGIS, so looking that up I find that the series diode is extra protection against flyback voltage egismobile.com/shop/… \$\endgroup\$
    – GodJihyo
    Commented Jan 4, 2023 at 16:36

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The voltage spike when switching off will be reduced by parallel resistor. The maximum will depend on coil relay resistance and this resistor ratio. The series diode could be useful in case one switch actuates 2 relays with different + 12 V supply circuits (for example ignition and auxiliary). That diode will block current going between them.

Another difference from a parallel resistor and reverse diode is the current (and also the contact) will decay faster, as more energy from the coil is lost when switching off. See simulation with resistor 3.3 x Rcoil and reverse diode, current and voltage spike. Anyhow it will be only a fraction of miliseconds. enter image description here

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