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GEN 2 (2017-2020) Ford F-150 Raptor Forums
Ford Raptor Lighting Forum [GEN 2]
Help on SPST relay to connect light bar to high beams
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<blockquote data-quote="mezger" data-source="post: 1311726" data-attributes="member: 28569"><p>The relay is controlled by a coil; when you have your high beams on, current will flow from the headlight tap through the coil to ground.</p><p></p><p>A coil resists time rate changes in current similar to the way a mass resists acceleration.</p><p></p><p>If you suddenly open a circuit with a coil in it (flip to low beams), if the coil doesn't have an alternate path to flow current, the coil will produce a very big voltage across its terminals attempting to keep the current going (in particular, it's trying to maintain its magnetic flux), and in this case the voltage on the headlight tap side will be negative. Depending on the circuit, this could exceed 100V; if it were connected to a switch, it'll cause arcing across switches' terminals. A flyback diode give the coil a path to gracefully dump its energy into the coil's internal resistance.</p><p></p><p>The mass analog would be using your brakes instead of hitting a wall.</p><p></p><p>I don't currently know what's on the headlight side of the circuit; there could be a path for the coil to dissipate its energy gracefully.</p><p></p><p>Given that I don't know what's on the other side and given the cheapness of diodes I'm taking a careful route until I either get a schematic or hear from someone knowledgeable indicating a reason why it's a non-issue to run without spike suppression.</p><p></p><p>It is notable that there are some disadvantages to running a flyback diode; in particular, the relay will be a little slower to open and it's harder on the relay's contacts.</p></blockquote><p></p>
[QUOTE="mezger, post: 1311726, member: 28569"] The relay is controlled by a coil; when you have your high beams on, current will flow from the headlight tap through the coil to ground. A coil resists time rate changes in current similar to the way a mass resists acceleration. If you suddenly open a circuit with a coil in it (flip to low beams), if the coil doesn't have an alternate path to flow current, the coil will produce a very big voltage across its terminals attempting to keep the current going (in particular, it's trying to maintain its magnetic flux), and in this case the voltage on the headlight tap side will be negative. Depending on the circuit, this could exceed 100V; if it were connected to a switch, it'll cause arcing across switches' terminals. A flyback diode give the coil a path to gracefully dump its energy into the coil's internal resistance. The mass analog would be using your brakes instead of hitting a wall. I don't currently know what's on the headlight side of the circuit; there could be a path for the coil to dissipate its energy gracefully. Given that I don't know what's on the other side and given the cheapness of diodes I'm taking a careful route until I either get a schematic or hear from someone knowledgeable indicating a reason why it's a non-issue to run without spike suppression. It is notable that there are some disadvantages to running a flyback diode; in particular, the relay will be a little slower to open and it's harder on the relay's contacts. [/QUOTE]
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GEN 2 (2017-2020) Ford F-150 Raptor Forums
Ford Raptor Lighting Forum [GEN 2]
Help on SPST relay to connect light bar to high beams
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