Why not use a latching relay to power the headlights instead of two regular relays?

[Anonymous1969]

I was wondering why why not use a latching relay to power the headlights instead of two regular relays? It seems less complicated to me.

 

[Anonymous1969]

Added note, I think the relay would be called a DPST relay.

 

[DPG]

Added note, I think the relay would be called a DPST relay.

A DPST relay is not necessarily latching, or could still be a standard relay.

 

[Avo Mk8]

Normally one relay for high beam, one relay for low beam?

 

[brianmoooore]

There's no practical reason why a change over relay couldn't be used instead of two single pole relays, but it's debateable whether it's less complicated.
It's largely irrelevant these days, since there's a move to electronic solid state relays, which aren't available as a changeover.

 

[Avo Mk8]

My first reaction to the question was that you couldn't do it with a DPDT relay, as that didn't provide 'off', but if you are prepared to feed the full current through the headlight switch, you can. Turns out Jaguar did something similar back in the 1980's (Mk vi)
http://www.jag-lovers.org/lumps/tech/basics/relays2b.htm (not secure, but seems bona-fide - I know, old website, could't see same content on current site!)
This describes their 'latching' relay for the dipping function.
But the photo's show what appears to be a single pole latching relay, so I don't think this is exactly what the OP meant.
I suspect this was not popular because a 'special' relay like this is more expensive, possibly less reliable, and maybe needs beefier cables in more of the circuit, and beefier switches to carry the full load current.

 

[Anonymous1969]

First, thanks for all the responses.

possibly less reliable, and maybe needs beefier cables in more of the circuit, and beefier switches to carry the full load current.

Really? I thought they were like regular relays that switched the high current via a low current switch.

 

[brianmoooore]

First, thanks for all the responses.

Really? I thought they were like regular relays that switched the high current via a low current switch.

If you used C/O relay to switch between main and dipped beams, you'd still need another single pole relay to actually switch the headlamps on or off. More complex than a single relay for each function.

 

[Avo Mk8]

First, thanks for all the responses.

Really? I thought they were like regular relays that switched the high current via a low current switch.

Yes relays switch a high current with a low current to actuate them.
But you were asking about using a single relay for headlights, rather than the usual two relays.

A way you can sort of make that work is as described in the link I gave, where the current for both left and right lamps goes through the dashboard switch, and the relay acts as the dip switch.
If you use just one conventional relay to switch between main and dipped beam lights, how do you turn the lights off if not directly from the dash?
With another relay! Back to using two!

When you do route the full current through the dash switch, as Jaguar did below, you are not benefitting from the advantage of a relay (as you describe), which is why I said "beefier wiring"!

I'm not clear that one could use a single relay for the function: off/on dipped/on main/ off

 

[Anonymous1969]

Whoa, I really don't know if I meant latching persay or not, but I see I made mistakes with either way. You guys addressed both. Thanks for helping.