12v Backfeed Sanity Check

[joshik]

Not sure if this is the right place to post this but I have a 12v lighting question but it can related to any general 12v circuit application I suppose.

I wanted to tap in a relay (coil) into the wiring harness of the high beam. Since it is ground switched, it would just become a part of the "highbeam circuit".
I'm wondering if the below scenario would back feed power to the battery. I don't think so, but for some reason I am racking my brain trying to make sure I get it this right. Maybe it's late and my brain is tired

In the diagram below, do you see any issue with the circuit through the relay coil and high beam going into the switched ground when the switched ground is off?


Thanks!!

 

[Buddha.]

You’re trying to control auxiliary lighting triggered by the high beams?
I usually use a “fuse tap” right at the fuse box so I don’t have to modify factory wiring.

 

[4000lbsOfGoat]

If you notice there is more to it.
The scheme depicted allows nr. 1&2 circuits to be enabled by the masterswitch in the event highbeam is not switched on.

That doesn't change the fact that the best option for tapping the high-beam circuit would be an add-a-fuse on the high beam fuse.

 

[joshik]

Aside from OEM negative switching the high beam circuit...
Everything else looks good.


Its not say or imply negative switching the highbeam is somehow wrong. In my various vehicles have over the years, not encountered that.
btw,
Even if the highbeam circuit were positive switched, your added relay scheme will work fine either way.

yeh, the neg switched highbeam is a OEM toyota thing. Apparently, the 4Rs with no DRLs have their highbeam neg switched :/

You’re trying to control auxiliary lighting triggered by the high beams?
I usually use a “fuse tap” right at the fuse box so I don’t have to modify factory wiring.

I would have but the highbeam fuse is constantly hot since it the highbeam is neg switched.

How does that work when the highbeam circuit is depicted as negative switched ?

precisely. The neg switched highbeam makes things more challenging.

And what Verkstad stated earlier, I am trying to engage the offroad lights (which i didnt include or mention in my OP) even when the high beams are off.



I guess my concern may be due to my lack of detailed knowledge of backfeeding and how DC current would flow in the case where the neg switch high beam is off, effectively cutting off ground. Is it not possible for current to flow thru the relay coil, back up the neg side of the high beam and back to thru the + wire of the high beam due to any voltage differences...? I only ask because there is a diagram from AOB on wiring a switch to a relay thru the neg switch of the highbeams. They put a diode between the switch and the relay coil. In my scenario I don't have a switch but constant hot feeing the relay trigger. Are they expecting backflow with the neg switch is off?

 

[GlennA]

I don't see a feedback loop issue. However, I don't understand your goal.

 

[joshik]

Yes. Were no diode used, backflow could happen when the vehicle is switched off.
If the 'aux switch were left 'on', current path exists thru highbeam bulb, thru relay coil, thru 'aux switch backfeeding into the 'ign circuit. The diode blocks that backfeed path.

are you referencing the AOB switch diagram or my diagram?

I don't see a feedback loop issue. However, I don't understand your goal.

In the diagram in the original post:
I have a switch panel that has switches that triggers relays to a lightbar and a pair of ditch lights.
To provide power to those switches, i have a "master arm" switch that allows/disallows power to the downstream switches. So when the master switch is off, the offroad light switches do nothing.
However, I want to also be able to provide power to the switches via the high beam, thus bypassing the "master arm".

 

[GlennA]

Sorry, I was unclear on my post. I understand your goal but see no advantage to switching with the high beam over switching with master switch. It seems to me that switching with the high beam adds a few potential failure points. However, it is your vehicle, your call. I wish you only success.

 

[joshik]

Sorry, I was unclear on my post. I understand your goal but see no advantage to switching with the high beam over switching with master switch. It seems to me that switching with the high beam adds a few potential failure points. However, it is your vehicle, your call. I wish you only success.

Ah. Prior to the idea of the highbeam trigger, the offroad lights were only engaged with the overhead switches. Sometimes I drive on really dark roads and its nice to have ditch lights + front light bar on. However, it's a pain to turn off/on with the overhead switch when an on coming car comes. Triggering it with the high beams is much easier, faster and natural.

 

[GlennA]

Carry a spare relay. My experience has been that using a high current (30a) relay to switch low current (<1a) load results in intermittent connections inside of the relay.

 

[2.ooohhh]

I played with adding aux lighting circuits to my '14 FJ in a similar manner. I didn't like what would have I had to do to really make it work like I wanted to integrate with the factory circuits so I just added my own. I used solid state control (feniex 4200) for the aux lights and utilized a '70s ford floor mount dimmer switch to change modes on my aux control without having to use my hands.

 

[Alloy]

One of the outputs from the Tekonsha ZCI can be used to trigger a relay

TEKONSHA | 119250 | ZCI™ Zero Contact Interface Universal ModuLite®

Tekonsha | Brake Controllers

www.tekonsha.com

 

[Alloy]

Maybe use better quality relay ?

Beginning to think everything comes out of the same offshore factory. I just replaced two Cole Hersey relays that I've had last 20years before. Both were installed in 2019 with failed coils. Replaced the first one thinking it was a one off then 3 weeks later the 2nd one failed. Replaced both with CH again so we'll see what happens the 2nd time around.

But why only one output to trigger a relay ?
Regardless, This thing is pretty cool. Too bad its low current output limits.
I wonder if similar current sensing relays what are not optimised for adapting trailer wiring exist ?
Btw,
Happy Birthday to our friend the transistor.
Thanks to our friends at Bell Labs !
75 years this month.

OP only needs one output.

I haven't found anything else. Not that I've looked to hard. Tekonsha says it is patented but it can't be the transistor so it must be for the use.

 

[shays4me]

To me it looks like the circuit is too complicated. If I had a ground feed switch for my high beam headlights, I'd run that feed directly to the negative 30 amp relays for the aux lights. I'd use my aux light switches with a fused input to control the positive side of the auxiliary relays. With the diagram below my high beam switch would only light up the aux lights with the aux light master switched on as well. It eliminates redundant components in your original drawing....Less to go wrong is better IMO. P.S. sorry for the poorly drawn schematic!

 

[Kevin108]

The relay could only backfeed when triggered. Remember what it takes to trigger the relay. You're thinking too hard. But it's good to work it all out before you've already run the wires.