Headlight Relays and Diodes Question

[Jocko_51_B3B]

I'm planning to use the dimmer switch to toggle between HI and LO beam headlight relays. Then I started thinking about the coils in these relays (that connect pin 85 to 86). When I switch from HI to LO beam or vice versa using my dimmer switch, the energized coil in the corresponding relay will suddenly be open circuited causing a big voltage spike that I suppose could burn the contacts in the dimmer switch. Has anyone experienced this problem? What about wiring in a diode between pins 85 and 86 to suppress the voltage surge. If so, is there a common diode part number for such a diode? Maybe I'm overthinking this, but I think it's a question worth asking about.

[Tooljunkie]

Yes. You are overthinking this. 
85 to ground. 86 to either terminal on dimmer. Three wires left. 30 to    battery 87 to high 87a to low. 
relay energized, one circuit, de-energized  For other circuit. 
you need not utilize both circuits on dimmer switch. 

[lostviking]

I don't see why you are using relays at all. They weren't used by the factory, which means the switch itself is plenty. As for diodes, they are used to protect a transistor from the inductive kick, not a mechanical switch.

Edited February 28, 2020 by lostviking

[JBNeal]

additional information - headlight relay instructions 

[Merle Coggins]

Many relays that I use have a built in quenching diode, or resistor, to suppress the induced voltage spike when the magnetic field is switched off. If the relays you use don't have them built in it would be an easy addition. Wire it up as shown here, with 86 being the + side and 85 is -.

 

 

Edited February 28, 2020 by Merle Coggins

[Jocko_51_B3B]

Merle,

I'm sure my relays don't have built-in diodes. Just out of interest I'm going to call the relay manufacturer (Temco) and see what they say about adding them externally which seems like a good idea. Maybe adding diodes is unnecessary, but physics is physics. The energy in the coil has to go somewhere when the relay is turned off and the only place it can go is to cause an arc across the contacts of the switch that turned it off.

 

LostViking,

Apparently, Dodge didn't think diodes were necessary. (Then again, semi-conductor diodes might not have been available in the early fifties.) But that doesn't mean they aren't a good idea. Using relays to take current load off the headlight, dimmer, and horn switches seems like a good way to prevent possible switch overheating (and a fire) especially in the headlight and dimmer switches. I'm using my original switches which test good, but they ARE OLD so I want to minimize the current they carry.

 

Edited February 28, 2020 by Jocko_51_B3B

[Tooljunkie]

Your “old” switches are far more robust than what you can buy today. 
as far as relays go, for the most part they are all the same. Siemens is the common one i used in remote starter installations,for activating solenoids in door locks and power trunks. Rarely has one even failed. 
running a 40 amp circuit through a 40 amp relay is looking for trouble, but they will cycle thousands of times in a 20 amp load circuit without fail. 
only time diodes are called for was usually a mult-plex circuit to prevent backfeed into the digital controls. 

[lostviking]

22 hours ago, JBNeal said:

additional information - headlight relay instructions 

And if you read that, you'll see they are stock on another brand, not a Dodge. There are no relays in the electrical diagram, take a look there. Also, as an engineer with over 40 years in the business, I understand why someone would want to use a relay. They are still not required though.

 

The Dodge design has been working for 70 years. Nuff said.

Edited February 29, 2020 by lostviking

[JBNeal]

I've made the switch to headlight relays on several vehicles, 6V and 12V, and light output almost doubles the visible field ahead without oncoming glare...that's why I put together a wiring diagram that incorporates a headlight relay years ago...are they required? Nawwww but I live out in the sticks so any advantage I can get to avoid the nocturnal varmint population, I'll take it  ?

[Plymouthy Adams]

I personally like using the relay and recommend them for the one simple truth, these switches though robust in the day and designed for the vary application for which they have worked all these years there are a few showing their age and with the many cycles of the witch the contact is grooving the other contact to the point there is less force and thus with the lighter pressure an increase in amp draw and switch failures.   If you do not have a good new switch or serviceable old switch to replace this, then the relay being such a low current device can continue to use the older now failing switch as there is little to no current involved switching the relay....the other side of the coin...

[lostviking]

Not trying to talk someone out of anything. The plain fact though is that if the switch you are using is faulty, it needs to be replaced. I've taken mine apart and the inside was new looking. There were zero wear marks, pits or any residue from arcing. The design is most definitely intended for high current. It's better than most I've seen you can purchase today, in a similar size.

 

I've got a very long time working with this stuff and a lot of experience. I won't tell you that putting a relay in your system is wrong, it isn't. But it adds non-stock wiring and complications that are simply not needed. Using one to band aid a worn switch is just asking for a failure, or worse a fire. Fix the problem, never cover it up.

 

If your truck is already modified, then do anything you want and makes you feel safe. If not, I'd think twice about making modifications. The median price for an original WD15 in my area is $22,600...high book is $50K. Modified trucks don't have that value.

 

But then I've never purchased anything with the intention of ever selling it.