Can a relay be wired to turn a circuit off?

[njoconnor]

I'm changing out the stock headlamps for the Valeo/Cibie H4 units A/R sells. I decided to get the model with the small "city light" in the bucket (a B9 type bulb). Since I have a North American 72, there is no wiring nor schematic for these lights. Got them because I've gone Euro with the turn/marker lights, as well as the tail lights. Figured I'd tap a pigtail into a nearby marker light feed and ground someplace nearby. Since I have a 3 position headlamp switch, I could have the city lamps on when the side/front/rear marker lights are lit. They would stay on once the headlights are switched on, but I figured no one's gonna notice a smal wattage lamp next to a lit H4 bulb.

Then I got to thinking (dangerous, that). Could I wire a relay so that the city bulbs lit when the marker lamps lit, but as soon as the headlamp circuit energized, the relay would open (?) the city lamp circuit, and they would turn off? I know enough about the car circuits that I can run down bad grounds and fixtures, and test for power. Relays are literally a black box to me.

Along with th H4's, I picked up a Greg Smith kit. That means I'll have an unused headlamp power source on the passenger side. Tap off the R side marker light for the city lamp circuit. What am I missing here?

Thanks, as always.

Neil

[aj81spider]

If you buy a normally closed relay you can do that. Run the circuit through the contacts and power the coil with the headlight drive circuit. When the headlights come on the coil will energize and the normally closed relay will open up, breaking the circuit.

[njoconnor]

"Normally closed" is the terminology I need. Thanks, AJ!

Neil

[DieselSpider]

A double throw relay would probably do too so you would only need one wiring it so when energized it would power the headlights and when in the rest position the other.

[toplessexpat]

Most of the relays sold as "bosch" are five pin, labelled with two numbers....

85 - relay ground
86 - relay power in (from the controlling switch)
30 - line power in (from the battery/alternator/fused power source)
87 - line power out (when 86 has voltage)
87a - line power out (when 87 has no voltage)

So you may have just what you need already. Some relays are 4 pin and missing 87a.

To do what you need for your circuit I think you simply need two five pin relays.

Andy

Edit: the 30 to 87/87a doesn't have to be at the same voltage as the relay 86 to 85 circuit. This is helpful if (for example) you need to switch something at a much higher voltage (eg a 24v circuit) or much lower (eg a speaker line for a telephone mute).

Edit 2: generally you put a fuse upstream of pin 30 to cover the accessory. When running several things off of this, put downstream lower amperage fuses for each downstream item on the 87/87a supply lines. Remember, combined load of downstream needs to have a high enough rated fuse on pin 30.

Edit 3: use the right calibre wire for the load on the line.

[76was124]

To do what you need for your circuit I think you simply need two five pin relays.

I agree with toplessexpat as you want at least three conditions:
1. No lights
2. city lights
3. Head lights, no city lights
One relay can only do two of these, the additional relay with contacts in series will give you the third condition you want.

[toplessexpat]

glad that I wasn't posting too early to make sense.

Depending on how main beam is supposed to work in your jurisdiction (eg main beam is supposed to turn off dipped headlights) then you put an additional relay infront (for main beam canceling dipped) or in parallel (main beam + dipped).

While you're playing around with 87/87a you could also rig a relay to the turn signal circuit (take a spur to the relay) and the running light.... And then the running light will also flash in reverse with the turn signal (when the lights are on).

Oh - and before someone comments - you can do the headlight relay mod either under the dash or on the fender inside wall.

I've done both and they work equally well provided you seal your connections right.

A

[spider2081]

I believe most folks who upgrade their headlights also install a headlight relay kit. The relay kit consists of a Hi beam and a LO beam relay for the headlight bulbs. The Hi beam relay supplies power to both headlights and the Lo beam relay supplies power to both Lo beam bulbs. Naturally only one works at a time.

The "city light" bulb in the headlights you purchased draw 0.33 amps each so the total draw added to the your cars parking light circuit would be less than 1 amp for both "city lights" to run at same time, I would think it safe to splice into the existing parking light circuit for this power.

To have the "city lights" turn off when headlights are "on" has 2 conditions, Hi beam, and LO beam. You would need to either add a relay for each condition to open the new "city light" circuit or install 2 steering diodes and one relay. I would go with the one relay 2 diode circuit.

[toplessexpat]

When I had a city light function I had it wired on to the front parking light circuit - and thus always on when the lights were on.

If I was going to try to turn it off when the lights were on - I think I'd just take a spur from both the main and dipped feeds and put them both to the switched feeds on a relay. Then have 30 wired to the parking light circuit, and 87a to the city lights on the units. Then the city lights turn on with the parking lights (one click on the headlight switch) and whenever the headlights are on the city lights are off.

Right now - I'm running 7" LED headlights - no relays and tiny current draw.

A

Edit: on the subject of relays..... Has anyone come across a decent 12v delayed relay (one that's behavior is when the relay 12v is applied, there's a 1-2min delay before the line power is opened)?. I'm having an urge to wire my second fan that way!

[nalle]

this is a heavy solution for your fan.
12 v time delay relays are not so common or they use to be quite expencive.

Use 12 V PLC (Programmable Logic Controller) like:
http://www.siemens.com.tr/i/Assets/logo ... ablo_2.pdf
in the matter of fact those are not so expencive.
Also othe manufacturers have those.

[toplessexpat]

Neat! Thanks.... So apart from the low load handling (10A isn't enough I think?), so I'll use the logic controller to drive a relay to power the second fan. Also - I can get a nice little text module to tell me when the second fan is cycling!

A

Edit: just found this HELLA 996152131 -that will do the job nicely!

[njoconnor]

Wow. Great info! Thanks for all the responses!

Hadn't considered the hi/lo beam factor. Back to the wiring diagrams!

What I believe I will do is this:

Run the passenger headlight power feed to the 85/86 poles of a 5 pin relay, and run a ground out the other side of the magnetic coil. Using the passenger side, since the Smith harness frees that up and renders it "extra" or redundant. Set the relay up as normally closed, with feed from the side marker light, feeding both city lights (using a B9 LED for even less draw). That should work with the light switch as position 1 OFF; position 2 (mid) parking/city/marker lights on; position 3 headlamps on, city off, markers on. To be determined: if I switch on the brights/hi beams, do I re-open the city light relay circuit, and thus need the second relay. I am suspecting yes.

Am I close? Any major issue I'm blissfully ignoring?

Neil

[toplessexpat]

I think I follow you.

My solution would be...


Relay A: Pin 30 from the original parking lights (yellow I think?), 87a to the city lights. Relay switched by .... And here's the rub - you don't want to have the low beam feed to this relay inadvertantly running the main beam circuit (and vice versa).... So you need a diode on these two feeds.

Relay B: Pin 30 from battery/alternator, pin 87 to dipped lights, feed from dipped lights circuit

Relay C: Pin 30 from battery/alternator, pin 87 to main beam, feed from main beam circuit.

I know not of the Smith Harness of which you speak(!)

You know there's a way of doing this with the relays under the dash and using the stock wiring right? Apart from the city lights, which could be done with a insertable mod.

A

[baltobernie]

I had a similar set of headlamps as you describe. The output of the "city" lamp is so feeble that it is insignificant when the headlamp is On. I wouldn't bother trying to extinguish it with the headlamp operating.

[njoconnor]

Bernie, I wondered about that. I had a SAAB 900T with similar lights, and yeah, they were just kinda "there" with the main beams on. Certainly a reasonable result.

The "Smith Harness" is the Greg Smith relay/wiring kit mentioned often in the Forum as the simplest way to get large power to halogen bulb upgrades. The harness uses the power feed from the driver side headlamp connector, and leaves the passenger side open (well, I planned on securing it against moisture, etc). In a typical (for me) spate of overthinking, I was trying to figure a neat way to cut the city lights out while the main beams (hi and low) were on. As Bernie suggests, probably not a big deal to just leave it alone. But your description of the pin assignments lines right up with what I was thinking, adding in the extra relay(s) and diodes.

Headscratching over a brew or two to ensue. Some other needs (front calipers, etc) on the work list first.

Thanks for the education on relays, as well as the ideas and suggestions!

Neil