simple relay switch wiring

Well dear C, for a noob you show much wisdom. Ok, the "neat trick"... First a diagram extract courtesy of Isuzu... . Note that in that pic/fig, the current flows from left to right (and top to bottom - the common convention in our culture). IE - the battery and master fuse is to the left of the fig, and the loads (lights) are to the right. The "RELAY; dimmer" is a 5-pin relay aka SPDT or "change over" relay. We WONs (Were Once Noobs) like Howard and myself - tho I wonder if Howard was ever a noob! - prefer to call them SPDT (Single Pole; Double Throw ie double posiTion) - because of the various 5 pin relays that Howard cautioned about above. They are all 5 pin but not all are SPDT - some are SPST (like your 4-pin relay but with 2 outputs terminals). The first relay could be a cabin switch like my car originally had old cars used to have. But as I wrote, who these days wants a 20A or larger cable into the passenger cabling thru a high current switch and out to the front headlights? Modern cars use relays. Old farts like me add relays to take the heavy Amperage load off the old switches so the switches last longer; and we get brighter old's cool lights (especially halogens but also tungsten). Hence the first on/off or power switch is usually a relay - eg "RELAY; lighting" in the above fig. Incidentally, that fig shows that the light switch (SW; lighting & the dip switch SW; dimmer - shown separately but is in fact a single swittch) is GND switching - they connect the -ve side of the relay coil (85) to GND. The +ve side of the relay coil (86) goes to +12V so if 85 is grounded, the relay actuates. Remember - a relay is simply a remote switch. A local low current (eg up to 250mA) powers a remote coil (86, 85) that causes HEAVIER contacts (eg, 30A, 40A, 400A) to close. A relay can do other things like invert a polarity (eg, a -ve or GND voltage can switch a +V or +12V voltage) or isolated different supplies (eg 12V relay to turn on a 230VAC or 110VAC etc load), but that's not relevant here. (Tho that fig uses inversion - a GND signal switches +12V to the lights, or switches the hibeam.) Yes, it's a neat "trick", but so what? IMO it is less reliable. Let's say the first power relay (RELAY; lighting, or beam power switch) fails - then you have no hi or low beam. Picture instead those same 2 relays but where +12V goes to both (ie, the horizontal +12V from the left that goes into the RELAY; lighting also goes to the 2nd relay (RELAY; dimmer). The output of the first relay RELAY; lighting goes to low beam. The output of the second relay RELAY; dimmer goes to high beam. If a relay fails, only one beam is effected. That's my point. Also, if the master fuse to the power relay fails, both beams are lost whereas in my "fuse & relay per beam" scenario, each beam can be separately fused. I could ramble on about theoretical reliability versus an actual failure occurs - like when 48 hours north of home doing in excess of 100kmh on dark twisting roads (I used to drive rally) and having the beam fuse fail (luckily we were both too drunk to care) - but I'll skip that for now. (Actually is was a mate's car. His dip switch melted and hence all beams were lost. His antichrist vehicle did not have my relay mods at that stage. But it's the same effect as a SPDT hi/low's neat trick main relay or switch or fuse failing. I'm sure the above would be simpler if I drew my circuit, but I don't draw. (Ok, I've drawn ONCE only since getting Win7, but that's enough for this year.) But I suspect you have the nouse to sketch it out or know what I mean. (If not, the kick me - ie, say so, or ask.) And again, as I said, even when you only need a SPST (4pin) relay, always buy a SPDT relay (5pin) because you don't have to use the 5th pin (87a) and it's probably the same price AND why carry two yes of spares when one will do? THAT assumes the relay mount can accept the 5th pin (some are blanked off) and that SPST and SPDT prices are the same, or close enough not to break from "one part suits both". FYI - even Japanese JECS & JIDEC etc relays once had SPST relays (and mounting bases) with a tab instead of the NC terminal (87a) to prevent substitution, but later they scrapped SPSTs and supplied all as SPDT (else omitted the "blank" tab) and downstream users like car manufacturers allowed for 5-pin SPDT relays in 4-pin SPST locations. That made many people very happy! Now to 87, 87a, 85 etc. 85 & 86 are the ends of the relay coil. Tho they might be connected to 87 or 30 or even 87a, they do not supply the power (CURRENT) to the load. They merely supply power (voltage; current) to the coil that closes the relay CONTACTS 30 & 87. If de-energised - ie, insufficient voltage or current to the coil 85 & 86), then contact 30 remains connected to the NC = "Normally Closed" contact 87a - assuming that exists (ie,, SPDT). Note that as a rule, the NC contact 87a cannot supply as much current as 87. F.ex, the SPDT (5-pin) equivalent of your relay is still rated for 40A from 30 to 87, but only 30A from 30 to 87a. The 70& (or 80%) rule is subjective. One problem is that some manufacturers have already taken that into account; others haven't. Hence someone's 30A relay may be someone elses 40A relay. But to say +ve comes off first, that is crap UNLESS talking about VERY old cars (especially 6V) or telco's. The simple rule is that whenever working on the HOT SIDE, the cold side (aka GND, earth, frame etc) shall be disconnected. It is simple logic. If your hot side spanner or tool touches (bridges or shorts to) GND/chassis/frame then you have shorted your power supply (eg, battery) to GND etc if that GND is still connected to the supply. Hence the simple rule - whenever working on the HOT side, ensure the return (GND/chassis/earth/frame) is disconnected. Mind you, I have seen that same rule posted as 12 rules, plus another 12 rules for +ve earth/GND/frame systems.

I have installed the (Individual/alter) light system now and funnily the need for a relay was never needed before, I purchased a switch that would handle the load, but the set up now will prevent volt/watt problems phweew! I think the groud connection was the problem before. Also I've read the relays can be used with multiple lights connected, like LEDs, if you've any ideas on this rig then we've got a great post!. Yes there are cheap tricks all over the place, unless you check everything your not safe, makes these forums very needed!. I really think the gear will hold up more than blown lights, the only fault I've ever found. THANKYOU for explaining the real relay use I would need this information .

Oh well, if you have already purchased a full current switch... The relays would then only be to (1) reduce heavy wiring in the cabin; (2) reduce voltage drop to lights; (3) increase the life of the switch; (4) enable any switch or uPC etc to be used; & (5) any other reason I haven't mentioned. But relays are usually not needed for LEDs. I find the notion of using a 250mA relay to switch 20mA or 50mA LEDs etc somewhat ridiculous except where voltage drops or isolation are an issue.

Yeah, who would have guessed, at the time I presumed it was part and parcel of using .... anything electrical, that everything on the wire to the product IS the same as each other component wise, associated with the maximum electrical capacity of that system. Saved me time and a dime. Led'SSSS.    There are under the car,on-all-the-time,interior,etc all connected to the one relay @ switch, ?if that's possible to run many different types of lights off of one. Assuming it has the same volts(fingers crossed) the wattage will figure itself out..?..yes? Well that's theory.

As long as all are 12V loads, their current and power sort themselves out when connected to a 12V source - assuming the 12V source can handle it. All vehicle 12V loads are connected to the battery that can supply hundreds of Amps, yet 20mA LEDs (with resistors) and 12V 3W (250mA) bulbs don't blow.

So the only detail left is whether two or more separate component earth/ground leads can be connected at the same 'bolt' gnd placement. Won't it back flow the 'fault' or risk excess charge down the other wire if its required to operate or gnd?. Or is there a fuse for that. Grounding equipment essentially is a nullifier, that's probably it.

GND/earth is GND & earth - aka "common". There is no backflow if there is no circuit (loop) path. And grounds are NEVER fused (excluding some rare exceptions).

Great news. Well thanks for your advice, I will need all that for future projects, things I needed to know. Removed a lot of confusion.