main power switch for monitors

Hey everyone. I'm looking to purchase 4 of these Farenheit monitors on Ebay: Click here My issue is powering all the monitors by tapping into nearby 12v sources. I hate that! I'd rather make a custom harness for the monitors and run them through the harness and have the harness get power from the fusebox or battery. I would want a main power switch on the dash for the monitors. Can I use just one SPST relay to send power to all 4 monitors?

my bad here's the link http://#/2-FARENHEIT-8-8-BEIGE-HEADREST-CAR-MONITORS-HEADPHONE-/350389871044?pt=Car_Audio_Video&hash=item5194dd21c4#ht_3319wt_1165

Yes.

Cool, thanks buddy

Alright so I laid out all the wiring. 12 gauge positive and ground wires going from the back rows to the front where I set up the relay. Is this the correct way to wire? 30 - To battery 85 - To ground 86 - To main switch 87 - 12 gauge positive wire 87a - N/A If the wiring is correct, I can put a terminator on the other end of the 12 gauge wires and I can t-tap the monitor's power wire and ground wire into the 12 gauge wires. Correct? Each monitor takes in 3/4amps so 4 monitors would take up 3 amps. Do I use a 3 amp fuse or a 5 amp fuse for a little bit of slack? What fuse rating would I use for the #30 wire that goes to the battery? This freakin' site is the greatest I swear.

I empathise with your last comment. Though I came in off-side for the strangest of reasons, I too am blown by the expertise through experience that the others have. And though I am not into a lot of the12volt stuff, I can now avoid a few hundred years of mistakes when I do.... Your relay wiring is correct, ie: 30 - from "fused" battery. 87 - to loads (monitor +12V) 86 - from a +12V "on" signal/switch 85 - to gnd. (So 85 to 86 is 12V to connect heavy power 30 to 87.) Your monitors are all grounded, and have their separate (whatever) video signals, and maybe their own local on-off switch etc. [ FYI - Monitors do NOT blow if they receive video/sound signals when they are powered off. (In part thru "good design" - ie, they SHOULD not blow up! - but also because of their circuitry - inputs of ~1.5V - 3V though passive components (capacitors, resistors) or solid state stuff (transistors and other foreign visitors) that tolerate those "small" signals. ] Fusing: Aha!! Yes = 3/4A x 4 = 3A hence 5A for extra margin - EXCELLENT! Maybe even 10A - it all depends on their inrush current - eg, maybe 3/4A average after a 165A degaussing current and a 47A "let's pssoff the owner" current. But de-gaussers are old-tek TV tube stuff. Besides, I'm just poking fun... unless it is a crap display that does not limit peak currents! But I assume each display has its own fuse? Usually equipment has its own "protection" fusing - the "user" is not expected to supply that. (Only crappy big-power amplifiers do that!) If so, fusing is simple. Chose a "copious" wire and relay t handle the current - eg, 3A average. That probably means a 15A or 30A relay (because that is the common/cheap relay), and some "extra meat" 5A or 10A or 25A wire... The fuse at the battery end (ie, #30's "fused battery") then has to be NO BIGGER than the reelay or wire - whichever is the smaller smaller. I say "no bigger" which means equal to or (preferably!) less than the smallest rating. EG - relay rated for 30A, wire is 10A, then fuse NO MORE THAN 10A. EG - relay 15A, wire 50A, fuse = max 15A. Minimum fuse size is so it doesn't blow under normal operation. IE - a 3A fuse could blow. Besides, usual "rule" is to fuse at ~70% load - ie, a 3A fuse for 3x0.7 = 2.1A. 5A fuse is better - eg, 5x0.7 = 3.5A "average". So hence a 5A fuse; or 10A fuse (if wire AND relay are rated at 10A or higher) or 15A fuse (if 15A relay/wire etc). Sorry for any confustigation, but that is a basic fusing lesson. Only ONE fuse is needed "at the start" to protect the "downstream wiring". [ If it's from the battery thru a 30A wire to a 15A relay thru a 10A wire to load(s), then could have 10A fuse at battery end to protect the lot (yay!!), else a 15A fuse at battery end and a10 A after the fuse (yuk!) or 30A at the battery, 15A at the relay, 10A after the relay... (double yuk!)... get the picture? ] If the monitors need protection fusing, then each needs its own fuse - eg, 750mA or 1A etc. (But then the same "power" aka "distribution" fusing as above still applies - a 3A fuse might protect "all" going faulty, but not one alone going faulty etc...) Maybe in summary - 2 fusing "types": Distribution fusing - protects the power distribution - eg, wires, relays etc. It may also protect the power source (alternator, battery). Equipment fusing - protects the equipment itself. Usually supplied by the manufacturer because who better to know what fuse is required, and why risk it to others with their two-inch nails? Besides, equipment protection is a design issue - it is often internal to the equipment - eg, internal fuses, current & thermal feedback....) BTW - circuit breakers can be good.... I often use ATC (blade type) breakers - especially for lighting circuits. I use an auto-resetting 5A circuit breaker in place of the 10A fuse specified for my car fridge (a 45L Engel with ~1.3A max draw (I think)). These cost ~ $7 in sizes from ~5A to 30A in "blade" style.

Thanks for the reply oldspark. I see what you're saying about the inrush current. But would that matter if the monitors are all off by way of the main switch on the dash? I would think that the inrush current would be a concern if the monitors were directly wired to the cig lighter or something? Do I have to worry about inrush current when the main switch is flipped on? If so, how much current can I expect from a standard 30/40A relay?

Ok so based on what you're saying, I better stick to one type of gague wire to the relay. Ok so check this out: Let's say I have 30A wire on the fused battery. That wire goes from the battery to #30 on the relay. Boom that's set. Ok, I have two 10 gauge wire laid out running along the sides of the whip down to the second row. All four monitors are in position to get wired at this point. I would assume all monitors have their own 1A fuse on their wires. I begin to t-tap the monitors' fused power wire into the 10 gauge wire I have already laid out. Are you saying that the fuse on the battery should be 10A and not the rating of the wire? Here's my setup, dont laugh lol

Me laugh? Never - except that I still have an open window for my reply to your previous post.... I was searching for an "all informative" fuse blowing graph that shows how a fuse takes time to blow depending on overload magnitude/current. EG - typically might sit all day at 100% overload, maybe 0.001 seconds at 10,000% overload, and maybe seconds or minutes at 200% overload etc. IE - fuses lack the "accuracy" that some expect, and designs are somewhat flexible . Why? Oh yeah - inrush currents.... Anyhow, the search found something else which led to.... and then... and... And that's the only reason I'll laugh in this thread! Good - closed that reply and info discarded (but saved in case you're interested...) And as a post-edit after below comment, you're fine - use any fuse between 10A and 30A. But below is more detail... Assumptions, conditions, and a GENERAL but all-inclusive lesson in protection.... Ok... Your diagram looks fine. (Excluding trivial but implicit "main switch" fused +12V and groundings etc. EG - #85 is ground is GND is chassis is body is engine is 0V is battery -ve is "a common connection" to everything and is un-fused and "where & how" is generally unimportant provided they are good connections (paint scraped off etc) and ground loops etc are not an issue. Good - legal disclaimer and stating the obvious has now been achieved... or has it?) Phew! (Oh! And a "circuit line" is electrically the same "point" or connection anywhere along that line. EG - the "horseshoe loop" is the same as "connecting everything to #87". Conversely, all the monitor "#85's" mean any GND point. In practice, such interconnections are whatever is practical- eg, you probably would not have 4 monitor grounds all connected to the one #85 relay terminal....) Phew again? Alas all above may be obvious stuff with a bit of experience, but confusing to some - like "can I move the top left monitor to under the bottom right monitor?". Electrically it is all the same - it has nothing to do with where or how etc. Oh no... brace yourself...    (That assumes reasonable cabling and loads - ie, no undersized cables hence no voltage drops, ground loops etc) I'm now officially all Phew'd out. Yay! Oi - I heard that!

paperboi wrote: Are you saying that the fuse on the battery should be 10A and not the rating of the wire?

No. It can be... A 30A wire from the battery to the relay. Hence a fuse NO GREATER than 30A to protect the cable (from downstream faults). But also everything between that fuse and the next downstream fuse must be 30A rated. A 40A relay? Cool. A 30A relay? Cool. A 20A relay? Reduce "battery fuse" to 20A.    The fuse now protects the weaker of the downstream components. Now, 10G from the relay (output). Damn - what's 10G...? $#%#@!! &*%%$**! ...&.. !!*&&*$#@!! A 4 meter return trip to bookshelf.... Aha! 12G is ~30A so 10G is above 30A (9G is 50A) - that's "my" source, but I never use those ratings anyhow. (Check - powerstream.com/Wire_Size... 10G is 55A for chassis; 15A for transmission. Cool - don't hang it in the air 'cos that gets hot if you run a more than a steady 15A load. Great! So paperboi being the clever 12volter that he (else she) has done a brilliant job (IMHO). That's worth another PHEW! (Trust me - thank you!) So assuming 30A cable and 30A or larger relay and 10G to the monitors, then a 30A fuse is fine at the battery. Certainly no bigger fuse - it cannot exceed its smallest downstream un-fused cable or device. (EG - hence if a 15A or 20A relay, reduce the fuse to 15A or 20A respectively.) But we can use a smaller fuse - it just can't be BIGGER than downstream.... How small? Not so small that it does not handle the load. And it should have at least 20%-30% "headroom" as a general ROT (Rule Of Thumb) that allows for hotter ambient temperatures (hence fuse and breaker deratings), minor overloads, etc. So, what did we say... 4 monitors @ 0.75A each = 3A total. Hence say a minimum fuse size of 5A? Lets use 10A - extra margin and still WAY under our distribution sizing. (I do like over-sized distribution - if cost and weight acceptable. It allows for later expansion; is more robust; with smaller voltage drops...) Or maybe IF you have 15A fuses anyway, use 15A. Thanks to your (say) 30A design, you have the flexibility of using any fuse from 5A to 30A. I'd suggest starting with 10A (if you have them; else 15A else 20A etc) and if that does not blow, then fine. If smaller fuses blow (maybe due to inrush; maybe just because), increase the size. Note that the 0.75A monitors may have a 1A or larger fuse. It may be that 750mA fuses are rare. Or that 750mA "average" plus the 30% ROT means "rounded to" the common 1A fuse. Or it may be a 2A fuse because - despite 1A being common and ROT'd for a 750mA average, they allow for inrush and other transients, or higher temperatures (especially in vehicle cabins!). The point is that arguably, the upstream fuse should not be LESS than the sum of the total monitor fuses - eg, if 2A, then 4x2A = 8A which means a 10A fuse (not 5A). That argument would be based on all monitors having an "up to" 2A inrush, hence an "up to" 8A inrush is logical. Or it may be based on statistical crap where random peaks MIGHT be up to 2A, hence theoretically a possible 8A even if statistically only one a year etc. (Hence one of many reasons that some fuses seem to randomly but rarely blow.)    But maybe it's a 2A fuse because they need a 1.1A fuse, and 2A is the next practical size. That reality makes nearly a 50% change in our upstream fuse assumption - ie, 4x1.1A = 4.4A => a 5A fuse is fine. That's an example of using equipment fuse ratings for dimensioning purposes. Add up all the fuses in your domestic loads and you probably get 10 times the value of your main fuse etc. Same for the fuses in your car as opposed to the main fuses (fuselinks aka flinks) - although they are (sub-) distribution fuses rather than equipment fuses. Whilst fuses may indicate the maximum current a load may take (eg, 110% of rating longterm, 200% for 10 seconds) and hence max power input (ditto times input voltage) and hence max power output (ditto less inefficiency), it does NOT indicate typical power. EG - I have seen amplifiers that quote RMS output powers based on stated voltage and hence running at 100% efficiency at greater than 110% fuse ratings! That's possible, but man those fuses will get hot.... Smaller is better to limit sparks or short-energy and relay contact damage etc. Larger is better for robustness against surge gremlins. And probably I am too fussy - after all, 30A relays often handle more than 30A; fuses have minimum response times etc. And we are not designing for a 20A or 30A load. Though it's a 30A distribution system, our load is only a few Amps... We... er you - are only fusing to protect against short circuits - not a high current that will normally flow and hence pre-heat insulation and components.... Just remain practical - use a fuse that you already use, or is common and cheap etc. The above is a classic case of "distribution fusing" or protection. And therein ends this short reply. But as to the monitor protection....

Ha ha - you thought that was it.... (anon-lesson-101 part B) Now - load protection... Distribution protection is very different to load protection - the load in this case being the end-equipment. Maybe I should use "equipment protection" - but distribution is also a load as is equipment... But your equipment SHOULD have its own protection. Almost every electrical appliance has its own fusing or protection. If not to protect itself, then to protect its manufacturers and retailers from law suits when it fails and kills or burns things to the ground (or chassis or earth or ... lol!). Usually the latter is mandated by Law etc. So your monitors have a 750mA or 1A or 2A fuse etc?    So what about between your 30A distribution and those fuses? Houston - do we have a problem? What is the monitor supply cable rated at, or does it have its own "upstream" fuse - eg, in a cig-socket? Or do YOU have to provide a 1A or whatever fuse for EACH cable? (Though their power cables are probably rated at more than 1A. I'd expect 5A or 10A...?) You may chose to provide a fuse for each monitor at the start of its run from the 10G distribution. Advantage - the faulty leg's fuse blows instead of taking out the bigger upstream fuse and killing all monitors. WARNING - if kids are involved, make sure one fault kills all monitors. The only thing worse than 4 grumpy kids is one kid without waring with those kids with. If kids can rotate and you have an established pecking order where the lowest is truly subdued else less noisy than the quartet, ignore this warning. If spouses are involved, likewise ignore this warning (see aforementioned pecking order). If outlaws or in-laws are involved, I provide zero advice. Disadvantages (with final leg fuses) - insignificant if the above warning applies. Otherwise, just the cost of the fuses & holders, the hassle, and more fuses. Alternatives? YES! Did I mention "protection"? This is all about electrical protection. And kids. Etc. Fuses and circuit breakers are the common forms of protection. So is rubber insulation, but it's pointless discussing that now if the kids already exist. However, insulation is the basis for the OTHER method for electrical protection. This might be called physical security or protection. EG - insulation is what stops you being fried and starting a fire (and probably dying) in domestic electrical systems - fuses will NOT prevent that! (IE - 10mA thru the heart to stop it, 1mA thru the heart to fibrillate it (which is worse than stopping it!). We do not have 1mA or 10mA fuses in our home AC systems! Most are 8A and higher - more than enough to kill!) I mention that because if fusing is impractical or certain faults are deemed unlikely - eg, the risk is worthwhile or it's the in-law else neighbour's kid anyhow - then physical protection is valid. I often cite telephone exchanges as prime examples - rooms full of batteries the size of 2-drawer filing cabinets (or half that size - namely mere 2V cells with AH ratings of thousands of Amp-Hours, not pathetic hundreds of BIG car batteries!). These normally deliver hundreds or thousands of Amps (typically at 48V) and not a fuse in sight (nor site)! Nor any other means of interrupting their current should a downstream distribution (buss-bar) fault occur - except maybe an axe or similar through strategic inter-battery cabling. Thousands of such systems around the world and no fuses or breakers etc, yet rarely a critical accident. In fact fusing such exchanges and dc systems is a hazard! The apparent common "fuse within 12" or 18" of the battery" rule becomes "fuse AT LEAST 12 or 18 yards AWAY from the battery" (with shields to protect personnel from the imminent explosion!).     Ah yes - the power of "physical protection/security" and the dangers of conventional interruption protection in high-current DC systems! (Hey - anyone seen exploding cars or fireballs at SPL competitions?) But in this case, any "end leg" fusing can be more of a gray area. You might decide that end fusing is not worthwhile - given a short circuit of a +12V tail to GND (somehow), despite the tail cable NOT being (eg) 30A rated, the upstream "master" 30A fuse might (should?) rupture before the tail has time to melt and burn. An equivalent similar example perhaps... I use a 50A rated current shunt (a special .001 Ohm 50W resistor or wire) to measure starter-motor cranking currents up to a few hundred Amps. Why? Because 100A and 500A shunts cost a heck of a lot more. But for me that is no problem - the 50A shunt is for testing only (not permanent) and its 50A rating is for CONTINUOUS use. I only pump a few hundred amps for a few seconds - maybe 10 seconds - not long enough for the mere 50A shunt to heat and smoke or melt or anneal. Plus it is external - if it does flame, nothing else will flame. The only risk is burnt fingers (but not yet!) or not having eye protection - but I'm used to weld craters. The example's relevance? That there is usually a big difference between continuous and short-term ratings - especially for raw "high inertia" stuff like copper cables etc. Though I feel I should add a caveat like "do not try this... do only if sufficiently qualified or stupid...."    But I have been sitting here sipping a full bottle of wine, and not via a wine glass. And my drinking vessel is almost empty. I'd better quit now before I'm too intoxicated to walk to my car. (Great - that's my legal "not in control" out!) Humour aside, I yet again apologise for my long tutorials.   One day I'll take the time that I claim I don't have (or take) to find and link to previous 'gurgitations of the same old rambl... er, "tutorials". Improvement of previous writings is one thing, but repeats of lost or buried stuff is IMO unacceptable! I consistently break my own writing and posting rules, and my experience with human interaction (aka HFE - Human Factors Engineering). My experience suggests a simple reply in lieu of these last (2?) replies - namely "Either is fine - just ask if unclear" - with a "specific (aspect or question)" embedded somewhere in the latter... The exec summary - a 10A to 30A fuse is fine. The detail - assuming "protection" is adequate. I hear your "phews", yet I can't seem to muster one.... LOL.