MogLite Posted August 24, 2005 Share Posted August 24, 2005 Modern vehicles use loads of relays. I understand this is because all their fancy small dashboard switches can't handle the current. Is there any other reason why I can't just use a 30A switch on a 30A circuit without a relay, as I was planning to use toggles I've got the option of man-sized switches, and I'd rather keep the wiring as simple as possible, and therefore the number of relays to a minimum. Quote Link to comment Share on other sites More sharing options...
Tonk Posted August 24, 2005 Share Posted August 24, 2005 a relay is only a switch anyway....... Quote Link to comment Share on other sites More sharing options...
minivin Posted August 24, 2005 Share Posted August 24, 2005 Only detail to remember is that running a 30A switch at 30A's means you are running it at it's maximum all the time, this will reduce the life of the item, if you can find a data sheet for the item it should break-down what the load-versus-life expectancy is Quote Link to comment Share on other sites More sharing options...
rtbarton Posted August 24, 2005 Share Posted August 24, 2005 Disadvantage of 30 Amp switches is that you are running 30 A wire everywhere which means voltage drop and cost, relays allow you to keep the high current runs short if you plan their location. I'm not in favour of centralised relay boxes, my SIII project has 11 relays scatttered all over the place. If you are aiming for road-legal minimum then use your free switches. Another advantage of relays is that you can build logic into your circuits, such as having headlights go out when the ignition is turned off and rear fogs on with headlights, driving lights on with main beam &c&c. As if you hadn't guessed - I stock relays too! Quote Link to comment Share on other sites More sharing options...
02GF74 Posted August 24, 2005 Share Posted August 24, 2005 a relay is only a switch anyway....... only?!?!?!?! it is more than a switch the short answer is yes you can. the longer answer is whether it is wise to do for 30 A? Without looking into it, I reckon a 30 A switch costs more that the same relay namely because you want a big contact area and that is ahieved easier in the relay. The relay contacts will me made of funky metal and be sealed in vacuum (depending on model ofcourse!) so less spark erosion hence longer life than your switch. Think about the cables too; you need fat bu88ers to cary 30 A , especially if they are long. With a relay you would position it close to the item(s) you are powering and have a thin cable between the relay coil and the switch; the switch method requires using fat cables all the way; thinner cables would loose power resulting in heat. Quote Link to comment Share on other sites More sharing options...
Tonk Posted August 24, 2005 Share Posted August 24, 2005 ok, i'll answer it a different way. Andy, i've had switches carrying heavy loads in my truck for about 3 years and they're still going strong. i've also had relays in there which haven't lasted as long, maybe thats due to how deep i drive (or should that be dive? ) my truck. Quote Link to comment Share on other sites More sharing options...
fisha Posted August 24, 2005 Share Posted August 24, 2005 i'd recommend the use of relays to for the reasons above . . . principally the attenuation of long runs on high amps. the fatter the cable, the shorter the better. Quote Link to comment Share on other sites More sharing options...
rtbarton Posted August 24, 2005 Share Posted August 24, 2005 Also I'd guess a 30A relay is cheaper than a 30A switch, and easier to replace, and you can get them with integral fuseholders. Quote Link to comment Share on other sites More sharing options...
will_warne Posted August 24, 2005 Share Posted August 24, 2005 Andy, why not use circuit breakers? That way you've got a switch and a fuse in one. No need for swich, relay and fuse to switch things. It just makes wiring much easier. They cost more but worth it IMHO. Quote Link to comment Share on other sites More sharing options...
Ivan Posted August 24, 2005 Share Posted August 24, 2005 One thing I did notice on my 300 Series body is that most of the relays tend to switch the earth. This makes sense as you can wire everything using very small cable and run the feeds (via a fuse) direct to the component you are switching. Also, it means you can add in extra "switches" (relays) to activate the component under other conditions and automatically. A classic example is the Air Con circuit. There are a number of cases where you weant to cut power to the compressor( i.e. Coolent temperature too high/low, Coolent pressure too high/low) by simply switchcing the earth circuit you don't have to run live cables everywhere. Ivan Quote Link to comment Share on other sites More sharing options...
rtbarton Posted August 24, 2005 Share Posted August 24, 2005 Andy, why not use circuit breakers? That way you've got a switch and a fuse in one. No need for swich, relay and fuse to switch things. It just makes wiring much easier. They cost more but worth it IMHO. Circuit breakers aren't designed for routine on-off cycles, they're there for emergencies and frequent use will wear them out. Quote Link to comment Share on other sites More sharing options...
minivin Posted August 24, 2005 Share Posted August 24, 2005 One thing I did notice on my 300 Series body is that most of the relays tend to switch the earth. This makes sense as you can wire everything using very small cable and run the feeds (via a fuse) direct to the component you are switching. keeeh? the earth line has to have the same current carrying capability as the feed line, therefore it must be the same size, if not larger if it also carrys the return path of other devices Quote Link to comment Share on other sites More sharing options...
LR90 Posted August 24, 2005 Share Posted August 24, 2005 Gosh Andy. look what you've started now. Anyone would think you'd asked a Q about tyres Quote Link to comment Share on other sites More sharing options...
02GF74 Posted August 25, 2005 Share Posted August 25, 2005 Modern vehicles use loads of relays. going back to original post, you may have answered your own question. ask yourself why? If it wasn't a good idea, they would be using fat cables and fat switches. Quote Link to comment Share on other sites More sharing options...
robhybrid Posted August 25, 2005 Share Posted August 25, 2005 If you are happy with thick cables and more expensive swithes and a slightly more limited range then I would do away with relays, in my experiance relays dont last very long if you are regularly dunking them in water. I would recomend putting a fuse box as close as possible to your bettery/ power supply. Thick cables rubbing on bodywork and arking tend to make a bit of a mess and smell funny. If you cant put a fuse box close to your power supply consider putting a fuse link on the wire from the battery to the fuse box. Quote Link to comment Share on other sites More sharing options...
timbo Posted August 25, 2005 Share Posted August 25, 2005 If you are happy with thick cables and more expensive swithes and a slightly more limited range then I would do away with relays, in my experiance relays dont last very long if you are regularly dunking them in water. you could just put your clump of relays (if they aren't scattered throughout the vehicle) in it's own little sealed box Quote Link to comment Share on other sites More sharing options...
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