don't reinvent the wheel. they have been using relays in cars for years.
That's not reinventing the wheel...that's just using the appropriate tool for the job. Relays are for switching large loads. Transistors are for switching small loads (or some non-switching jobs like amplification).
Don't use a sledgehammer to pound a finish nail. A transistor is the right tool, keep at it until you get tired of trying, THEN use the sledgehammer.
you do ralize that you can get a relay that is as small as a pencil eraser. I am not talking about a huge device.
he is trying to engineer a system within his vehicle to work with somewhat unknown values. if they were known then things wouldn't be getting warm.
there are a million ways to get where you want to go. the question is which is the best for what is available. a relay (very small one at that) will do the job and nothing else has to be taken into consideration.
also, fets are used to switch large loads. variable speed motor drives have been using FETS in their output stages for a while now. field effect transistors rated for 1200 amps(each one weighed about a pound and a half ). that is a pretty big load.
honestly, either way would work but the question is how much time do you want to invest? have fun figure it out or just buy a relay (the little one).
Be the change you wish to see in the world
1. you didn't switch the leads (most have to switch the red lead to the amps/ma socket)
2. blew the fuse because you didn't break the circuit to measure something
3. cant measure current (havent seen one but do know of some crappy ones that cant)
(didn't think the link to the search would show up right)
this is just an example of one. the current that they can handle may be the biggest drawback and yes they are supposed to be mounted in a PCB but so is the transistor that is being used. I think that current through the device may also be making it hot.
transistors are used for switching but the reason they use them for switching is usually for #1 no moving parts but #2 their switching speed is much faster. the second one goes along with the first but it doesn't have much to do with the size. a solid state relay is just a transistor (be it a big one) in a package that fits in a relay socket. it does have all the resistors and other good stuff built into a neat little package so you don't have to do anything but plug and go. they last longer because of the less moving parts which makes them very valuable in industry.
also, those boards that you can find the transistors on, look at them more closeley and you may find relays on them too. most are square and long (the small ones) maybe a qtr to a half an inch square by about 1 inch long. two pins on one side (the coil) and 2 or 3 pins on the other side (the actual contacts) they are used more than people realize in electronics. not necessarily computers but definitely for controllers. old PLCs used them for their outputs. they were very low current though. that may be a problem with them though in this application.
*edit* the way I see this is a low-tech problem with a high-tech answer. that is the reinventing the wheel thing. you want a switch to flip off so that the lights won't come on when the car is off. a relay is simply a switch. flip off the switch to the coil and the contacts open. low-tech but effective. nothing to figure out (other than current capcity you would need) if you use a 20 amp one, you have nothing to worry about but you can figure that out and use an appropriately sized one if you care that much. I guess also remember to use one with a 12VDC coil but that goes without saying.
*edit again* http://relays.tycoelectronics.com/datasheets/OMR.pdf
here is one from mouser. this pdf shows a lot of things. it shows physical size but it also shows maximum switching rate. these things get really small but also the current handling is small as well. a lot of good data on this data sheet. I wouldn't use this relay but you see what I am saying about relays being small.
sorry for the book ^^^
Be the change you wish to see in the world
*edit* the way I see this is a low-tech problem with a high-tech answer. that is the reinventing the wheel thing. you want a switch to flip off so that the lights won't come on when the car is off.
That's when I realized that using a relay or transistor is probably more than necessary, due to an idea that I got:
The headlight switch gets power from somewhere and puts that power through a relay which powers up the headlights. Why not just change the source of the switch's power? You could just tap a wire that's only on when the car is on. This avoids needing any relay or transistor, because you're using that big honkin' ignition switch that's made to control exactly this sort of thing!
According to http://www.bulldogsecurity.com/New%2...stiva91-93.htm OP's Festiva could use the Black/White (ignition) or Blue/Red (accessory) wire from the ignition switch harness. The only gotcha would be if the headlight switch runs on the opposite polarity, which is a distinct possibility...that would pretty much break this idea.
Nope, doesn't matter if it runs in the opposite polarity, the headlight switch could switch positive or negative side of the coil, doesn't matter and making that positive controlled by ignition will fix the problem in either case. But I already outlined above why I'm not doing that.
And thanks for all your thoughts, that small relay is a cool idea and quite easy to use. I may just do that. I don't want a big transistor because I'm not under the dash, I'm in the steering column, no room. But the small one will work. I'd just prefer solid state if I've got a million of them. To me it feels like the right solution and to me it seems stupid to give up because we just doing know what we're doing. It should be the thing used for the job we just don't know how.