Still working on my car pc project and finally starting on the pic based digital HVAC replacement. I need to replace the blower switch and work some cables via servo and a few other odds and ends. This isn't only for convience, but also because with my LCD in, I had to relocate the controls and they are far from convienent in location. The thing I'm worried the most about is the blower switch itself. It supplies power (13-15v) to one of 4 leads. The 4 leads go to the blower motor, three of them (low, med low, med high) go through a resistor network to the blower motor, the 4th (high) straight through the motor to ground. Looking at the schematics, the blower motor switch is powered by a 30amp circuit/relay. An automotive Ammeter put inline says at max blower speed, it's ~15amps. I'm assuming it's somewhat accurate. My DMM only does 10amps so an automotive ammeter seemed the cheapest way to measure. I'm assuming until the motor get's up to speed, it's drawing more initially and the needle doesn't have time to really register it. Should I use 20 or 30 amp relays? I'm trying to keep it as small as possible as I need 4 relays for the blower switch alone and three more for the other controls. I was thinking of these 20 amp relays 817-FTR-K3AB012W at mouser. They have a absolute max current of 25amps, so it seems like I'd have some margin (assuming the ammeter isn't really, really off). The other question I have is on coil voltage, these are 12v relays, while It's really 13-15v while the car is running. Will this be a problem? I see the same relay is made in an 18v version that has a must close rating of 12.6v. Any comments or suggestions would be appreciated. Thanks, -Lee
My Maplin catalogue quotes coil voltage ranges for a few of their 12V DC coil relays, and it varies quite a bit, from as tight as 9.6-13.2V up to as wide as 7.2-20.5V (the wider ranges with the "automotive power relays"). The Fujitsu datasheet doesn't help any, but there's a general "ENGINEERING REFERNCE - RELAYS" pdf that gives some de-rating info for coil over-voltages in already-warm conditions, suggests 15V won't kill the relay but will reduce the 20A rating. They also suggest bringing OTT supply voltages nearer the relay voltage with resistors or a zener. Simple way is to see if the relay does get warm and if so put a couple of 1N4001 in the coil leads to lose 1.5V.
You are the man. Many thanks as always, just the info I was looking for. One more question if I may, I'm trying to avoid cutting up the harness if I can as it's part of one massive $1000 loom for the front of the car. So if I sell it later, will be just easier to revert to stock if I don't hack it up. The switch this is replacing has six spade connectors, arranged 2x3. hey are approximately, 1/4" (6.35mm). I was thinking if I could get the right size, I could just plug the loom into it. I see them on rapid as "PCB Blade connectors", are they known as any other name? Can't find them for the life of me on mouser and I'd rather order from them as that's where I always order from and where I plan to order the other stuff from. Thanks again.
If I may ask one more probably silly question for the sake of my curiosity. Seeing I'm going to be using these as SPST switches, figured it would make sense to use SPST relays. So I was looking at these small NEC automotive relays. EQ1-22111S The thing is they are listed as for lamp loads, while they have another SPDT listed for motor loads, EQ1-31000S. (Both are in this data sheet http://www.worldproducts.com/pdfs/eq1.pdf) The price is the same, the form factor is the same. Though the EQ1-31000S is 30amp as opposed to 25amp, so I'll probably just go with the EQ1-31000S and leave the NC contacts disconnected. But anyone care to venture if there is any actual difference in the design of the relays or is it just because most people would want fwd/rev in most motor applications? It would seem to my tenuous understanding, that a switch is a switch outside of contact rating.
If you're going as far to use a PIC, have you considered using a solid state relay or a MOSFET, and just connecting it to the "high" connection, and using PWM to control the motor speed? Then you have basically any speeds you want, and if you use the hardware PWM, it's pretty easy to setup. My guess, is that a single SSR or MOSFET with that kind of current rating, will be cheaper than buying 4 relays, and also be a cooler design.
At peak a car battery can shove out well over 50-60A, not sure if that changes any of your relay plans...? dragon
Amperage shouldn't matter (not sure what you mean by a battery shoving out 50-60A, that just doesn't make since, a battery does not shove out current, it only allows the amount of current out of it that a device, in this case, a motor, can take, based on it's load--but speaking in your terms, the battery can "shove" out voltage, because that is not dependent on what the load is, of a particular device), except what the motor can draw, if the motor draws 15A, then you should be okay with a 20A relay, but to be on the safe side, I'd probably consider rating your relay the same as the fuse rating, that way, the fuse protects the relay.
Here are a couple SSR's that look like they would work well. The 20A one would be at your own risk, but it's slightly cheaper... http://www.mouser.com/search/ProductDetail.aspx?R=D1D20virtualkey55810000virtualkey558-D1D20 http://www.mouser.com/search/ProductDetail.aspx?R=D1D40virtualkey55810000virtualkey558-D1D40 I guess I was wrong, in thinking it would be cheaper...but, some of those, you can parallel together, so if you find some that are rated at 10A maybe, you can put 3 of them together, to get a 30A rating, for maybe $30-40...
Driving the motor directly is out as I'm trying to integrate it into the factory harness. There's an RC network inside the motor itself and connections to the ECU as well. So SSRs seem way too expensive considering how many I need. I also need to operate a few other relays for defrost, recirc, etc. As long as the profile is low, there is plenty of room in the center console and I'll still have 3DIN of space. It's also where I want it as I need to operate two cables by servo and they are PITA to reroute. Basically this board will be a little smaller than the stock assembly it replaces, but it will fit an inch or two down, freeing up that DIN of space. The circuit is fused at 30amp (though it feeds some other stuff), so even though at full blast, it draws 15amps, think I'll go with 25A or 30A (and fuse appropriately). I guess I'll go with the EQ1-31000S, still curious if there are any physical differences between relays for inductive loads. Thanks for the suggestions.
geogecko > A modern car alternator will kick out in excess of 60A without breaking a sweat, 200A if you're trying. If you get bored and you're sick of the interior decoration of your car, unhook the battery when the engine's running. For best results, have spare bulbs for every socket in your car to hand and don't worry about the rest of the electrics working for the ride home. Oh, and stand back I take no liability for injury etc. If you're *really* serious about trying it, get a job for an automotive electronics startup. It's part of initial e-marking for a new product which you will require to sell the product within the UK. This means you *have* to do it for your first test ... but your kit has to survive! (plus they provide free paramedics ... sometimes )
Hmm...once again, basic electronics here... Amps mean nothing, unless the electronics can provide a load. This is the same reason that you can mod, say for instance, an iPod, with a larger battery. That battery can provide more amperage, or if you want to say it correctly, amp-hours. Just because it has that ability, doesn't mean it's going to force the iPod to DRAW more CURRENT (AMPS), it just means the iPod will last longer. You are correct, in that a car alternator can support a 60A load, or even some heavy-duty alternators, can support a 200A load. The key word here, is support, meaning, the car doesn't have to provide a load that would require 200A, even if the alternator is capable of providing it. I'm not sure what you mean by having spare bulbs if you were to unhook the battery while the car is running. I've never tried this, but if what you say happens, I find it a little confusing. Every car has a voltage regulator, to keep the voltage from staying at an unsafe level for any length of time, at least, I think most modern cars have them. The only way for a bulb to blow up in a car, is for the voltage to spike, not the current. The bulb's load characteristics are not changing. If you don't believe me, here is the equation you need: Voltage = Current * Resistance In a simple circuit, containing a car battery, and a light bulb, we'll say is a 24W bulb, which means it's a 6 ohm load (Power = Voltage * Current or Power = Current ^ 2 * Resistance). In this particular case, the bulb draws 2A (Current = Voltage / Resistance). The only way for this bulb to blow up, would be if the voltage were to surge, which, if you looked at the equation, would cause the current to then surge (the load would generally stay the same, in this example, but could become a larger load, due to heating, in real life), causing the bulb to draw excessive current, and therefore, blow up.
