Electrical question about the direction of D/C current flow
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CobyRupert
Super Member
Yes true, but......shorting to power is extremely rare. When the genie gets out of the bottle he usually falls to the ground (- hey I like that saying! (Trademark!!)) What is more commn is that the contacts of the switch will weld together, or the mechanical device that moves the contacts will fail when contacts are closed, or the power side will short to ground (i.e. the insulation of a wire is cut and conductor rubs on a grounded frame).
However, if the positive side of the load (w/ switch) shorts to ground:
1. There's a big spark and (hopefully) a fuse blows. Load device stops. (i.e there'd be true "short circuit" (from pos to neg) with little or no resistance in the circuit. Following Ohm's law, when the resistance value approaches zero, the current approaches infinity.)
2. Even if the fuse doesn't blow, both sides of the load are now connected to ground. Load device stops.
This concept as it regards to switches in control circuits is important from a safety aspect. Not only does one have to consider how a circuit will work (when it's working) but what happens when it fails. Sorry if my last repose seemed confrontational. Cheers :drink:
However, if the positive side of the load (w/ switch) shorts to ground:
1. There's a big spark and (hopefully) a fuse blows. Load device stops. (i.e there'd be true "short circuit" (from pos to neg) with little or no resistance in the circuit. Following Ohm's law, when the resistance value approaches zero, the current approaches infinity.)
2. Even if the fuse doesn't blow, both sides of the load are now connected to ground. Load device stops.
This concept as it regards to switches in control circuits is important from a safety aspect. Not only does one have to consider how a circuit will work (when it's working) but what happens when it fails. Sorry if my last repose seemed confrontational. Cheers :drink:
LD1
Epic Contributor
US makers are doing that as well. ITs been awhile since I been "up" on it. But on newer stuff, (maybe 2000 and newer) you see alot of the relays being controled with the ground side instead of the hot side.
In a direct switch like that, I dont like switching the ground. because then there is always potential at the device. Not such a big deal in 12v, but in higher voltage AC systems, the switch no longer eliminates power from the device. be it a light or whatever.
But in the automotive world, with lots of relays, one big advantage to switching the ground side of the relay is running fewer "hot" wires up into the dash area to all the switches. So if one "were" to short to ground, you simply have something that stays on (like wipers, lights, etc). Rather than the potential for melting wires, blowing fuses, and item totally not working.
Sometimes not being able to turn one on can be more dangerous. If the headlight wire shorts to ground in the middle of the night, with the control switch on the negative side, you simply have lights that will no longer turn off. With it on the hot side and the same short, you now have lights that do not work, a blown fuse, and greater possibility for wire damage and/or fire.
I cant think of too many things on a vehicle where leaving it ON can be MORE dangerous than not being able to come on at all.
Soundguy
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i never like anything in the ground path.. fuses, switches.. etc.
Soundguy
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starter? electric solenoid pto engagement?
i don't want either of those turning on unless i tell them to be on. imagine a tractor in gear and a fualt makes the starter engage and run you over!
Great points LD1. Current automotive wiring doesn't really use a "lot" of relays anymore. There will always be some, but it's far more typical to find a computer controlling multiple circuits based on input from the switches. Doing so drastically reduces the "hot" wires away from the passenger area.
LD1
Epic Contributor
In those cases, then no, a switched negative side wouldnt be good. And I dint think they are. I was more talking about most of the things that are relay controlled with one of them small relays. Horn, lights, wipers, etc.
As I said, its been awhile since I have been "up" on things. I actually have schooling for working on cars. bit that was ~15 years ago. And the newest that I have owned is a 2008. But since late 90's or so, relay circuits began being controlled with the ground. So the 85 and 86 coil terminals, one will be hot at all times. The ground is the one switched. The benefits as I understand them have already been explained.
As to going to computer controls now, I dont doubt that. But I would think there still has to be a relay?? Something to handle the higher current of headlights, horn, etc. Cause I woudnt think they are passing all that through the ECM or BCM. Rather I would think the switch feeds the CPU, then an output feeds a relay still??? But thats just my thinking. I have no idea how they actually are doing it.
I think you're doing just fine! I'm not trying to correct you in any way, just adding to the conversation. I am currently still ASE certified as a master technician and I was teaching automotives at a state college until a couple of years ago. As you suspected and as I said, there are still some relays but probably only 10-20 as an average for a new car. Back in the 80s through the late 90s there were probably twice that many on the average car.
The computers don't control the high current directly, the computers replaced the relays. Or, more correctly, the transistors inside the computers do the switching just like the relay used to. Transistors don't have moving parts, and can turn on/off far far faster than relays...and of course they are much smaller than relays. So as the age of computer controlled cars grew so did the need for smaller components that lasted longer and worked faster. Among other things, this allows the high current to stay outside the passenger area, closer to the components being controlled and only much lower current is wired to the computers inside the passenger compartment. On current cars, there are actual networks of computers "talking" back and forth along data transmission lines...it's really all quite amazing (to me at least).
Heh...good trademark
. And don't worry, no hurt feelings here! :drink:
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LD1
Epic Contributor
One question I have (and am surprised of) is that there is no "network" or cat5 cables on cars yet. At least that I am aware of. But again, my experience of "new" stuff ends ~2008. I am sure lots has changed in the last 7 model years
boggen
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to much over head, in network protocols. other words. for example your long "sig" of what you have (tractors/attachments, etc..) having to be sent every time ya pushed pedal down a little bit. vs just sending what is needed example my sig that just has my name. cars/trucks/tractors are not multi tasking computer, the computer chips have a very distinctive code they run, and nothing else. KISS (keeping it simply stupid) and just send/recieve whay they need. vs video, audio, mouse commands keyboard commands, advertisments, etc..)