1. #1
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    Default 12v Electrical Questions

    I have a pretty basic knowledge of 12volt wiring. I've done quite a few light and siren repairs, radio and flashlight charger installations etc. I know what things are by sight and name but cannot necessarily explain their purpose in a technical fashion to people that ask. Being that I like to understand what I am doing, here are a few questions, with more likely to follow.

    That said, can someone explain the difference between volts, amps and watts relative to 12vdc systems. I've heard analogies made to water flow in hoselines but can't remember.

    What is the difference between a solenoid and a relay.

    When you use undersided wiring, does the load at the other end try tp "pull" too much power therefore causing wiring to heat up?

    How do you determine the max load (and is it in watts amps etc) and longest distance you can run for each guage of wire?

    A siren speaker is rated in watts. To me this is equivalent to audible level, however I'm pretty sure thats a wrong assessment.

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    Quote Originally Posted by MG3610
    I have a pretty basic knowledge of 12volt wiring. I've done quite a few light and siren repairs, radio and flashlight charger installations etc. I know what things are by sight and name but cannot necessarily explain their purpose in a technical fashion to people that ask. Being that I like to understand what I am doing, here are a few questions, with more likely to follow.

    That said, can someone explain the difference between volts, amps and watts relative to 12vdc systems. I've heard analogies made to water flow in hoselines but can't remember.

    What is the difference between a solenoid and a relay.

    When you use undersided wiring, does the load at the other end try tp "pull" too much power therefore causing wiring to heat up?

    How do you determine the max load (and is it in watts amps etc) and longest distance you can run for each guage of wire?

    A siren speaker is rated in watts. To me this is equivalent to audible level, however I'm pretty sure thats a wrong assessment.
    Boy, you aren't asking much, are you?
    I'm going to try to do this.

    Hose theory: Voltage is the hose, water is amperes. The bigger the hose (Cir.mill of the wire) the more amperes (electrons moving) capable. does that make sense?

    Solenoid- electromagnetic device typically used to actuate a pneumatic valve body i.e. air/hydraulic/water. But also could actuate a mechanical pin/device of sorts.
    Relay- What I call a relay is a device that is used in control circuits to switch higher power controls, or create sequence of events in equipment. i.e. using a 12v. relay to switch a 220v. motor circuit. the relay looks for the presents/absence of 12v energize/de-energize the main circuit. this will allow you to use safe voltage to control high voltage flows. (small set of contacts pulls in a bigger set of contacts).

    undersized wiring- YES. since DC doesn't alternate as does AC, there tends to be more heat. Size of wire vs load = important. doing this make more amps flow through too small of wire...like trying to flow 1000gpm through a single 1-3/4 hose. You can only push(friction) so much through the cir. mill of the wire before destroying the insulation value of the wire and resulting in wiring failure/fire(heat). A wire that is close to ampacity will result in the expanding/contracting of connections which will then result in higher resistance due to poor connections-which then will result in ba...well, you get the idea.

    How do you size a wire? NEC table 310-16 (I believe) is the ampacity table for conductors.
    Basically, it is determined by Insulation factor, ambient temp, size wire, whether or not it is solid or stranded, single conductor, multiple conductor....open air, burial...

    Length of wire allowable? depends on voltage drop and other factors...there are calculations for voltage drop (consider this friction loss in a hose.) Not going to go into that here.

    Watts= resistance db= audible volume (can't think of a better description)

    120v light bulb @ 60 watts.
    Current= power/volts
    I=P/E (Ohms law)

    60 Watts / 120v = .5 amps.

    Now that I have butchered this....can somebody else help?
    Last edited by spearsm; 11-12-2005 at 03:24 PM. Reason: because it's me! correct mistake
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    Since I wrangle electrons for a living Iíll take a crack at this:

    1). Volts, amps and watts have very definite meanings but theyíre also so interrelated that it gets very confusing trying to explain them. In simple terms (very simple) here goes.

    Volts: Also called potential, is the pressure of electrons in the wire and can be very accurately related to the water pressure in a hose. Ever had to hold a 2 Ĺ that some chuckle head pump operator pumped at 200psi? Well try holding a wire with about a thousand volts on it. By the way, volts is a pressure only and does not have any real meaning to the amount of work being done on that thousand volt wire I just asked you to grab. If the current flow is low than you would survive just fine, think static shock. Same thing with that 2 Ĺ at 200psi, reduce the gpm to about 1 and youíd have no problem.

    Amps: Think flow. Amps is a measure of the amount of electrons flowing through the wire and pretty much defines how much work you can do with that wire. Lot of amps means lots of energy and lots of work. Same on the fire hose right? If I have lots of pressure but no flow then I am limited in the amount of fire I can put out, crank up the flow though and I could put out a volcano.

    Watts: Watts describes the amount of work being done. I canít come up with a good analogy to fire hose. The rating on your siren speaker means nothing more than the amount energy the speaker can absorb before it burns up. Use the watt designation for comparing one device to another, although efficiency is so important that a device that looks better from a watts consumed comparison may not perform as well.

    Any direct question about the three terms above usually has to be answered with ďit dependsĒ. As I already mentioned the three terms are related to each other in that one always has an impact on the other. As an example (for anybody still awake):

    A derivative of Ohmís law is P = E I Where E = Volts, I = Amps and P = Watts.

    So, a 1000w device would draw about 9 amps at 110vac. Change the voltage to 220vac and the device draws 4.5amps. The device still uses the same amount of energy regardless that two of the terms (in this case volts and amps) changed.


    2). Wires heat up because there always have some resistance to them so they resemble an electric stove burner coil. Sure, the resistance is low but if the wire is very long, or you try to flow too much you get heat. Itís almost exactly like friction loss in a fire hose (except the heat). There is no one explanation for line loss, it depends on hose diameter, length, flow and pressure, elevation and sometimes the fire Gods. Same with a wire.
    As Spearsm says there are tables that show line loss in electricity, try Radio Shack or a good electronics house (very hard to find these days).

    For anybody still conscience after all that please realize that all of the above is very rudimentary and there are always exceptions to the rule, not like in English, but because itís impossible to explain all the variables in this kind of discussion. Heck, Iíve been at it for 25years, in one specialty (engineering has specialties just like doctors), and there days when I think I donít know anything.

    Bill.

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    Dr. Whflfff,


    Did I pass?
    YGBSM!
    Eagles may soar, but weasels don't get sucked into jet engines.

    If all you have is a hammer, then your problems start to look like nails.
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    The field of electricity is challenging yet rewarding when you have that problem you worked through. If these answers posted are great. However if it still seems a bit overwhelming go to your local book store and pick up an electricity text. McGraw-Hill makes a good one that covers alot of dc as well as ac. Best part is that they really break it down for the novist.

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    Looks good so far, thanks for the help guys!

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    Oh, and something I forgot to specify. Everything I gave you is based on DC current and a purely resistive load (which is kind of redundant). That means that Ohms Law is OK for filament lamps and other similar resistive loads but donít bother applying Ohmís Law math to a common AC motor. The math wonít work.

    When you switch to AC current, things get really interesting!

    Spearsm, you pass.

    Bill.

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    Default 12 volts

    Rather than try to explaina lot of technical stuff coming from a 38 year mechanic I would rather refer you to things like the ASE certification type training stuff www.ase.com. Mitchell manuals and Motor Age manuals produce study guides in all areas of automotive and truck testing. These manuals would be great source of information for any questions you might have on electrical and other aspects of automotive and truck systems. Its not always having the information memorized in you head but knowing where to find it that counts.

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