--- In dmcnews@xxxxxxxxxxxxxxx, "John Hervey" <john@...> wrote: > Maybe one of the EE will explain the difference between resistive load and > inductive load. Here's kind of a simple explanation... There are a couple of varieties of "resistance". What is known as DC resistance is a resisatnce to the flow of current, regardless of whetehr that current is unfluctating, such as a steady DC current flowing through a resistor, or changing, as in the case of an AC circuit. There is also a kind of "resistance" known as reactance. This is a "resistance" to CHANGES in a current or voltage across certain kinds of loads. Inductive and capacitive loads present this type of "resistance" to CHANGING currents and voltages. Changing currents may be in the form of AC currents, but currents also are changing from 0 amps or no current flow, to some non-zero value when a switch or relay completes a circuit to a reactive load such as a motor or a capacitor. In most cases when we are discussing automotive electrics, we're talking about inductive loads such as motors. Motors have what is known as inductive reactance. When a switch is first thrown completing a circuit to a motor, a very heavy current flows at first. This current is several times the amount of current that the motor draws when it is up to speed and running at constant speed and load. So components such as relays and switches used to control inductive loads must be rated to carry several times (at least 4 times, in most cases) than the steady-state current of the motor. In relays the extra capacity is achieved by a variety of mechanical and electrical design choices made by the relay designer. These include contact size, contact pressure, wiping action of the contacts, and the metallurgy of the contacts. In spite of these design choices, the relay's contacts are still under considerable stress when dealing with inductive loads and must be protected using one or more techniques designed to prevent the contacts from burning, sticking, or degrading with use. By the way, it might surprise some to learn that an ordinary incandescent light bulb can be a much more demanding load in terms of current inrush than even a motor. The reason is because the filament has an extremely low resistance when cold. When you throw your wall switch and turn on an incandescent lamp, the lamp may draw TEN OR TWELVE TIMES the steady state current it draws after the filament has heated up! SO when you turn on a 100W incandescent lamp, it may actually present a 1200 watt load for a very brief period of time. -Joe Kuchan To address comments privately to the moderating team, please address: moderators@xxxxxxxxxxx For more info on the list, tech articles, cars for sale see www.dmcnews.com To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/dmcnews/ <*> To unsubscribe from this group, send an email to: dmcnews-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/