Basic Electronics - DC Voltage on the Loop
By Douglas Krantz
A pair of batteries, just by themselves, might measure 27.2 volts.
I know that it seems trivial, but pay attention to the polarity of the voltage measured at the battery. The positive terminal will be positive and the negative terminal will be negative.
Just adding some wire to the battery will still show the same positive/negative 27.2 volts.
The wire can be lengthened quite a distance, and the measured voltage is still the same positive/negative
With the installation of a resistor to complete the circuit, and allowing electrons to flow, the voltage measured on the leads of the resistor is still the same positive/negative 27.2 volts that is measured at the battery terminals.
The source (the battery) is where the voltage comes from that pushes and pulls the electrons through the circuit. The load is a constriction or partial barrier to the flow of electrons (here, it's a resistor).
The electrons come out of the positive terminal of the source
and they go into the positive side of the load
The opposite is also true.
The electrons come into of the negative terminal of the source
and they go out of the negative side of the load
The voltage on the load will always be an equal and opposing voltage to the source of power that is pushing the electrons through the load.
In this case, the source is a battery. The source could be utility power, the terminals of a panel, parts of the circuitry between the battery and the load you're looking at, etc.
In this case the load is a resistor. The load could be a transistor, diode, strobe, horn, a whole circuit, etc.
Real Life Application
When you don't know which loop goes to where in the building
When trying to figure out where in the building a wire-pair (loop) goes, try measuring exact voltage on the screw terminals of the panel. When checking all the loops, from one loop to the next, the exact voltage on the loops will vary. Sometimes loop voltage from one to the next can vary by as much as 0.75 volts.
Use this exact voltage you're looking for as an identifier. Usually the loop can be identified out in other areas of the building because the voltage will be exactly the same as the measured voltage at the panel. If the voltage is different, many times the loop you're on is the wrong loop.
Just make sure that the loop is complete at the time that it's measured, or what you read won't be accurate.
Example: The screw terminals on the loop that you want measure 20.03 volts. Out in the west wing of the building, you measure 20.05 volts on the loop. That is the wrong loop - the voltage is too high.
Out in the east wing of the building, you measure 20.03 volts on the loop. This is the right loop - the voltage is right on.
Real Life Application
Battery Voltage is Nominal
Battery voltage is stamped on the side of the battery, but that number is NOMINAL; the voltage stamped on the side of the battery is IN NAME ONLY.
For instance, two fully charged batteries wired in series won't be 12 volts times two (24 volts). The batteries will be more like 13.6 volts each, or 27.2 volts total. The exact voltage varies according to how well the batteries are charged. This voltage slowly decreases to about 10 volts each before the batteries are basically discharged.
Even though two batteries in series say 24 volts (2--12 volt batteries times 2), the real voltage can be anywhere from 20 volts to 27.5 volts.
I'm choosing to use 27.2 volts because that voltage seems reasonable.
Having serviced fire alarm systems for nearly 20, Douglas Krantz has compiled his knowledge of the causes of Ground Faults and how to reliably detect them into the book Make It Work - Hunting Ground Faults
. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.
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