Originally posted by jibbguy
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The term comes from "current shunt" ; a very common method of determining Current in a circuit path through reading the Voltage across a "Shunt", and then using Ohm's Law to calculate the Amperes (since there really is no direct way to read "current").
As examples, a "1 Ohm" shunt resistor with "5 Volts" reading across its ends, represents "5 Amps" of current. A "10 Ohm" shunt R reading "5 V" would represent "0.5 Amps".
Most Shunt Resistors are very low Ohms, and rated for high wattage. For these reasons, "regular" resistors used in circuity usually won't work in their place, because they are too high a Resistance (and so could significantly affect the circuit's performance skewing the data), and/or have too low a current rating (so they burn up under load).
Large current shunts used in industry are often simply rectangular blocks of steel or iron; it doesn't matter much, as long as the precise Resistance is known so the calculation can be done, they can handle the current, and the R remains stable as temperatures rise.
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202:
Current shunts are often "floating off ground", for that reason it is important to watch where you put the ground lead of scope probes when reading them (or any other "Single-Ended to Ground" instruments that are not specifically stated to be "Input to Output Isolated"). Hand-held DMM's ("digital multi-meters") and some "scope-meters" are battery powered, with no ground connection, so it is not a problem with them. But it can be a BIG problem with "single-ended" scopes that get a ground through their power supply / wall plug connection, and could cause them to break the house "No Smoking" rules
As examples, a "1 Ohm" shunt resistor with "5 Volts" reading across its ends, represents "5 Amps" of current. A "10 Ohm" shunt R reading "5 V" would represent "0.5 Amps".
Most Shunt Resistors are very low Ohms, and rated for high wattage. For these reasons, "regular" resistors used in circuity usually won't work in their place, because they are too high a Resistance (and so could significantly affect the circuit's performance skewing the data), and/or have too low a current rating (so they burn up under load).
Large current shunts used in industry are often simply rectangular blocks of steel or iron; it doesn't matter much, as long as the precise Resistance is known so the calculation can be done, they can handle the current, and the R remains stable as temperatures rise.
___________
202:
Current shunts are often "floating off ground", for that reason it is important to watch where you put the ground lead of scope probes when reading them (or any other "Single-Ended to Ground" instruments that are not specifically stated to be "Input to Output Isolated"). Hand-held DMM's ("digital multi-meters") and some "scope-meters" are battery powered, with no ground connection, so it is not a problem with them. But it can be a BIG problem with "single-ended" scopes that get a ground through their power supply / wall plug connection, and could cause them to break the house "No Smoking" rules
The term "Shunt" means "to bypass" and represents a parallel pathway. Metering systems are very sensitive to current, it only takes a small amount, usually microamps to push them full scale. So to measure the amperage in a circuit it is necessary to provide a parallel path for the larger currents to flow through and a ratio is determined between the meter current and the "shunt" current. Most current meters (aka ammeters) today incorporate internal shunts and are simply placed in series with the current carrying wire.
When instantaneous current measurements are required, it becomes problematic to use meters that provide internal integration. In this case a CSR or current sensing resistor is used which converts the current flowing through the resistor to a voltage across the resistor. These resistors must be noninductive and noncapacitive to work properly, otherwise the instantaneous voltage will not be commensurate to the instantaneous current. A CSR is a series device and the voltage across it can be measured with field effect equipment where zero parallel current flows through the measuring device. For this reason a CSR should not be called a shunt.
These are semantics and I am probably a purist in that regard but the proper use does reflect directly whether a reputable institution would cite the use of it in their works, in my humble opinion.
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