Why is the word "shunt" in the name "current shunt resistor"? - Page 1
Forum section: electronics / beginners

author: bitshift
author_rank: Regular Contributor
timestamp: March 25, 2015, 02:02:42 pm
content: Hey guys,So as far as I understand, in order to measure the current through a circuit, you can place a current shunt resistor in series with the circuit you would like to measure. There will be a voltage drop over the current shunt in proportion with the current through the current shunt.Then in terms of linear regulators, you get two basic types, series and shunt regulators. A shunt regular places the regulating device in parallel with the load where a series regulator places the regulating device in series with the load.So why is the term "shunt" used to describe the resistor when in reality the resistor is in series with the load?

author: BradC
author_rank: Super Contributor
timestamp: March 25, 2015, 02:12:16 pm
content: Quote from: bitshift on March 25, 2015, 02:02:42 pmBecause the resistor is shunting the meter. Without it the entire current would have to pass through the meter movement/electronics.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2015, 02:13:57 pm
content: The resistor is in parallel with the meter / measuring device.Back in the day, you'd either have a panel meter with a high amperage coil, wound for direct use at whatever the load current range is; or, a panel meter of rated current (1mA and 50uA, among others, were common) and known resistance (therefore, known voltage requirement). To use the latter with the former, you must shunt away almost all the load current, leaving just the tiniest fraction to deflect the meter.I suppose I shouldn't phrase that in the past tense. Analog movements are still available to this day! A typical panel mount device of reasonable size (~50mm?) runs $20-30 for the basic models I think.Tim

author: rsjsouza
author_rank: Super Contributor
timestamp: March 25, 2015, 02:15:21 pm
content: Or perhaps it is an excuse to joke about it, as mentioned inthis thread(not being a native speaker, I had to google for the expression)

author: IanB
author_rank: Super Contributor
timestamp: March 25, 2015, 02:19:35 pm
content: Quote from: bitshift on March 25, 2015, 02:02:42 pmThis is not a currentshuntresistor, this is a currentsenseresistor. A current shunt resistor is placed across the terminals of a sensitive ammeter like a milliameter or a microammeter to bypass the majority of the current round the meter. Thus you end up with, say, 1% of the current going through the meter and 99% of the current being diverted round the meter. This is the meaning of the word shunt in this context, to divert or bypass the current.

author: TimFox
author_rank: Super Contributor
timestamp: March 25, 2015, 03:00:01 pm
content: For panel meters to measure high currents, especially at a remote location, there was a standard 1 mA, 50 ohm d'Arsonval movement that was used with an external "50 mV" shunt, i.e. full-scale current produced 50 mV at the meter. For current > 1 A, the 1 mA through the meter was negligible. A proper shunt was designed for four-terminal connection: two heavy bolts at the outside and two machine screws near the calibrated resistance (usually flat bars of a resistance alloy). The high-current connections were made to the bolts and the meter connected to the screws to avoid calibration issues with the high-current wires.In this modern era, one can find surface-mount low-value resistors with four terminals, often used in high-current switching power supplies, for similar reasons. One should use a differential amplifier with careful layout of the sense leads with these devices.

author: bitshift
author_rank: Regular Contributor
timestamp: March 25, 2015, 03:00:19 pm
content: I can again sleep at peace. Thanks everyoneQuote from: IanB on March 25, 2015, 02:19:35 pmFantastic clarification! Thanks IanB

author: bitshift
author_rank: Regular Contributor
timestamp: March 25, 2015, 03:06:52 pm
content: Quote from: TimFox on March 25, 2015, 03:00:01 pmVery interesting thanks TimFoxWikipediahas a nice image of the exact device described.

author: TimFox
author_rank: Super Contributor
timestamp: March 25, 2015, 03:31:55 pm
content: Yes, that is a typical ammeter shunt. The screw connections shown are important: I had to help a junior employee who had found one of these and was using it to measure the current on a high-current 5 V supply, but one of the small screws was missing so he made that connection to the large bolt at that end. The resulting measurement was off by a large factor. If I remember correctly, the shunt was a 50 A full-scale unit, and the resistance element was flat instead of the rod in your photo.