Whats the function of a shunt resistor?

[little_carlos]

why and when should i use one?

[Dago]

Shunt resistor is usually a low value (low resistance) resistor that is used for current sensing. A current going through it causes a voltage drop (as per Ohms law) which is then usually amplified and monitored. The idea is to keep the resistance as low as possible to cause as little power loss (and other disturbance) for the circuit as possible. The value is usually chosen to be as low as your amplifier (usually op-amps) permits (input offset voltage etc.).

[fivefish]

If you want to extend the range of your ammeter, you can use a shunt resistor across the meter.  The idea is to divert some of the current to your shunt resistor, and let the rest flow to the ammeter. This effectively increases the range of your ammeter. For example, if your ammeter only has a 100mA full scale reading, it can now become a 1Amp full scale meter.

[Brumby]

If you want to extend the range of your ammeter, you can use a shunt resistor across the meter.  The idea is to divert some of the current to your shunt resistor, and let the rest flow to the ammeter. This effectively increases the range of your ammeter. For example, if your ammeter only has a 100mA full scale reading, it can now become a 1Amp full scale meter.

Absolutely true.

You can do this on your meter's voltage or current ranges - you just need to do the maths and be able to get a high precision value resistor of suitable power rating to do the job.

The voltage drop across the shunt is the bane of any technician - professional or hobbyist - and it is the one of the reasons Dave came up with his micro current unit: http://www.eevblog.com/projects/ucurrent/ 

Measuring really, really tiny currents was another.

[eas]

If you want to extend the range of your ammeter, you can use a shunt resistor across the meter.  The idea is to divert some of the current to your shunt resistor, and let the rest flow to the ammeter. This effectively increases the range of your ammeter. For example, if your ammeter only has a 100mA full scale reading, it can now become a 1Amp full scale meter.

Note for new players: A configuration with two parallel shunts introduces an additional source of error. Differences in the current carried by each shunt means that they have to dissipate different amounts of power. Differences in the cooling of each shunt means they may heat at different rates. Differences in the temperature coefficient of each shunt can mean that their resistances change at different rates due to self-heating (and also long-term drift). Taken together, the % of total current carried by the measured shunt may (will likely) change over the full measurement range, as ambient temperature changes, and as the components age.

[Tim T]

The other day I was looking at the circuit of a Saab P/N 4109070 headlight lamp control which contains a pair of Atmel U479B automotive lamp outage monitor ICs. The data sheet of the U479B says “If the voltage drop across the shunt resistor exceeds 8mV, the output is turned off.” The output here goes to the base of a PNP BJT.


tim t.

[TimFox]

Also note that a good shunt resistor often has four terminals:  separate pairs for current and voltage sense.  This is to avoid errors in the external voltage drop across the high-current wires.  Traditional ammeter shunts had a pair of heavy bolts at the ends, with a pair of smaller screws near the actual resistor element.  Modern SMT shunts have what appear to be equivalent pads at the corners to separate the internal paths.

[macboy]

"Shunt" comes from the use of the resistor as a shunt, which means that it is connected across something. When mechanical/analog meters were used, they usually deflected full scale at some small voltage (commonly 75 mV). So you would choose a shunt resistor that would have 75 mV of drop at the required full-scale defection of your meter, and connect it directly across the meter input terminals, creating a shunt across the meter terminals, for current to flow through. They are still used this way, but usually the meter is digital. And "shunt" resistors (as opposed to generic power resistors used as such) are often still specified with 75 mV full-scale, so e.g. 75 mV @ 100 A. This is awkward for digital meters, which would be better served with a decimal scaling like 100 mV @ 100 A.

[Simon]

Thread cleaned up!

[little_carlos]

"Shunt" comes from the use of the resistor as a shunt, which means that it is connected across something. When mechanical/analog meters were used, they usually deflected full scale at some small voltage (commonly 75 mV). So you would choose a shunt resistor that would have 75 mV of drop at the required full-scale defection of your meter, and connect it directly across the meter input terminals, creating a shunt across the meter terminals, for current to flow through. They are still used this way, but usually the meter is digital. And "shunt" resistors (as opposed to generic power resistors used as such) are often still specified with 75 mV full-scale, so e.g. 75 mV @ 100 A. This is awkward for digital meters, which would be better served with a decimal scaling like 100 mV @ 100 A.

Thanks, helped me a lot, I was wondering that because im designing a linear psu and I needed one for feedback at the current control