Current Sense Resistor

[hassaan1309]

Hi.
I am facing a problem in my Final Year Project. I want to develop a circuit for current sensing as a part of that project. Now the series resistance that is used to measure current (current sense resistor) is normally of a very small value (i.e 0.1 or 0.2 ohms). Can somebody tell me where can I get such a small value resistance. Normally the resistances available in the market starts from around 10 ohms... I cannot use such a large value as it will cause a large voltage drop in the circuit. So how can I get a resistance of a smaller value (e.g. 0.1 ohms)?
Thanks. :-x

 

[tpetar]

You can buy new and cheap resistor in almost every market 5W 10W 0,1Ω or put two 0,1Ω to get 50mΩ.

Resistive current shunt is not good, its worst way to measure current. Shunt have impact on circuit in some cases is very dangerous for usage (on higher amperage). Better use Hall sensor such as ACS712 or any similar or maybe A1302 (with magnetic flux - without direct contacts with circuit and there is no impact on circuit). Also current can be measured with current transformers.

Also resistive current shunts are sensitive on higher temperatures, and need to recalibrate from time to time, specially if shunt is used on high currents and higher temperatures. You cant make resistive current shunt from any material, some metals change their properties when high currents flow through them (such as steel).

:wink:

 

[Tunelabguy]

I must disagree with tpetar's conclusion. A resistive shunt is a fine way to measure current and it is done all the time in all sorts of current-feedback circuits, like motor controllers. The Hall effect method does have less impact on the circuit being measured, but it has its own problems. It is more expensive and subject to magnetic interference, and it also has to be calibrated. With a proper choice of shunt resistor the impact on the circuit being measured is usually acceptable. You do have to watch for temperature effects. But that can usually be handled by using a large enough wattage resistor. I use a 30-watt 0.1 Ohm resistor to measure current in the range of 0-10 amps. The shunt resistor dissipates much less than 30 watts, but remember the wattage rating is just for keeping the resistor from burning itself up. To avoid even the slightest bit of heating you will want to use a resistor with a wattage rating that is many times larger than the power it actually dissipates. That way it can dissipate the needed heat without substantially changing its resistance. It also helps if you use resistors with a low temperature coefficient. But this is all part of the engineering calculation.

 

[tpetar]

I must disagree with tpetar's conclusion. A resistive shunt is a fine way to measure current and it is done all the time in all sorts of current-feedback circuits, like motor controllers. The Hall effect method does have less impact on the circuit being measured, but it has its own problems. It is more expensive and subject to magnetic interference, and it also has to be calibrated. With a proper choice of shunt resistor the impact on the circuit being measured is usually acceptable. You do have to watch for temperature effects. But that can usually be handled by using a large enough wattage resistor. I use a 30-watt 0.1 Ohm resistor to measure current in the range of 0-10 amps. The shunt resistor dissipates much less than 30 watts, but remember the wattage rating is just for keeping the resistor from burning itself up. To avoid even the slightest bit of heating you will want to use a resistor with a wattage rating that is many times larger than the power it actually dissipates. That way it can dissipate the needed heat without substantially changing its resistance. It also helps if you use resistors with a low temperature coefficient. But this is all part of the engineering calculation.

Example : A1302 Hall sensor no calibration needed, ACS712 also no need for calibrations, price 0,5-0,6eur, very simple to use, no wasting of power in heat. Lets say you have 0,1R shunt and need to measure 50A what is power losses on shunt resistor ? Can you imagine losses in solar system or car system on resistor shunt on higher currents, and about voltage drop I dont even want to talk:smile: ?

:wink:

resistive shunt is a fine way to measure current and it is done all the time in all sorts of current-feedback circuits

Its old and cheap way, but of course do the job.

 

[Tunelabguy]

I looked at the datasheet for the A1302. I don't know why you say it does not need calibration. The sensitivity can be anything between 1.0 and 1.6 mV/G. Even the typical sensitivity ranges from 1.25 to 1.38 mV/G over the temperature range of -50 C to +125 C. A resistive shunt can do much better than that. Then there is the need to precisely position the Hall sensor near the wire carrying the current, or use a coil to concentrate the magnetic field. This is another source of inaccuracy. If there are external magnetic fields this can also affect the output. Perhaps the Hall device is better if you need to measure 50 amps. But you will have to calibrate it and control the temperature and the positioning and the external magnetic fields.

 

[tpetar]

I looked at the datasheet for the A1302. I don't know why you say it does not need calibration. The sensitivity can be anything between 1.0 and 1.6 mV/G. Even the typical sensitivity ranges from 1.25 to 1.38 mV/G over the temperature range of -50 C to +125 C. A resistive shunt can do much better than that. Then there is the need to precisely position the Hall sensor near the wire carrying the current, or use a coil to concentrate the magnetic field. This is another source of inaccuracy. If there are external magnetic fields this can also affect the output. Perhaps the Hall device is better if you need to measure 50 amps. But you will have to calibrate it and control the temperature and the positioning and the external magnetic fields.

A1302 have typical sensitivity 1,3mV/G at 25C. On page 6 of datasheet you can see graphs for each package UA and LH.

Example of usage of A1302 sensor (there is no need for wires, just ferrite ring core) :





Example of usage ACS712 sensor (no need for ferrite, this type of hall sensor have contact with circuit, but have small resistance and small impact on circuit) :




Just look impact what resistor current shunt have on circuit on voltage drop on higher currents and specially on wasting of power, speacially if battery is used. For each resistor current shunt manufacturer declare max allowed temperature. After some time of usage current shunt should go on recallibration because changing in material properties specially if works on higher temperature (manufacturer recommendations have priority).

I speak for professional current shunts, maded from special brass legure and precisely calibrated, they looks like this :



And this is ordinary power resistor with 0,1Ω resistance with tolerance usually 5-10% :



This first type is expensive and more accurate then second below, which is much much cheaper and of course less precise, and cannot be compared with each other in quality and precision.


Please dont mix ambient temperature with heat developed by resistor (power loss) in circuit its not the same heat source. Reading datasheet require some attention.

Ohm Law is charge for everything else.

I do not want to convince you in anything, you can use what you want, one thing is for sure you will have warm home at winter. ;-)

 

[hassaan1309]

Thanks to all for replying..