Hello Everyone,

I am doing a project related to current sensing for which I need a 500 micro ohm resistor.
However 1milli ohm resistor is also fine in worst case. Resistor should be capable of dissipating 3W.

I am measuring current in order of 50A-75A and I need to have resistor as low as possible and also the resistor must be capable of dissipating atleast 2W just to avoid resistance fluctuations.

I also need a INA139 IC along with it.

I need 5 sets of these.
Hope I find it asap.
Thank you.

What is your connecting wire ? Calculate the resistance of the wire, by 4 wire method (preferably kelvin's double bridge if possible). Then measure from precise distance, and make one.

I wanted to design the circuit to reduce my cost. If I had to go through kelvin double bridge then it will increase my cost further.

Whatever will be the material, the purity will be definitely make grate percentage error.
You have to measure it once. Unlike high value resistance, you cannot neglect connecting lead/connection resistance, and materials of the same resistivity over the batches of resistive materials, for low value of resistance.

As your wattage will be 70A x 70 x .0005 = 2.5 watt appx. Specially high current.
To be precise, I am using 2000 watt heater coil (readily available).
It's normal current = 8-10A. So I should allow 4A only (to avoid variation due to heat).
So, for Max 80A, 20 nos of parallel path considered.
So, resistance of each path = .0005*20 = .01 Ohm.
Now total resistance = 26.5 ohm
So, measure 1/2650 times weight of each resistance path by weight.

However for high current, you may use DCCT principle (Magnetic amplifier).
It is far better, and used in DC Clamp Meter.

what exactly you tring to measure.. there is huge difference between 500 milli and 1 milli ohm though both are considered as current sensing resistors.

what exactly you tring to measure and category of application ..?
is 50-75 amp avg or peak. ?  as in continuous ..?

what is your project - 230 AC relevant applications -or-  low power 3 phase drive developments (say 12 to 48v bldc motor power drive as  in ESC development) -or- DC motor current measurements for motion control.

what is deadline as in time to complete your project/objective ..?

INA139 from aliexpress in retail of 10pcs/pack would cost you cheap than TI estore. ..!!

2 to 3 watt smd resistors are available from again aliexpress. smd size 2512 or bigger say.
you may find current sensing ceramic resistors from local market but may not be of precise value.
Industries prefer Manganese copper allow resistors for high precisions and better temperature stability. again you may hunt for these alloys in local market else you may procure from ebay or aliexpress. note - these are through hole ..!!  

filtering parallel to current sensing resistor may appear significant  if measuring path has current through PWM switching of mosfets/IGBTs. since more harmonic contents due to switching.
if using AC lines and relevant applications - say geyser , filtering may not be that critical. low harmonics.

finally , select current sensing resistor [CSR] carefully.

use 500 milliohm resistor only if you application has high voltage. high voltage networks (even 230 mains) usually have high resistances of order in ohms. If you working on low voltage motor drives (BLDC motor drives as in ESC developments ) then use current sensing resistors under or upto 10 milliohms else CSR will create loading limiting current and thus torque. BLDC motors usually have very low resistance of milliohm order.

500 milli and under 10 milli -  tremendous difference in magnitudes.. understand application then proceed ..!!
sensor instrumentations will be handled by INA139 offcourse.

Good wishes ..!!  

Thanks for your reply.

I have no where talked about 500 milli ohm but its 500 micro ohm.
My current draw is 100A peak.
I am working on DC wattmeter as open source project from end to end.

These components are parts APM power module. APM power module will have current and voltage sensing circuitry and I am combining that power with an embedded device in order to have wattmeter with much more feature like PC interface, graphical view and much more.

Using copper wire as a shunt resistor will have its own problem because of high temperature coefficient.
How about Hall effect sensor?
You don't have to waste power and worry about heat dissipation.
Yeah cost may be a factor.
If you want to go with shunt resistor technique then you may consider MOSFET's RDSon resistance as shunt resistance
IRFZ44N has 17.5mOhm of RDSon.

I have found a solution. I bought cheap APM power modules which have both of these ICs and cost was very low. They do have breakout pins in order to measure current flowing as well as input voltage.
Now I can use them directly with any development board and extract the readings.

Anyway thanks for your reply.