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DC Current shunt resistor selection

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james.larkin

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I need help selecting the right shunt resistor to measure DC current.
The current will vary depending on the watts I need. I will be going from 35W to 120W using a 12.5Ω heater element and a programmable dc power supply. I am currenly using a dc current transducer and it is not accurate enough for my needs. Any ideas?
 
What type of accuracy do you need?

What do you do with the measured current value? Is it for automatic or manual control of the current?

Why not use a digital multimeter?
 
Q. What type of accuracy do you need?
A: In need +/- .01%.

Q:What do you do with the measured current value?
A:Log it with a data acquisition module

Q: Is it for automatic or manual control of the current?
A: Neither...I use it to calculate watts.

Q: Why not use a digital multimeter?
A: I have to log the measurements at 1 sec. intervals and place the data into a .csv file
 
Here are some expensive shunts.

Can you put the shunt in the ground lead from the heater, or does it have to go into the high side? How many amps flow in the shunt?
 
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I actually meant to put 0.1% accuracy. I can put the shunt in the ground lead. I actually have 3 heaters in parallel I am supplying with one dc power supply. The max amp draw will be 5 amps.
 
Is the resistance of heaters non-linear? Don't you trust the voltage accuracy of the programmable supply? Can't you just divide the programmed voltage by the parallel resistance of the heaters?
 
The heaters are in Ultra low temp freezers operating in ambient conditions at -80 to -150C. At those low temps the resistance typically varies +/- 1ohm. I must know precisely how many BTU's I am putting into each unit. I use this to calculate the BTU reserve capacity of each freezer. Therfore I must have this data collected live from each individual heater.
 
If you want to measure the actual heater current in conjunction with the supplied voltage to derive the heater power I would use a precision shunt like those suggested by MikeMl configured like the attached image. You don't mention much about your data acquisition module which could play a roll in the accuracy you get.

Your data acquisition device should allow for isolated analog inputs. That will allow measuring voltage at the load and across the shunt. Using a .1 Ohm shunt with 5 amps the output voltage (voltage drop across the shunt) will be .5 volt. If you are measuring the voltage across V Supply the voltage measured will not be an accurate representation of what is across the heaters.

Additionally you need to look at your data acquisition module. For example if it is a 12 bit device with a range of 0 to 1 volt for your shunt input you have 4096 quantization levels for 1 volt. Therefore 1 / 4096 = 24.41 mV per step. Using a shunt with 0 to 10 amps = 0 to 1 volt into a unit like this will give about 1/4 amp resolution which isn't very good for what you want. What is your data acquisition module?

Ron
 

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Here a method of making a voltage proportional to the current through the heater(s). I scaled the gain of the opamp to get 5V/5A, and put a trimpot in gain setting path, because the gain-setting resistors are likely to be no better than 1% tolerance. You will have to calibrate the gain.

Note that in order to use just a single supply 10V to 24V for the opamp, it must be the type whose input common-mode allows differencing of signals at ground, or even a few mV less than ground. An LM358 and similar ones allow that. If you were to use "normal" opamps, you would need to add a split +15V -15V dual supplies to run the opamp. Note the grounding method.

I was assuming that you needed three separate current to voltage converter channels, connected to three different A/D channels. If so, use a quad opamp, and build three of these circuits.
 

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