Trying to make a constant current source with Opamp [SOLVED]

Thread Starter

anishkgt

Joined Mar 21, 2017
529
Hello All,
I am trying to make a jig to measure the voltage drop on a 4 AWG weld cable to determine its resistance. What i have in mind is to supply a constant current on the cable and measure the voltage drop at either ends. In the simulation when i reduce the 'Load' resistance between 1 to 3 mOhm while keeping the Rsense constant at 7.641Ohms the current stays constant.

To measure a weld cable more amps would be needed to see a resonable voltage drop so i change the value of the Rsense and everything goes crazy. What am i doing wrong here.

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Thread Starter

anishkgt

Joined Mar 21, 2017
529
Rite so here is what i think i would get.

The opamp here subtracts the supply voltage from the voltage seen at the mosfet's drain to get the voltage drop of the load. The voltage is off by a couple of mV at the 'ADC'. I could add a trimpot and adjust it. Is there a better way to accomplish this task ?

Unlike the earlier one, the AD8675 has good precision that i am looking for but the price is 3$. are there any equivalent spec'ed ones.
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crutschow

Joined Mar 14, 2008
25,264
Unlike the earlier one, the AD8675 has good precision that i am looking for but the price is 3$. are there any equivalent spec'ed ones
Possibly, but you'll need to do a search to find them.
Mostly, the cost of an op amp is proportional to its precision.

Possibly just easiest to add a pot to adjust the current and calibrate out all the circuit tolerance errors.
 

crutschow

Joined Mar 14, 2008
25,264
Besides the slewrate, what other parameters should i be watching out for ?
Since this is a low-frequency application, slew rate or frequency response are not significant parameters of interest.
The main parameter for DC accuracy is input voltage offset.
 
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Thread Starter

anishkgt

Joined Mar 21, 2017
529
Thanks missed that. The MAX44251 is rated 2.7-20v with an offset voltage of 3uV guess that offset would be fine. can't find a better balance between price and offset.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
529
The max current for the weld would be between 800A to 1500A. Doing the schematic, would it be ok to drive the MOSFET driver via the opamp this way using a BJT to trigger it via the microcontoller ?

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Thread Starter

anishkgt

Joined Mar 21, 2017
529
Managed to keep it at 1kA assuming that the weld cable is 1.5mOhm. Haven't considered the pcb trace and others in the path. Not sure if the opamp can be switched with a BJT.

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ci139

Joined Jul 11, 2016
1,675
the mosfet drivers seem to be a "jewel find"
however the http://ww1.microchip.com/downloads/en/DeviceDoc/22135C.pdf states
Electrical Characteristics: Unless otherwise indicated, TA = +25°C, VDD = 5.0V, VSS = GND, VCM = VSS, VOUT≈VDD/2, VL=VDD/2, and RL = 10 kΩ to VL
Common-Mode Input Voltage Range VCMR -0.3 min 3.7 max V
Output Short Circuit Current ISC ±20 mA \(\leftarrow\) driving fast 6x 25pF = 150pF (Pg.2) ... RL never specified under 10k
Slew Rate SR 2.3 V/μs \(\leftarrow\) although the input mosfet and Schmitt make the edge sharper your undetermined timing is somewhat 1µs range

you could ask Microchip for the capabilities/substitute of the MCP6L1
, also a while (some years) ago there still were old folks active in https://cr4.globalspec.com/
 
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Thread Starter

anishkgt

Joined Mar 21, 2017
529
Thank you @ci139. For now i've decided to go with the MAX44251. Once i have the prototype ready and get it working the way i want then i will start consolidating the BOM that includes cutting down on components and making compromises on the cost to precision. Still optimistic if the above opamp would be able to drive the drivers when microcontroller sends a pulse.
 

sparky 1

Joined Nov 3, 2018
299
Making it yourself and using a mosfet with a good heat sink can give very impressive performance.

Looking at the LM317 configured as an adj voltage regulator the darlington is placed into the feedback loop.
By comparing the functional block diagram 8.2 page 9 to precision the current limiter figure 9.3.3 that is how I built my first op amp current control.
This finally made sense the output to the zener's base.
https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1595556862234&ref_url=https%3A%2F%2Fwww.google.com%2F

LM317 fairly good but I needed a lab bench PS controller to get more serious with both voltage and current I wanted a way to dial in quickly and accurately so I could adjust circuits precisely. The LM317 current limiter had greatly improved my circuits I was excited when I found this.

Original Hiland DIY 0-30V 0-1A LM317 (the keywords for search)
Finally I studied this kit after reading endless jabber on CV CC. All the semiconductors chips and flavors of the month.
Unbelieveable It could accureately adjust 1.5-27V and 2mA-3A using heat sink fan, it worked with a linear supply.
you can see in the video the huge 0.47ohm 5Watt resistor. For me this circuit looked like everything I was trying to build and understand so I needed specifics if I was ever going to figure this out. I wanted to study something known to work, that had a good reputation and was worth my time studying.
 
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