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I built a constant current load to test out some battery banks that I'm building. The constant current load is based off Dave Jones' "Dummy load". When I built it, the op-amp I had on hand was the LM358 and the circuit works as expected. Now I wanted to replace the op-amp as it was a chinesium branded LM358 one for a locally sourced NE5532 of which I have about 20. According to the datasheet the layout is pretty much like the LM358 so I thought I could just replace the fake IC with a proper op-amp. With the NE5532 in place, I cannot control the current. It just draws the maximum the DUT can put out, even with pin 3 (set-current) grounded. I know it's not the perfect op-amp for the job but using it for such easy non-critical & non-precision jobs has to be possible.

I'm clueless why it won'w work in my circuit. Please check if I'm missing anything. enter image description here

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The common-mode input range of the NE5532 does not include the device's most negative rail (unlike the LM358). Given that the most negative rail of the circuit is 0 volts, you have a problem. Choose an op-amp that can handle inputs at the negative rail AND produce outputs that can get close to that same rail.

Or, provide a 3 volt negative rail.

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  • \$\begingroup\$ Okay, testing that right away. Will report in about 30. Thanks for the heads up on that. \$\endgroup\$ – wiZZmnma Mar 20 '18 at 17:56
  • \$\begingroup\$ Okay, works like a charm. Added a LiPo-Battery for the -3.7V and it seems I may have to re-do my pcb to cater for such situations where there is a split rail or not. Thanks again. Any suggestions on what/where to read on op-amps so that such snafus don't happen again? \$\endgroup\$ – wiZZmnma Mar 20 '18 at 18:49
  • \$\begingroup\$ Look for common mode input voltage range. Virtually all opamps carry this parameter in the main performance table. Also look for output voltage range. \$\endgroup\$ – Andy aka Mar 20 '18 at 19:57

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