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| Oscillations with LM358 electronic load |
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| MarkF:
The only thing more I can suggest is trying a different Op-amp or MOSFET. Also, have a look at what @Jay_Diddy_B did for more ideas: https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/ Notice the 2.2Ω 2.2uF RC damping circuit on his output. |
| MarkF:
I just noticed where you indicate you have the scope probes. You can NOT put your scope ground where you show it. The scope GND should be on the circuit GND and the scope probe on the MOSFET source or drain. |
| salami738:
--- Quote from: MarkF on July 13, 2019, 11:25:00 pm ---I just noticed where you indicate you have the scope probes. You can NOT put your scope ground where you show it. The scope GND should be on the circuit GND and the scope probe on the MOSFET source or drain. ... --- End quote --- Hi, is this also a problem with galvanically isolated power supplies? The circuit under test was driven by a 12V wall adapter (230V). I also tested the circuit with a battery driven oscilloscope and got the same result (JYETech DSO 150) |
| Kleinstein:
The scope and thus PE connection would not be a real problem with an isolated supply (for the constant current circuit and the DUT). However even an isolated supply adds some capacitance that would influence the circuit (though not necessary a bad thing). With a SMPS there can also be common mode current that is injected. Usually it should be enough to watch the voltage over R5. |
| salami738:
--- Quote from: exe on July 13, 2019, 08:01:20 pm ---Wait, you have three mosfets driven from one opamp? If so, I suggest drive them individually. If you drive fets independently, does it oscillate when you use only one channel? As of stability, try 1) increasing the cap 2) lower R3 3) add resistor from opamp output to ground (100-2k, experiment here). --- End quote --- 1) which capacitor? 2) will try that later - i lowered R3 to 100 Ohms and got worse results 3) I tried a 910 Ohm from opamp output to ground. now I have oscillations way earlier at >= 300mA. |
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