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I designed a programmable constant current power supply to control two solenoids(electromagnetic).For the solenoid to open and close proportionally(it can move between fully open to fully closed and in between) i need to change the current from 0 Amps to 0.5 Amps under 24V. My problem is after few runs the opamp tend to heat up and it malfunctions(sometimes one valve stop working) and once the op-amp burnt up(literally smoke came out and was split in the middle)

What am I doing wrong and how to fix it ?

FYI: the Arduino and 24V power supply grounds are connected even though its not clear in this diagram.

MOSFET data sheet : https://www.jaycar.com.au/medias/sys_master/images/9346270003230/ZT2468-dataSheetMain.pdf

Opamp data sheet : http://www.ti.com/lit/ds/symlink/lm358.pdf

Solenoids control circuit

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  • \$\begingroup\$ It seems like you have shown a resistor for an inductive load and the C1 into 10 Ohms with high gain may have been unstable so a power Darlington would be better suited with 12W heatsink. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 21 at 0:09
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    \$\begingroup\$ Plus the 100nF on the output could be degrading the phase margin and causing oscillation. \$\endgroup\$ – John D Feb 21 at 0:10
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 . Yes in the place of load it is an inductive load i left it blank there. The 10 Ohm resistor is the sensor resistor. Since the voltage supplied from Arduino varies from 0-5V at the non inverting end i used the voltage on sensor resistor to control the circuit from 0-0.5 Amps . Isnt that the right way to go ? \$\endgroup\$ – Malika Navaratna Feb 21 at 0:29
  • \$\begingroup\$ @JohnD I thought the 100nF capacitor stabilise the output since the Opamp is driving a capacitive load(MOSFET gate) \$\endgroup\$ – Malika Navaratna Feb 21 at 0:32
  • \$\begingroup\$ If it oscillates at 24V into 0.1uF +10R load at high frequency worst case it sees a the cap as a short CCT and a 10 Ohm load @ average 12V drop @ 60mA Isc =720 mW which is the worst case AC series loss at max current if the osc. was symmetrical sine, decouple gate with R to work. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 21 at 0:43
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To stabilize the circuit change as follows (X marks the break):

enter image description here

Across the top X add a 100 ohm resistor (to isolate the output from the gate loading)

Across the bottom X add a 1K ohm resistor

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  • \$\begingroup\$ Ok thanks for that . Does that mean i have to get rid of the 100nf cap. Im new to electronics so apologies for dumb question but would the 1K resistor at the bottom X change the Sense voltage ? \$\endgroup\$ – Malika Navaratna Feb 21 at 0:42
  • \$\begingroup\$ No you need the capacitor, the resistors are what make it work, but it could be quite a bit smaller, maybe 10nF or less even. The 1K resistor will not noticeably change the DC feedback voltage because the op-amp input only requires nA and the cap does not allow DC current to pass. \$\endgroup\$ – Spehro Pefhany Feb 21 at 1:01
  • \$\begingroup\$ Awesome !! I'll do that, cheers. Do you think that was the reason the opamp burnt up ? What should I search for(articles or keywords to search for) to understand how the resistors and cap make this setup work better \$\endgroup\$ – Malika Navaratna Feb 21 at 1:13
  • \$\begingroup\$ Maybe due to oscillation. It's unusual to have those burn up at all, unless supply voltage is too high or reversed. I would also add some capacitors across the LM358 power supply, like 470uF electrolytic in parallel with 100nF ceramic. \$\endgroup\$ – Spehro Pefhany Feb 21 at 10:38
  • \$\begingroup\$ As far as my earlier suggested changes, have a look at this application note for "in-loop compensation". Rx represents the 100 ohm and Rf represents the 1K ohm resistor in my suggested changes. CL is the gate capacitance of the MOSFET. \$\endgroup\$ – Spehro Pefhany Feb 21 at 10:40

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