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I have a Pi Pico project I am working on and I ran into a hiccup. I have a 12VDC soft start that I used from this thread. I pretty much used the same thing, but I changed the MOSFET to IRF9620 . I am using the soft start because the datasheet of the 5VDC Converter down stream suggested one was used.

the full circuit below: SoftStart_5VDC

When I turn on my power supply to 12V and .07A, it goes into current limit mode with voltage only reaching about 4V, and current obviously limited to .07A
The R7 resistor is very hot and the output of the MOSFET is only 1.5-2V.
I blew R7 previously by connecting a scope to the C7 input and to ground.

Any thoughts? Is the MOSFET not fully closing? It is not getting hot - no noticeable heat when touched.

Thanks, Pete

peterman
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    With 0.07A current limit you are not able to get higher voltage on such a small resistance (24ohm). – Michal Podmanický Jul 09 '23 at 22:09
  • I increased the current limit to .5A and burned out R7. Should R7 be a higher value? – peterman Jul 09 '23 at 22:20
  • There is no reason to R7 be so small. It just drains power. Use R7 = 4k7 or 10k. – Michal Podmanický Jul 09 '23 at 22:28
  • With 12V R7 would be pulling .5A, no? Increasing the value to 5k would bring that current across R7 way down. I can deal with the small voltage spike initially, but would this solve the problem? Currently, R7 is spec'd for .25W, so it is quite small. – peterman Jul 09 '23 at 22:29
  • @MichalPodmanický - Thanks! I will try that – peterman Jul 09 '23 at 22:30
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    The 24ohm in original post was placed as load resistor just to show circuit behavior in chart. But in your case the load is a 5V converter so use a R7=10K just to discharge the capacitances after power-off. – Michal Podmanický Jul 09 '23 at 22:37

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