Thanks for mentioning the Voltage Drop, I wasn't aware of it ,should have read the data sheet more carefully.
I was totally confused with getting 0v after the diode. Without the nodemcu the diode shows the same behavior meanwhile I think it is broken after all.
You're right. I should have explained the whole setup from the start. Maybe there is a better solution for my probem.
I have a Nodemcu that drives a servo and gets data from a few sensors. To power everything i use an LD33.
This setup works fine until i use the usb port of the nodemcu to upload a sketch or get serial data. I always cut the Power to the LD33 when doing this because i don't want 2 power sources (thats a bad thing right ?)
if i then connect the USB the nodemcu onboard regulator tries to power everything and fails because it can't handle it (nodemcu doesn't boot properly, cant even upload a sketch ). I fix this by disconneting the nodemcu 3.3v Pin from which i get power when not using usb.
What i hoped to achieve was that power can go to the nodemcu but not from it to the rest of the components.