Electronics > Beginners

LM7805 unstable

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bogdant:

--- Quote from: alex-sh on February 18, 2019, 09:03:56 am ---Is your 7805 operating the same at, say, 24V? If yes, perhaps it is damaged?

--- End quote ---

If Vin <20V the LM7805 works Ok. I did try another sample, but it is the same. There are from ST.

ebastler:

--- Quote from: bogdant on February 18, 2019, 12:05:21 pm ---There are from ST.

--- End quote ---

But where did you buy them? Can you be sure they are genuine?

bd139:
None of the voltage regulators work properly above about 20V if you want 5V out. If you need to drop more, add a preregulator.

I found this out the hard way as well dropping 28V to 5V DC for logic in an RF PA driver. Currently waiting for a board spin. I only needed 50mA and didn't want RF noise so I went with a simple MOSFET/zener/resistor preregulator to drop it down to 18Vish. Because the current requirements are relatively low I don't need to heatsink either then as the load is spread evenly across both MOSFET and 7805.

Edit: also if you value the circuit it's driving, stick a 5W 5.6V zener across the regulator output. That will smoke the prereg / regulator not the load!

wraper:

--- Quote from: bogdant on February 18, 2019, 12:05:21 pm ---I did try another sample, but it is the same. There are from ST.

--- End quote ---
If it's from ebay/ali, 95+% of them are counterfeit. Post a picture.

bogdant:

--- Quote from: ebastler on February 18, 2019, 12:15:01 pm ---
--- Quote from: bogdant on February 18, 2019, 12:05:21 pm ---There are from ST.

--- End quote ---

But where did you buy them? Can you be sure they are genuine?

--- End quote ---

They are from tme.eu. I have no idee if there are contrafit.

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