I'm powering my circuit from 5V USB. Since USB is already regulated, I don't need a voltage regulator.

Will adding a modern regulator like L78M05C offer protection (besides regulation)? Concerns:

  1. Short circuit - The datasheet says it protects against this. (Is it needed? Won't any USB source have this?)
  2. Reverse voltage - I can't tell if the L78M provides it; this post suggests modern regulators do. The L78M datasheet doesn't seem to have any info about this either way, but does say "Each type employs internal current limiting, thermal shutdown and safe area protection, resulting it essentially indestructible" (emphasis added). If that doesn't include reverse polarity protection, what does it include?
  3. Current limiting (an auto-resetable fuse) - the reg. provides it, according to the data sheet. Is it needed for a USB source? (Sources may be cheap knock-off chargers.)
  4. What other protections should I have? I want to protect both my circuit but more importantly the USB source, and, since I'm a neophyte, I have to anticipate design and construction errors.
  • \$\begingroup\$ The only one you might want is short-circuit protection. Not all USB sources have it. When I'm powering from USB, making a prototype on a breadboard or something, and not needing high current, I put a 500mA polyfuse in series with the USB wire. It drops about .2 volts under normal conditions, drops all the volts in a short circuit, and self-resets after some time (if you don't want to wait, put it back in your bag of polyfuses and replace it with another one) \$\endgroup\$ Sep 13, 2023 at 16:34

1 Answer 1


Despite the 2020 revision date of the PDF, that is an extremely old regulator and the dropout voltage of 2V means that you will not be able to use it as you intend - the input voltage must be at least 7V to get 5V out of it.

As far as protections go, it depends on your circuit. Overcurrent is easily and cheaply done with a fuse. A resettable fuse has much higher resistance (and subsequently, a much higher voltage drop) than a standard one-use fuse and in most cases it's fairly implausible that your circuit would fail into an overcurrent situation and then spontaneously recover from it.

Reverse polarity is mostly avoidable by using a polarized connector, which USB is. You could use diodes (voltage drop, heat), or ideal diode controllers (cost, complexity) but if it's being powered through a USB connector, this isn't really going to be an issue.

Current limiting: see above for fuse discussion. If you are worried about the source somehow "pushing" extra current into the load, this is not something to worry about.

If you really wanted to, you could get a buck/boost switching converter and feed the USB input into that. Then you would have whatever protections are built in (overcurrent, short circuit, maybe reverse polarity) plus some additional noise rejection and input voltage variance flexibility at the cost of literal dollars, complexity, and switching noise.


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