Linear voltage regulator + reverse polarity protection

[bdunham7]

It's even to say that a fuse burnt isn't a common issue if things are done properly... so an alternative could be to use a polyfuse? the problem is it's not fast, right?

You would need to think about what scenarios you want to protect against before deciding on how to do it.  The zeners-and-fuse approach I detailed above was a sort of belt-and-suspenders that I think would be robust enough for a situation where the power supply was being connected and setup repeatedly by different people and you wanted to protect at least your microcontroller against any misuse--up to and including accidentally using mains power as an input.

Fuses are disfavored in production use because in todays throw-away no-service world, dead is dead and salvaging or repairing the unit is a non-issue.  Fuses are often only considered to prevent fires and the ensuing liability.  In a lab or development environment, a simple fast-blow fuse will save the day as or more effectively than more elegant solutions.  I would even solder the fuse in because taking the time to find your soldering tools and desolder the fuse will allow you to reflect on what went wrong!

[wizard69]

It has a lower thermal throttling limit so in effect it does have a lower heat tolerance even thought they're in the same package.  Still I think the lm78m05 is the best choice given the max current draw of the device of 350ma, in my thermal testing on a  breadboard of a lm317@12v input-5v output 200ma was around the max current  before it would thermal throttle.  I'd guess in normal operation the device will draw well under 50ma so a heatsink wont be needed but it really depends how much current it actually draws in normal operation. 
A resistor on input would work at limiting current but it could lose some flexibility on what supply voltage you use, I don't like the fuse idea because it would need to be replaced if it burned out from over current.

Most three terminal regulators are fairly rugged so a fuse might not do much to protect the regulator.   However a fuse might protect something else so the need to implement such really depends upon your application.   It is likely advisable to protect the power source, for example if the supply comes from a simple step down transformer a fuse might be advisable.

It's even to say that a fuse burnt isn't a common issue if things are done properly... so an alternative could be to use a polyfuse? the problem is it's not fast, right?

I'm not sure what the negativity with respect to fuses are.   In normal operation they should not blow.   When they do one should figure out why.

[Lucky-Luka]

I'm not sure what the negativity with respect to fuses are.   In normal operation they should not blow.   When they do one should figure out why.

If they are not fast enough to open the circuit there should be damages to the other parts. At least that's what I thought. Am I wrong?
btw can anyone point out a thorough hole fuse that should be a good idea to put in this kind of circuit?

[ohms law]

Could you not just use a bridge rectifier on the input of your circuit to sort out reverse polarity protection issue?

[Zero999]

I'm not sure what the negativity with respect to fuses are.   In normal operation they should not blow.   When they do one should figure out why.

If they are not fast enough to open the circuit there should be damages to the other parts. At least that's what I thought. Am I wrong?
btw can anyone point out a thorough hole fuse that should be a good idea to put in this kind of circuit?

Yes, that's true, but the regulator's overcurrent protection should prevent that to some extent. The fuse is only there to prevent the battery catching fire, if there's a short circuit, i.e. the regualtor catastrophically fails.

Looking at the data sheets for the LM78M05 and LM7805, for the TO220 packages, the LM78M05 has a lower junction to case thermal resistance of 1.3°C/W vs 1.7°C/W of the LM7805, but a lower maximum junction temperature of 125°C, vs 150°C for the LM7805.
https://www.ti.com/lit/ds/symlink/lm340.pdf
https://www.ti.com/lit/ds/snvs090g/snvs090g.pdf

TI thermal resistance numbers in datasheet are quite low. Rja =23.6 °C/W for TO-220 case is very, very optimistic. Who knows all the tricks they have used to achive this measurements....

ST Micro gives Rja = 50 °C/W for a comparable L7805, and god only knows what one will get with generic 7805 from shenzhen market.
https://www.st.com/resource/en/datasheet/l78.pdf

I agree. Thanks for pointing that out. The thermal to ambient specificication on TO-220 transistors is often aounr 60°C/W.

The product will not be serial produced. It's for a dev board for a college course. Some will be produced if the prototype will be good enough.
It will be something like this:
https://www.rhydolabz.com/development-boards-development-boards-c-206_99/pic18f4580-can-development-board-rhydolabz-p-298.html
I've attached a new shematic. Better now? Right now I have 7805 at home, I will later think of other 78x05 solutions.
Will it work even using 9V power supply, right?
the power supply will be something like this:
https://www.amazon.com/LEDMO-Power-Supply-Transformers-Adapter/dp/B01461MOGQ/ref=sr_1_3?dchild=1&keywords=12v+power+supply&qid=1612361847&sr=8-3
or maybe:
https://www.amazon.com/outstanding-Battery-Holder-Storage-Switch/dp/B081395LK3/ref=sr_1_3?dchild=1&keywords=6+battery+holder&qid=1612361958&sr=8-3
or
https://www.amazon.com/LAMPVPATH-Pack-Battery-Holder-Switch/dp/B07T83B4SW/ref=sr_1_1_sspa?dchild=1&keywords=9v+battery+holder&qid=1612362003&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzM1Q2WlI5SzBJSEdKJmVuY3J5cHRlZElkPUEwODQ1NTMyMUdDWFZFMFpQSzlLSyZlbmNyeXB0ZWRBZElkPUEwMzE3NjIxMUY4UlpKNkVVM0RPMSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

The 12V power supply is fine, but if you're going for six AA cells, you might want to consider a low drop-out regulator and Schottky diode, or use eight AA cells. The LM7805 will stop regulating, when the voltage falls below 7V, plus 0.6V for the diode, giving a minimum voltage of 7.6/6 = 1.27V per cell, at which point alkaline cells still have a fair amount of energy left and it won't work very well with rechargable batteries, which are 1.2V. With a low drop-out regualtor and a Schottky diode, it will work down to 6V, giving a longer battery life and compatbility with rechargable batteries.

A common, cheap, Schottky diode is the 1N5820, will do. For the low drop-out regulator, the are plenty of options, but beware of fakes from the likes of ebay.
LD1117V50
LM2940CT-5.0
LF50AB
TLE 4274 V50

[bdunham7]

If they are not fast enough to open the circuit there should be damages to the other parts. At least that's what I thought. Am I wrong?
btw can anyone point out a thorough hole fuse that should be a good idea to put in this kind of circuit?

It depends on how the circuit is designed, but what I described above would protect at least the MCU (and likely everything) from almost any lab-accident misconnection.  Lightning strikes might be pushing it.  Fuses from reasonable manufacturers all have detailed specifications on fusing energy, time-to-blow at specific overloads, etc. 

https://www.mouser.com/datasheet/2/240/Littelfuse_Fuse_370_Datasheet_pdf-357362.pdf

The 12V power supply is fine, but if you're going for six AA cells, you might want to consider a low drop-out regulator and Schottky diode, or use eight AA cells. The LM7805 will stop regulating, when the voltage falls below 7V, plus 0.6V for the diode, giving a minimum voltage of 7.6/6 = 1.27V per cell, at which point alkaline cells still have a fair amount of energy left and it won't work very well with rechargeable batteries, which are 1.2V. With a low drop-out regulator and a Schottky diode, it will work down to 6V, giving a longer battery life and compatibility with rechargeable batteries.

I completely agree with using something like the LM2940 if he is using a battery pack, but it is worth pointing out that the behavior of an LM7805 as the input voltage drops isn't that it completely stops working, but rather it becomes less and less effective at filtering, which becomes less important with a battery since there's no input ripple.  It also will have the effective output impedance go up, so load regulation goes away too.  But the output voltage doesn't start dropping until quite a bit lower than 7 volts if the current is as low as this project is likely to be, less than 200mA.  The exact performance will vary a bit from example to example and brand to brand.