Trying to make a simple but reliable 15-12v to 5v circuit to power the ATTINY85 from an automobile (15v-12v), wondering if I have the right components and ratings, I don't know a lot about circuit designs so I've been living through google and google images to get to this point. I originally was going to use the LM7805 but the R-78E was recommended after much searching and states a 91% efficiency, with no heatsink required but similar size and same pinout. This will be housed inside a project box and mounted inside/behind the dash and powered when vehicle is on/running, also the ATTINY does not require a shutdown time its acting as a switching mechanism. LED is merely to show the unit is powered.

Ultimately I'm confused on if this circuit will work as planned and if the components are rated properly also what type of capacitor should be used, was planning on Alumnium Electrolytic but I'm open to anything thats through-hole soldered, or an easy to solder SMD. Any suggestions or insight would be greatly appreciated.

Diode - 1N4001

Capacitors - P975-ND

Datasheet for the R-78E5.0-1.0 - https://www.recom-power.com/pdf/Innoline/R-78Exx-1.0.pdfenter image description here

  • \$\begingroup\$ Move this question to SE Electrical engineering \$\endgroup\$
    – MatsK
    Aug 2 '18 at 8:52
  • \$\begingroup\$ there are automotive to USB sockets available that will do the conversion for you (and have proper filtering built in). \$\endgroup\$ Aug 2 '18 at 9:22
  • \$\begingroup\$ @ratchetfreak I'm aware, but I'm trying to put everything on a single board and haven't yet found a sample schematic for those style adapters \$\endgroup\$
    – dawm
    Aug 3 '18 at 3:06
  • \$\begingroup\$ Use a AEC-Q100-qualified regulator. \$\endgroup\$ Aug 3 '18 at 21:48
  • \$\begingroup\$ Also, this. \$\endgroup\$ Aug 3 '18 at 21:59

disclaimer this answer was writing from the point of view that this is to be a simple hobby circuit, placed somewhere where it is always visible and controlled. For something to be permanently installed you need an answer from someone who knows how to make automotive equipment.

You need a fuse on input in case you have a short. The power supplies by car batteries is huge and a short can lead to a fire.

Otherwise this looks good, although that SMPS is quite noisy looking at the datasheet. You might want to add a 100 nF cap on the output, parallel to your 10 uF.

Also that output cap can easily be rated for 6.3V which will make it physically smaller.

Also you might want to look for a diode with a lower voltage drop for less power wasted.

  • 4
    \$\begingroup\$ I think you need to be careful when saying it "looks good". It's for fitting behind a dashboard in a car and not mentioning connecting it via a fuse or using a TVS diode for over-voltage protection against spikes is, in this type of application, asking for trouble. This is an automotive application and I would not be inclined to say it "looks good". \$\endgroup\$
    – Andy aka
    Aug 2 '18 at 9:55
  • \$\begingroup\$ @Andyaka true that - this answer was a bit thoguthless - but what should I do about it? Remove it? Since I am not qualified enough for automotive. And considering OPs level of knowledge full fledged automotive is way out of their depth. \$\endgroup\$ Aug 2 '18 at 14:20
  • \$\begingroup\$ My intention was to defuze the answer so people didn't naturally think it was a good idea as suggested by the OP. You can do with it as you feel - it is defuzed now hopefully. \$\endgroup\$
    – Andy aka
    Aug 2 '18 at 14:23
  • \$\begingroup\$ @Andyaka probably a disclaimer then. \$\endgroup\$ Aug 2 '18 at 15:46
  • 1
    \$\begingroup\$ You really need to do some research into automotive supply line noise. \$\endgroup\$
    – Andy aka
    Aug 2 '18 at 16:10

Load dump is like 100V. it's probabaly possible to ruggedise a 7805, 78E05, 78SR04 etc sufficietly by adding pre-filters etc. but this problem now has a simple commercially available solution.

an automotive USB charger socket.

  • \$\begingroup\$ Not viable if I want everything on a single board, have a circuit design for those USB adapters? Seems to be the go-to reply but no info on building it into one's own board.. \$\endgroup\$
    – dawm
    Aug 3 '18 at 21:43
  • 1
    \$\begingroup\$ Do you actually expect any automotive USB charger sockets available to have proper filtering? I don't. I expect the worst rubbish Chinese electronics producers have to offer. \$\endgroup\$
    – Janka
    Aug 3 '18 at 23:34
  • \$\begingroup\$ I would expect the ones automobile manufacturers fit to be a quality product. \$\endgroup\$
    – Jasen
    Aug 4 '18 at 0:56

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