Is it safe to connect linear voltage regulators in parallel to a common source?

Could work if you need only small amounts of 5+ current.  Read about the ratings of your parts.

Do you plan to use an input transformer?  Seems so.  You need to run some formalisms.  The 12V regulator needs a little overhead.  You should sim in LTI spice.  Your 5V regulator will consume more power than its load.

Yeah, 95% of the system is powered off the 12V, and there's only max 100mA draw on the 5V, which is why I didn't want to put the 7805 after the 12V regulator.

Yeah, 24VCT transformer. ~17V peaks going into the rectifiers and I specced everything after to provide at least 12.5V to the 12V LDO under worst case conditions (temperature & line fluctuation).

If your 5+ reg does get warm, in this case you can use an intermediate regulator to share the thermal load if this is a one-off.  Leave room for a heatsink, should you need one.  There are some pretty good application notes published for these devices.

Agree with most of the comments and I think these days, a simple buck regulator for the 5V would be way more efficient but at the end of the day, if you only need 5V at 100mA and its not battery powered and you don't care about the wasted power then it will dissipate about 17-5 = 12V @ 100mA = 1.2W, if you're using the 7805, not a 78L05 then it will take the current no worries but will still get quite warm, a small clip on heatsink would help the situation

Aside from that, no worries about connecting both inputs to the same source, it can take up to 35V on the input

small clip on heatsink

Do you have a picture or example?

If your chip package doesn't accommodate a heat spreader readily, use a ten or twelve volt regulator to feed the five volt regulator.  Then your heat will be dissipated by two devices.  My advice about checking your component ratings was motivated by a desire for you to be confident rather than have you accepting my authority. I am aware of the BV of a 7805 regulator.

The way you have drawn it is the way it should be implemented. You don't want the 12 Volt supply to affect the 5V rail at all, that is, if the 12 Volt rail is damaged, you've just lost the 5 as well. As for what regulators to use, you have to specify how much noise you can handle. If you are working on analog electronics, do not, and I really mean do not, use a switching regulator. I have seen switching noise show up too many times in analog applications. If you are just working on digital electronics, by all means use a switching regulator.

Hi Stephen,

I did a blog a while ago on the difference between the linear and the switched buck converters. If you don't check out anything else, check out the link at the end of the blog to Recon's Book of Knowledge. It is free to down load and has a lot of good information on DC to DC voltage conversion and regulation. Here is a link to the blog I mentioned.

https://www.element14.com/community/people/jw0752/blog/2015/04/30/bench-testing-the-recom-r-78e50-switching-converter-against-the-lm7805

I have had to edit this post as the link to the Book of Knowledge from Recom wasn't working in my blog post. Here is a direct link to the book:

http://www.recom-power.com/en/emea/downloads/book-of-knowledge.html

John