I'm building a linear power supply that converts 120VAC mains voltage into multiple 9VDC, isolated outputs. I'm using this transformer that has the proper primary winding and 8 secondaries that I'll use for the 9VDC outputs. Each channel will look like this:
However, I'll be making a couple of these, and I want one of them to have an 18V output. My first instinct was to wire two of the secondaries in series and use an LM317 to get the 18V like this:
Then, I thought in order to avoid designing two PCBs and dividing my order (only making a few), I could just connect to of the 7809 outputs together to get 18V like this:
At first glance, this seems ok. Am I overlooking anything? Are there any nuances with the 7809s that would keep this from working? Am I forgetting something stupid?