7809 Linear Voltage Regulator Outputs in Series

Question

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:

^{simulate this circuit – Schematic created using CircuitLab}

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:

^{simulate this circuit}

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:

^{simulate this circuit}

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?

Answer 1

It should work. Spreading the heat dissipation between two regulators may make things easier.

You may wish to put an inverse-parallel diode across each of the 9V outputs that you are going to series. The reason is that if you consider what happens when the output is short-circuited - there will be two regulators each trying to drive the other one negative and one will win. That may harm the regulator that's driven minus and it also could harm the output capacitors (if they're polarized types). Something like a 1N5403 would work fine.

Answer 2

It probably works, but it just feels wrong to me ;-) However, you could design a single PCB that can be used with either the 7809 or LM317. The difference is just the presence of R1 and R2 (and the regulators have a different pinout, but just take that into account in the PCB layout).