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This may be a beginner question but I am kinda lost here. My Vin is either 24VDC or 12VDC - controlled by a rocker switch. If 24V is selected it gets regulated by a buck to 12V ~7A max. If 12V is selected it skips the regulater and I just connect it to the 12V port.

Now I was wondering for a protection in 2 Way:

  1. User has switch set as 12V but connects 24V - Could I put in a fuse to make it "break" on purpose?
  2. User sets switch to 12V and connects 12V - What happens to the output side of the 24V regulator, it get's 12V and loads up the inductor ? Do I need to stop the 12V from going there and if how?

Example schematic (no realistic values):

schematic

simulate this circuit – Schematic created using CircuitLab

Thanks for your help !

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  • \$\begingroup\$ You can control Vin with a relay in turn controlled by the rocker switch (if you like it, if it is already in the panel, etc.), but by sensing the Vin you can disconnect for safety if you detect e.g. 24V when it is supposed to be 12V. (In place of the fuse. You are never sure if the fuse will work because I am afraid that the short circuit current is not well set, unless you put a -big- zener to help it) \$\endgroup\$
    – andrea
    May 7 at 12:48
  • \$\begingroup\$ In my opinion, whatever you try to protect, fuse or switch, your are just building a killer/suicide circuit. Just get a readymade buckboost regulator, something like 5-30V to 12V. With less than 20€ you'll be safe whichever supply you connect. \$\endgroup\$
    – carloc
    May 7 at 13:37
  • \$\begingroup\$ I thought about that but they are mostly linear regulators that come in this size. In worst case I need about 80W constant load for a long time. Linear regulators would get way to hot or need way to big coolers in this size I guess. I.e. couldnt find one that fit my specs. \$\endgroup\$
    – Daniel Do
    May 7 at 13:46
  • \$\begingroup\$ m.banggood.com/… This is the very first one popping up, there are tons out there \$\endgroup\$
    – carloc
    May 8 at 7:26
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User has switch set as 12V but connects 24V - Could I put in a fuse to make it "break" on purpose?

If you just want to ensure that the load is not going to see 24V, instead of sensing the source, why don't you add an over-voltage protection at the load side. Maybe a crowbar or a back-to-back switch should work fine. See circuits below

User sets switch to 12V and connects 12V - What happens to the output side of the 24V regulator, it get's 12V and loads up the inductor ? Do I need to stop the 12V from going there and if how?

You could add diodes at the output of the buck-converter and at the 12V line, thus decoupling them. See circuit below.

How it works?

If the output voltage is high enough to turn on the zener diode, some current will start to flow through it thus trigerring the SCR, thus short-circuiting the input and blowing the fuse.

If the zener's tolerace does not suffice your needs, you could replace it with a reference voltage e.f. TL431. The sensing logic would have to be adapted accordingly.

schematic

simulate this circuit – Schematic created using CircuitLab

Here is another option with back to back FETs.

How it works?

Initially both FETs M1 and M2 are pulled to ground through Q1, therefore they conducting. If the output after the diode D2 is big enough to turn on the diode D4, the output MOSFETs will be turned off.

schematic

simulate this circuit

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  • \$\begingroup\$ Is it on purpose to only use 1 output of the switch here ? \$\endgroup\$
    – Daniel Do
    May 7 at 13:33
  • \$\begingroup\$ What is D1 supposed to do? If 24 V comes in it will pass it to the output. \$\endgroup\$
    – Transistor
    May 7 at 13:34
  • \$\begingroup\$ @Transistor you are right, it has no purpose. Only if the OP would like to have the same voltage drop in both lines. \$\endgroup\$
    – vtolentino
    May 7 at 13:45
  • \$\begingroup\$ @DanielDo No, it was a mistake. I did not notice that when I was dragging the switch :) \$\endgroup\$
    – vtolentino
    May 7 at 13:47
  • \$\begingroup\$ Slow down. You're short-circuiting the buck converter! With 24 V in you get 24 V out. \$\endgroup\$
    – Transistor
    May 7 at 14:01

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