I am designing a circuit that will run on 5 V. The input however may vary from 5-12 V. Should I use a voltage regulator or an overpower protection circuit. I have found the 7805 voltage regulator but the input voltage is between 6-30 V. I understand that when you use a voltage regulator the input voltage is has to be 1 V above the output voltage. How would I work around this to get a 5 V output from a 5-12 V input?

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    \$\begingroup\$ Welcome! What's your max output current? Without any other details, you'd probably be best off with a buck regulator or buck-boost regulator. \$\endgroup\$
    – winny
    Commented Mar 31, 2023 at 9:20
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    \$\begingroup\$ Classic x-y problem. What you're actually trying to do is convert 5-12 V in to 18 V out, so find a converter that will handle that range. \$\endgroup\$
    – Finbarr
    Commented Mar 31, 2023 at 9:31
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    \$\begingroup\$ Most likely the boost converter will like 12 V or 16 V even better for an output of 18 V: Check/quote the specification. \$\endgroup\$
    – greybeard
    Commented Mar 31, 2023 at 9:32
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    \$\begingroup\$ What is the boost converter you are using? It does make no sense to decrease the voltage and then increase it again - just increase it to begin with \$\endgroup\$ Commented Mar 31, 2023 at 9:36
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    \$\begingroup\$ @fKelectronicR The chip knows the output voltage because of R1, R2 and pin 5. It will automatically adjust itself to keep the output voltage how you want it - there is no problem if the input voltage changes - the output will stay the same - within reason. The limits here are: it's a boost converter so the output voltage can't go lower than the input voltage. And the chip is designed for up to 40 volts and might blow up if the input or output is higher than that. \$\endgroup\$ Commented Mar 31, 2023 at 9:51

1 Answer 1


You don't need either because you want to power a boost converter from the output of this. It makes no sense to reduce 12V down to 5V and then up to 18V - just convert it directly to 18V.

Switching converters (buck, boost, buck/boost, whatever) generally have flexible input voltages, that is, they are voltage regulators that will self-adjust to keep the output voltage how you want it, no matter what the input voltage is (within reason). Yours should be able to take an input voltage from about 3 up to 18 volts, and still output 18 volts. You don't need to make sure the input is 5 volts.

Some general info on voltage limitations: Because yours is wired as a boost converter it can't output a voltage lower than the input, which means if the input goes up to say 20 volts, so will the output. Also, the MC34063A chip is designed for up to 40 volts; if the input or output is higher than 40 volts you risk blowing it up (which is to say, it could break - it's unlikely to make an actual explosion). The number 40 comes from the "maximum ratings" table in its datasheet. And I didn't see where it says this specifically, but based on Figure 7 I think it needs at least about 3 volts to work.


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