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I'm building just a basic 12v regulator circuit with a LM7812 (it will be knocking a 15v wall wart down to 12v) and pretty much every example I see online uses 100uF caps in the circuit. How critical is that 100uF value?

I have some a little lower and some a little higher on hand but I'm not clear on why its 100uF or if that can change. (Generally, I'm not clear on when to use what value cap yet in circuits...)

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  • \$\begingroup\$ Higher never hurts unless the ESR is less. \$\endgroup\$
    – Brian Carlton
    Commented Apr 29, 2011 at 20:51
  • \$\begingroup\$ ESR is irrelevant in this situation. \$\endgroup\$
    – Leon Heller
    Commented Apr 29, 2011 at 21:01
  • \$\begingroup\$ well .. when you energize the circuit, the cap will look like a short circuit for a short time. larger capacitors present more stress to the devices that charge them up, potentially shortening their lifespan. you could probably use 470uF as well as 100uF; just don't stick a 50,000uF on there. \$\endgroup\$
    – JustJeff
    Commented Apr 29, 2011 at 21:40

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If you look at the data sheet you will see that the minimum requirement is a 330 nF capacitor on the input if the regulator is situated some way from the rectifier reservoir capacitor and a 100 nF capacitor on the output to improve stability and transient response. Those are the minimum requirements for the regulator itself. Additional capacitance is often provided for the circuit that is being powered; it depends on the characteristics of the load, and 100 uF is a popular value for most applications. I wouldn't worry much about the value, and use whatever you have.

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  • \$\begingroup\$ I can't find anything like that on the data sheet(s) I'm checking - can you tell me what to look under and possibly a link to the sheet you're looking at? Thanks \$\endgroup\$
    – rfusca
    Commented Apr 29, 2011 at 20:53
  • \$\begingroup\$ Try fairchildsemi.com/ds/LM/LM7812.pdf, page 23. \$\endgroup\$
    – Leon Heller
    Commented Apr 29, 2011 at 20:54

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