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Voltage Regulator Question?

Hi Guys,

Looking a bit into voltage regulators. When I'm looking at the wiring diagrams on the datasheets, most of them indicate that it requires a 30V+ish input voltage to work. I was hoping to get a small adjustable voltage regulator to run off the 12v range. I really don't need anything over 12volts anyway. Can I get this to work or do I have to find a power supply capable of providing 30V+ for the voltage regulator?

I was looking at models such as the lm117 if it helps.
Also, If I do need 30v+, where can you suggest I get that kind of power supply? I usually use 12v wall adapters.

Thanks!

You do not need an input voltage of 30V to a linear voltage regulator to make it work.  This number near 30V DC, is likely the maximum input voltage.

In contrast, the minimum input voltage is usually the output voltage plus a constant of about 3 volts.  E.g. if you want the output voltage to be 12V, then the minimum value of the input voltage should be about 15V. 

This voltage difference, between the output voltage, and the minimum input voltage, is called the dropout voltage, and that number (typically around 3 volts) is probably mentioned in the data sheet somewhere.

The other thing to consider when considering using a linear regulator, is the amount of current you want to supply to your load.  The amount of power dissipated by the regulator, as heat,  is essentially that current multiplied by (Vin-Vout), the voltage drop across the regulator. 

So basically what that says, is you want to make (Vin-Vout) small to not make heat and waste power, but at the same time you cannot make (Vin-Vout) smaller than that minimum dropout voltage, because in that case the regulator won't be doing a good job of regulating; you know the noisy noise on Vin will start creeping into Vout, and the whole point of using a regulator was to keep Vout smooth and constant.
transistorguy (author)  Jack A Lopez5 years ago
Thank you very much! That makes much more sense now!
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@ author
There are alternatives (slightly more expensive) in the family of low-dropout regulators that need as little as half a volt to provide reliable regulation, but standard off the shelf parts keep with that 3v number.
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