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How to Select Rated Voltage for Power Circuit

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kuyu

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I am designing a power circuit, using a voltage regulator to transfer 18V input to 5V output. I am a bit confused about the rated voltage for the regulator. Can I use 19V or 20V regulator for this circuit?

Of course there is transient protection and over voltage protection for this 18V input before this power circuit. In theory 19V rating should be sufficient. Is there any standard on how high the rated voltage should be?
 

MikeMl

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What type of regulator? Linear or SwitchMode?

What is the input? Unregulated DC that comes from what?
 

dknguyen

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Most Helpful Member
I am designing a power circuit, using a voltage regulator to transfer 18V input to 5V output. I am a bit confused about the rated voltage for the regulator. Can I use 19V or 20V regulator for this circuit?

Of course there is transient protection and over voltage protection for this 18V input before this power circuit. In theory 19V rating should be sufficient. Is there any standard on how high the rated voltage should be?
You usually want your regulator's maximum input voltage to be at least a little bit higher than the highest voltage you are going to put into it when you run it. No standard that I know of but its completely possible I just don't know about it. I don't think you really need to go beyond 20-25% though unless there is some unusual circumstance and in that case you probably want to add transient protection rather than rely on the regulator to survive it. 20V is probably fine since it's 10% higher than 18V. 10% seems reasonable to me.
 
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kuyu

New Member
What type of regulator? Linear or SwitchMode?

What is the input? Unregulated DC that comes from what?
It is an LDO CMOS type of voltage regulator. The input comes from a synchronous buck converter, which is supposed to have a low output ripple, +-0.3V. Does that mean the output from the converter, or input for the voltage regulator, should be relatively stable? Therefore it is acceptable to use 19V rating for the voltage regulator.
 

kuyu

New Member
You usually want your regulator's maximum input voltage to be at least a little bit higher than the highest voltage you are going to put into it when you run it. No standard that I know of but its completely possible I just don't know about it. I don't think you really need to go beyond 20-25% though unless there is some unusual circumstance and in that case you probably want to add transient protection rather than rely on the regulator to survive it. 20V is probably fine since it's 10% higher than 18V. 10% seems reasonable to me.
Thanks a lot. I also think 10% looks reasonable given the power input appears to be pretty stable.
 

dknguyen

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Thanks a lot. I also think 10% looks reasonable given the power input appears to be pretty stable.
Mind the heat. 18V to 5V is going to generate a lot of heat for a linear regulator 13W for every 1 amp that is output. At that rate, your typical IC will start getting too hot to touch at around 50-100mA output.

Linear regulators work by just burning off all excess voltage as heat so power dissipated is (Vout - Vin) * Iout.

Iin on a linear regulator is Iout + a negligible amount of current required to operate the regulator itself so Iin is almost the same as Iout on a linear regulator. It doesn't take higher voltage at lower current to produce lower voltage at higher current or vice versa (as a switching regulator would).
 
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ChrisP58

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I'm concerned about where the 18 Volt number came from in your first post. If it came from the datasheet for your 5 Volt LDO, then that is probably it's maximum input voltage. In which case running it from a 19 or 20 volt preregulator is too high, regardless of how stable it is.

Please post the part number or datasheet for your 5 Volt LDO.
 

boro3

Member
you should make the input voltage of the linear regulator as low as possible becaus of the power loss. 13V (18-5=13V) are going to heat up a resistor in the ldo.
 
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