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Forum Index : Microcontroller and PC projects : Voltage Regulators

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asknik2022
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Joined: 26/03/2022
Location: United Kingdom
Posts: 94
Posted: 07:27am 09 Oct 2024
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I have a general question about Voltage Regulators


Ref: 7805 for 5V DC Output.
Should the input be the minimum voltage input for less heat or does it not matter.
 
phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2448
Posted: 08:00am 09 Oct 2024
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The power dissipated in the regulator = (Vin - Vout) x I(A). Watts

So a lower input voltage = less heat but there is a minimum input for good regulation.

Eg  if the "dropout voltage" is 2.5V (typical for a 7805 but may depend on the letter after the number) then for a 5V reg. you need at least 7.5V in.
 
oh3gdo
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Joined: 18/11/2021
Location: Finland
Posts: 47
Posted: 11:23am 09 Oct 2024
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asknik2022

You can use also low dropout regulatrors lik LM2930
It has only 0.6V minimim dropout, but remenber to put 10 uF capaitor to outputput.
I remeber when I did't do so, i statrs to oscailate!
It is similar to LM7805

Pekka Ritamaki

 
asknik2022
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Joined: 26/03/2022
Location: United Kingdom
Posts: 94
Posted: 03:56pm 09 Oct 2024
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So I could use a Transformer that has 7.5 AC secondary as that would give approx 10V DC Output
Is it better to use a fixed regulator for 5V eg. 7805 or something like a LM317 with restistor setup?
 
stanleyella

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Joined: 25/06/2022
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Posted: 04:04pm 09 Oct 2024
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7805 been out years. 10uf tant caps in/out to ground. zx spectrum used one, 8v dc supply. there's a lower dropout version which runs less hot.
 
jwettroth

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Joined: 02/08/2011
Location: United States
Posts: 75
Posted: 04:50pm 09 Oct 2024
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  asknik2022 said  So I could use a Transformer that has 7.5 AC secondary as that would give approx 10V DC Output

Is it better to use a fixed regulator for 5V eg. 7805 or something like a LM317 with restistor setup?


7.5vac RMS would be good.  You have to maintain >2v through low line and transformer output droop, etc.

A 7805 would be better if your needs are just 5v. The output will be more precsise with no fuss.

Another tip if you're running out of power or dealing with heat is Morningsun and others make little 7805 plug in compatible switchers that are quite efficient and low power.  The standard ones are good for .5 amps.  

Note also that voltage drop times the output current creates power dissipation in the 7805, it starts to matter at about 400 mA out (1W).  Look at "theta" in the data sheet if you're up in this area.  Might need a heat sink.

Good Luck
John Wettroth
 
stanleyella

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Joined: 25/06/2022
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Posted: 05:22pm 09 Oct 2024
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why not just a 5v wall wart phone charger? I use a 240vac to 5v micro usb to power olimex board
 
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