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# Determination of voltage regulator and capacitors ratings to be used in dc supply

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#### snazzy c

##### Junior Member level 3
i need a regulated DC supply of 12V to power a device, but when i use LM7812 voltage regulator there is always a normal voltage drop of 1V, thereby having 11V instead of 12V. Is there any way i can get exact 12 V DC. Or, is there a realistic calculator or formular to use to determine the value of voltage regulator, and input capacitor(s) to be used! thanks.

The datasheet gives a min of 11.5v and max of 12.5v but these are the extremes, the output should be close to 12v.
The 11v output is not correct, there is something wrong, have you used decoupling capacitors in the input and output?
A capacitor of 100nF is typical for the output and for the input use a small electrolytic in parallel with a 100nF-330nF.

Alex

snazzy c

### snazzy c

Points: 2
If output voltage is dropping, maybe output current exceeded rated limit.

+++

snazzy c

### snazzy c

Points: 2
see the datasheet of LM7805

snazzy c

### snazzy c

Points: 2
To check if the IC is faulty (I mean half faulty since it gives 11V after all) and not your input voltage has a large ripple or the regulator is oscillating... try this:
Add a diode (as 1N400?) between its ground pin and ground node. If the minimum peak of the DC input voltage is higher than Vout+2 , then your new output would be about 11.65V.
But if you still get 11V (if not lower) then...check if there is a high ripple at the regulator input by using an AC voltmeter. For perfect DC the reading is zero.

snazzy c

### snazzy c

Points: 2
What are you using as the input to the LM7812?

If you are using an unregulated 12V supply as the input, it will not work, as the input to the LM7812 needs to be ~2V higher than the output to achieve stable regulation - so you'd need a 14V or higher unregulated input.

snazzy c

### snazzy c

Points: 2
am using an unregulated voltage of 12.7V OR 13.6V from lead acid baterry source. And there is no where i can make the voltage become 14V or more!

there must be a low dropout regulator (LDO) that can work with a difference of about 0.5v, maybe someone knows some models to recommend

Alex

there must be a low dropout regulator (LDO) that can work with a difference of about 0.5v, maybe someone knows some models to recommend

Alex

Even then, if the battery becomes discharged below 12.5V, the regulator will drop out of regulation and the output will fall.

What is it that you want to run from this circuit? Are you just trying to ensure that it doesn't receive a voltage above 12V? Will it tolerate a range of input voltages, say from 10 to 12V? If so, you can go ahead and use a suitably rated LDO. When the battery is full, above the LDO's dropout voltage, the supply will be regulated to 12V. When it falls below the dropout voltage, the output voltage will fall as the battery voltage falls.

If you really need to ensure that the output is exactly 12V, with an input that can be either above or below 12V, then you will need some form of buck-boost circuit, which is probably more complicated than you want or need :wink:

Edit: As an example, a quick search found the Micrel MIC29151-12WU (**broken link removed**) which has a dropout voltage of ~0.2V (worst case 0.6V) and can supply 1.5A. With this, you could achieve a regulated 12V until your battery voltage fell to 12.2V. Of course, it costs a bit more than a bog-standard 7812 but you get what you pay for!

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Keep in mind that 78XX regulators want to "see" almost 3V higher voltage at the input than the output, 12V output means 15V (and more) input ..
Go for LDO, some of them offer [Vin-Vout] as low as 0.2V ..

:wink:
IanP

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