Voltage Regulator Schematic Help...

Thread Starter

mattd860

Joined Jan 9, 2016
58
Hi Everyone - I'm working on a voltage regulator schematic and I found one that I'd like to use but have a questions. Can you please identify why there is a 12 volt power source circled in red? I can understand why there is the 12v-raw but I'm not understanding this second 12v wire and why it's even there to begin with.

I'm trying to create a voltage regulator so I can safely use an Arduino in an automotive application.

Thanks

Power Supply Schematic2.jpg
 

Kermit2

Joined Feb 5, 2010
4,162
It is flagged after it has passed through a diode. It will be a slightly lower voltage than the 12 volts labeled "raw".
Nothing to be concerned about.
 

dl324

Joined Mar 30, 2015
11,000
Power source can be AC or DC. Putting DC input after the rectifier saves a diode drop and allows it to be as low as 5V plus dropout voltage.
 

Thread Starter

mattd860

Joined Jan 9, 2016
58
Sorry I should have been clearer - The power source is 12 volts DC.

So the 12v circled in red is just a Flag telling me that the voltage will be slightly lower at that point due to the diode drop? Or is this a second 12v input? And if it is a 2nd 12v dc input, then where do I get this input from if not from the same power source as the 12v-raw?? Does the diagram want this 2nd input to be raw or regulated?

I'm hoping it's just a simple Flag as Kermit2 states - and I'm sorry Kermit2 if I'm second guessing you but I'm just trying to better understand all this. Thanks!!!
 

Kermit2

Joined Feb 5, 2010
4,162
You can input your 12 volts at that point as has been said. The symbol indicates a schottky diode so the voltage drop would be very small if it remains in series with the input.
Less than 1/2 a volt. Either way you choose to connect to 12 volt power would be okay.
 

Thread Starter

mattd860

Joined Jan 9, 2016
58
Okay so either input is fine and I now understand this a bit more. Since it's an automotive application would it be safer to use the 12v-raw input or is it redundant because the voltage is probably already cleaned and regulated somewhere else?
 

dl324

Joined Mar 30, 2015
11,000
D15 would protect from reverse polarity and U2 would protect from high voltage transients.

If your supply can be disconnected, adding a bridge rectifier for DC input would give you auto polarity.
 

crutschow

Joined Mar 14, 2008
25,131
Okay so either input is fine and I now understand this a bit more. Since it's an automotive application would it be safer to use the 12v-raw input or is it redundant because the voltage is probably already cleaned and regulated somewhere else?
Automobile 12V is definitely not clean and regulated.
 

Thread Starter

mattd860

Joined Jan 9, 2016
58
Alight I got it now. Perfect!!

Another question - If I wanted a 9.0v output instead of 5.0v dc, do I just change the voltage regulator in the diagram above to a LM2940T-9.0 and leave the rest of the circuit the same? Or will this require a different set of capacitors and components?
 

dl324

Joined Mar 30, 2015
11,000
If I wanted a 9.0v output instead of 5.0v dc, do I just change the voltage regulator in the diagram above to a LM2940T-9.0 and leave the rest of the circuit the same? Or will this require a different set of capacitors and components?
Everything the same except for the zener on the regulator output; which I would remove anyway. If you want output over voltage protection, you need to use something that can take more current; like a TVS diode.

If you don't use an LDO regulator, you need to be concerned with drop out voltage.
 

dl324

Joined Mar 30, 2015
11,000
Surprise! Automotive batteries are normally at 12.5 volts when properly charged.:D
When the car is running, it will be closer to 13.8V. And it isn't clear that the circuit in question was only for auto powered apps...:rolleyes:
 

#12

Joined Nov 30, 2010
18,210
I measured a car 2 days ago. The running voltage was 14.5V
Though I must admit, the battery was in perfect condition and Florida is much warmer than where most of our members live.
And I felt confident this is an automotive application because TS said that in post #9.
My bad.:oops:
 

Thread Starter

mattd860

Joined Jan 9, 2016
58
Yes - It's an automotive application and I really appreciate everyone's responses. When the vehicle is on voltage reads about 13.8 volts but when the vehicle is off voltage drops between 11 and 12 volts DC. This obviously varies between vehicles and alternators and doesn't take into account the voltage during the operation of the engine starter or if the battery fails while the alternator is running - which is why I need a regulator.

Should I swap the D16 zener to a TVS Diode whether or not I go with an output of 5v or 9v? If I go with 9v, which zener or TVS diode should I use?
 
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