Feeding a 7812 with 12V.

[Pommie]

I have a board which I want to use for two different projects. They need different input voltages - 12V and 18V. Part of the board can be safely powered from 7-12V. I'm thinking of using a 7812 to regulate the input voltage. When fed 18V it'll produce 12V but what will it produce if fed 12V - I'm (guessing) hoping it'll be around 10V, the datasheet states a dropout voltage of 2V,

Will this be OK?
Will I need to add additional smoothing capacitors?
Are the caps in note 1 sufficient?

Thanks,

Mike.

 

[Musicmanager]

Hi Mike

I guess suck it and see is the obvious answer .. .. ..

I've have a 7812 set up on a stripboard with the smoothing caps according to the datasheet and nothing else (load) connected to it .. .. I put exactly 12vdc in and 11.27 vdc came out .. .. .. .

Does that help ?

MM

 

[crutschow]

I suggest a low-dropout type regulator which typically have a dropout voltage of less than a volt.

 

[JimB]

I put exactly 12vdc in and 11.27 vdc came out .. .. .. .

Not the basis for a well designed product.

Part of the board can be safely powered from 7-12V.

So why not use an 8 or 9 volt regulator?

JimB

 

[Pommie]

So why not use an 8 or 9 volt regulator?

Why didn't I think of that. Doh.

Thanks,

Mike.

 

[Pommie]

Hi Mike

I guess suck it and see is the obvious answer .. .. ..

I've have a 7812 set up on a stripboard with the smoothing caps according to the datasheet and nothing else (load) connected to it .. .. I put exactly 12vdc in and 11.27 vdc came out .. .. .. .

Does that help ?

MM

Thanks, it does help but I'm wondering how stable the output is and what will happen under load.
Going to see if I can find a suitable 9V regulator - the obvious choice now.

Mike.

 

[Nigel Goodwin]

The linear regulators are usually pretty good, but do you need any great stability?.

What sort of current are you needing?, bear in mind you might need large heatsinks and run very hot.

Have you considered nice cheap switch-mode Chinese modules?, they cost very little and far out perform an LM7812, plus they are often variable.

 

[Pommie]

I will never need more than ~100mA so I've settled for a (lm)1117 in TO 252 set to 9V. With copper pours (as heatsink) top and bottom it should manage well. Not figured out how to connect top and bottom pours with vias yet. That's for tomorrow. 8pm here - beer O'clock.

Mike.

 

[danadak]

Just a side comment, when a reg goes into dropout its ripple and noise rejection
are compromised. So really never a good idea for a design to use this characteristic.


Regards, Dana.

 

[Pommie]

I've now got a 1117 with, what I think is, an adequate heatsink. I've drawn a separate schematic to demonstrate,

On the circuit board I've define 2 copper pours around the 1117 and a Ground pour on the top layer,

The top and bottom layers are connected with 12 pads arranged as shown.
If I turn off the top layer then the bottom area is shown,

The area is 25mm (1") by 20mm (0.8") with the 12 connecting vias (actually pads).
Is there a way to calculate the wattage this package/heatsink combo can dissipate?

Any comments/advice greatly appreciated.

Mike.
Edit, just discovered I can change the "Pad Connection" on the pour to solid (from spoke) to get a solid area.

 

[For The Popcorn]

It there any chance the output will have voltage on it while the input does not? A forward-biased diode from output to input will protect the voltage regulator if this ever happens.

 

[danadak]

And -

Not to forget input Reverse V protection -

https://fscdn.rohm.com/en/products/databook/applinote/ic/power/linear_regulator/linearreg_reverse_voltage_appli-e.pdf

Regards, Dana.

 

[eTech]

I have a board which I want to use for two different projects. They need different input voltages - 12V and 18V. Part of the board can be safely powered from 7-12V. I'm thinking of using a 7812 to regulate the input voltage. When fed 18V it'll produce 12V but what will it produce if fed 12V - I'm (guessing) hoping it'll be around 10V, the datasheet states a dropout voltage of 2V,
View attachment 136832
Will this be OK?
Will I need to add additional smoothing capacitors?
Are the caps in note 1 sufficient?

Thanks,

Mike.

I don't have a LM7812 but I have an LM7815.

If the input voltage is below 15v+2v(DO voltage), the output follows the input minus a few millivolts, if unloaded.
If loaded, the output voltage drops relative to the load current (an increase in load current causes a greater voltage drop). So basically, it doesn't regulate the voltage if the input voltage is less than 2V greater than the spec'd output voltage.