7805 regulator capacitors

[HellasTechn]

I have an 7805 regulator feeding a microcontroller for some very important project of mine.
I have seen many different schematics of 7805 regulator circuits and i see that they use different capacitor values on the input and output pins of the 7805.

My question is: how should i choose the input and output capacitor values ? I do need it to be as reliable as possible ant to last as long as possible. Also would it be a better idea to replace the aluminium capacitors with tantalum ?

the Circuit is fed with 12V through a diode and a 20 Ohm resistor and the microcontroller consumes maximum 300ma of current. as input capacitor on the 7805 i use a 10uf and as output an 100uf aluminium electrolytic.

Thank You.

 

[Minder]

See the ST app note on the IC, I always use the recommended .33uf on the input, and a .1uf on the output, if the circuit being fed is not near to the regulator, I add a 15uf to 20uf across the ,1uf.
For 300ma you could use the 79L05 series.
M.

 

[HellasTechn]

For 300ma you could use the 79L05 series.
M.

Why would i use a negative Vreg over Positive one ? I Dont think its a good idea to put the supply line on the tab of the 7805 (to-220) together with the heatsink. I risk touching the ground plane.

Any thoughts about tantalum caps ?

Searching the web i came across the Solid aluminium electrolytic capacitors.
I think the last can directly replace the traditional electrolytic having only pros. Right ?

 

[Minder]

Sorry, misprint, should be 78L05.
Tantalum in this application should be OK. Solid Aluminium also, both typically smaller than the typical electolytic.
M.

 

[Hunter64]

How can you use a 78L05 at 300mA?

The 78L05 is rated for an output current up to 100mA.

 

[bertus]

Hello,

The 78M05 is capable of 500 mA, so that can be used for the mentioned 300 mA.

Bertus

 

[HellasTechn]

Thats right. the TO-92 package can deliver up to 100ma.
The TO-220 package can deliver up to 1A if mounted on a large enough heatsink.

 

[Bluejets]

Why R1..??

 

[davenn]

Why R1..??

exactly

 

[Hunter64]

Please take some time and read the correct datasheets, yes, that's plural. There is a significant difference between the 7805 and the 78L05, so please try to not mess up the numbers.

 

[HellasTechn]

The 7805 comes in TO-220 package while the 78L05 comes in TO-92 package. They do the same thing but have different current and heat dissipation capabilities.

Like i mentioned in post 1 i use the TO-220 version. R1 is there in order to drop the 12V down to about 10V thus reducing the thermal stress on the Vreg.

Any ideas about the original question ?

 

[bertus]

Hello,

If you do not want to wast to much power, you could have a look at the RECOM R78 series:
https://www.digikey.com/en/product-highlight/r/recom-power/r78-series#VTabs3

Bertus

 

[HellasTechn]

Thanks. I already know of them.

My question is: how should i choose the input and output capacitor values ? I do need it to be as reliable as possible ant to last as long as possible. Also would it be a better idea to replace the aluminium capacitors with tantalum or with solid aluminium ?

 

[bertus]

Hello,

Aluminium capacitors will dry out over time.
Tantalum capacitors are solid capacitors and will likely be more stable.
On the following pages you can read more about them:
http://www.iequalscdvdt.com/Aluminum.html
http://www.iequalscdvdt.com/Aluminum_construction.html
http://www.iequalscdvdt.com/tantalum_and_niobium.html

Bertus

 

[Hunter64]

National Semiconductor doesn't recommend the way you want to use your circuit. I presume you already know that because it's in the application hints of the datasheet.

If you really want to proceed this way, use a protection diode.

 

[Minder]

You actually only need .1uf on the out pin if the rest of the circuit is close to it, and the rest of circuit fed has capacitive smoothing.
The recommended on the input is 0.3Uf.
M.

 

[bertus]

Hello,

Here is the schematic version of what @Minder said:



Bertus

 

[Bluejets]

The 7805 comes in TO-220 package while the 78L05 comes in TO-92 package. They do the same thing but have different current and heat dissipation capabilities.

Like i mentioned in post 1 i use the TO-220 version. R1 is there in order to drop the 12V down to about 10V thus reducing the thermal stress on the Vreg.

Any ideas about the original question ?

So why not simply use a lower voltage supply to start with?
9v is fairly common and cheap.
Resistor defeats the whole purpose of using a regulator .

 

[Harald Kapp]

Resistor defeats the whole purpose of using a regulator .

Not completely. In this circuit the resistor will help to dissipate power which then doesn't have to be dissipated by the regulator, thus keeping the regulator cooler. The output will still be regulated.

 

[bertus]

Hello,

The 20 Ohms resistor will be to large, as it will drop 20 X 0.3 = 6 Volts, leaving 5.3 volts at the input of the regulator when the input voltage is 12 Volts and the diode drop is 0.7 Volts.
The regulator needs at least 2 volts overhead to be able to regulate.



Bertus