Convert 7805 to 9v ?

[neazoi]

Hi, I think there is a way to convert a 7805 regulators to 9v using some diodes in the ground terminal. Is that true, and how is it accomplished?

 

[FvM]

If you can accepts some voltage tolerance, use 3.9V zener diode in series with ground terminal. Bias current is almost constant, around 5 mA.

 

[neazoi]

If you can accepts some voltage tolerance, use 3.9V zener diode in series with ground terminal. Bias current is almost constant, around 5 mA.

So just an ordinary low power zener, it does not need to be a high power type?
The input/output capacitors grounds will be connected to the zener, or to the actual ground?
I am going to draw something like 200-300mA out of this regulator, fed with 13.8v at it's input.

 

[FvM]

Bypass capacitors should be connected to ground as before. You may want to add a small capacitor (e.g. 100 n) parallel to the Z-diode, but it shouldn't be necessary.

 

[dick_freebird]

Isn't a 7809 dirt cheap? Why mess with a sloppy diode
cobble when you can change theBOM or rework the board?

https://www.digikey.com/en/products/detail/nisshinbo-micro-devices-inc/NJM7809FA/805766

 

[KlausST]

Hi,

I am going to draw something like 200-300mA

inside the 7805 the (load) current goes from INPUT to OUTPUT.
Only a small current for the internal regulation goes to GND.

You may read the datasheet.
The ST datasheet (DS0422 - Rev 36) shows a "quiescent current" of max 6mA. This is when the output current is zero, so the only possible path is from INPUT to GND.
And the datsheet shows a "quiescent current change" of max 0.8mA.
So you may expect a maximum of less than 7mA of GND current.

Also see Figure 22 for "high input and OUTPUT voltage" ... and read the description.

Klaus

added: I agree with dick_freebird ...I´d also prefer 7809 ...

 

[neazoi]

It is just I do not have any 7809 currently around. Waiting, cost (postage basically, or gas to go to the distant shop) is not worth it.
I will build it like suggested in this thread. Thank you very much.