Convert 7805 to 9v ?

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neazoi

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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?
 

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

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FvM said:
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.
Click to expand...
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.
 

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.
 

    neazoi

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Hi,
neazoi said:
I am going to draw something like 200-300mA
Click to expand...
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

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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.
 

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