7818 Voltage Regulator question...

[Jasonmatthew911]

Hi guys, I'm about to make a 2 in 1 build pedal, but I basically want to operate 1 of the circuits on 9V, and the other one on 18V, through the same power supply...So, my idea was to use an 18V Dunlop supply and put a 7809 Regulator for the 9V circuit, and then put a 7818 Regulator for the 18V O.D. circuit, just because it has a 4558 Chip which I think handles up to 18V, and I would like to protect it from ever exceeding 18 Volts....The thing is that I saw that the 7818 should have an input voltage of at least 21V to regulate at 18V, and that if you feed 18V or less it will reduce the output voltage....So, I guess my question is, if I feed 18V into a 7818 regulator, will it still output 18V or will it output less, if less, than how much less would it reduce the voltage with an 18V Dunlop supply???...Also, if I was to run the 2 circuits with a 9V supply, how would that affect the 7809 and 7818 voltage regulators?

[PRR]

> it has a 4558 Chip which I think handles up to 18V

The spec is plus and minus 18V (each side), and can just as well be 36V total (one supply).

If your "18V supply" has a known brand name on it, there's nearly no way it will ever exceed 36V.

If you are paranoid, be sure your caps are good for 35V or 50V.

> if I feed 18V into a 7818 regulator, will it still output 18V

No. Expect around 15V-16V.

> run the 2 circuits with a 9V supply

Then the circuit fed through 7809 would get closer to 6 Volts. Which may or may not be happy (I don't know what it is).

It does not pay to get too clever with power arrangements. Pick a GOOD supply, which will always be available, do the minimum cleaning and dropping needed, and go with it.

[Jasonmatthew911]

> it has a 4558 Chip which I think handles up to 18V

The spec is plus and minus 18V (each side), and can just as well be 36V total (one supply).

If your "18V supply" has a known brand name on it, there's nearly no way it will ever exceed 36V.

If you are paranoid, be sure your caps are good for 35V or 50V.

> if I feed 18V into a 7818 regulator, will it still output 18V

No. Expect around 15V-16V.

> run the 2 circuits with a 9V supply

Then the circuit fed through 7809 would get closer to 6 Volts. Which may or may not be happy (I don't know what it is).

It does not pay to get too clever with power arrangements. Pick a GOOD supply, which will always be available, do the minimum cleaning and dropping needed, and go with it.

Thanks PRR!....So, basically if my 4558 circuit can handle up to almost 36V, there's really no need for the 7818 Regulator, as long as all my parts are rated above 35V, and I use my 18V supply or anything below 36V, right?...I'd just use the 7809 regulator for the other circuit that uses a 555 chip, since I will feed both circuits with an 18V supply.

[antonis]

I use my 18V supply or anything below 36V, right?...

Not quite right because you have to take in mind the current consumption of the circuit(s) to calculate the Wattage of the regulator and (possibly) the series resistor (if any)..

The voltage drop "inside" 7809 from 18V inpout is 9 Volts and from 36V is 27V (which is 3 times larger and you will need a device capable to handle the TRIPLE power for the same current..)

[antonis]

.So, basically if my 4558 circuit can handle up to almost 36V, there's really no need for the 7818 Regulator, as long as all my parts are rated above 35V,

It depents also on the design of the circuit...

If, for example, the 4558 is powered from 18V and GND, you MUST use that way or else you propably will change the virtual ground bias (and the headroom of overdriving it ..)