I am trying to regulate the voltage. Please help.

[Iawia]

I have a Texas Instr. voltage regulator (UA 7805). I am looking at the schematic but still am unclear on how to accomplish this.

My source is 18v Dc, and I want to drop it to 12v. Even if the voltage at the source drops down, I would like to keep the output at 12v. Is the schematic that I attached the correct way on going about it?

View attachment 61253

If I regulate the R2 I can acheive the 12v I want, but if I vary the voltage source higher or lower, my ouput changes. How can I keep it at a constant 12v from a variable source (~12-18v)?

 

[bountyhunter]

A 7805 is a three terminal regulator that requires no external resistors. The schematic you posted is for an adjustable reg like an LM317. Different part. A 78XX reg has no "adjust" pin, it is only input, ground, output. Read the data sheet for the 7812.

And if you want 12V out, you need a 7812 regulator not a 7805.

 

[ronv]

The 7805 can be used that way, See fig. 5 in the data sheet. It needs 2 volts above the output voltage to regulate so you can only expect 12 volts with the source above 14 volts. What values have you used for the resistors?

 

[Iawia]

I am reading the datasheets for the 7812. Datasheets say 35 V for input voltage, but it doesn't say a range. Does that mean it requires 35 volts to function?

I used 500 ohm for R1 and a potentiometer (0-45k). But with this setup, the voltage varies with the source.

 

[ronsimpson]

The 7812 will break if the input voltage is over 35 volts.
The input voltage needs to be 2 volts or more than the output. So it needs 14 volts or more.

14Volts <input voltage<35 Volts

 

[audioguru]

I am reading the datasheets for the 7812. Datasheets say 35 V for input voltage, but it doesn't say a range. Does that mean it requires 35 volts to function?

No.
It says that its ABSOLUTE MAXIMUM allowed input is 35V. It says its RECOMMENDED max input is 30V. It says its RECOMMENDED minimum input is 14.5V.

 

[Iawia]

Ok. Thanks for the info guys. I am wondering why I even purchased the 7805 now. What is it useful for if it can not accomplish this?

 

[ronv]

Use the 7805 and 240 ohms for R1 and 500 ohms for the pot. You should be ok for a source of 14 volts and above for an adjusted output of 12 volts. They need a minimum losd supplied by the 240 ohm to regulate properly.
The 7805 will go down to 5 volts the 7812 down only to 12 volts.

 

[Jugurtha]

The 7805 and 7812 are positive voltage regulators that give +5 volts and +12 volts respectively. The 78 is for positive (+) , so if you had a 7905, the output would be negative.

https://en.wikipedia.org/wiki/Linear_regulator

One thing to watch for in the datasheets is called "Dropout Voltage" or [LATEX]V_D_R_O_P[/LATEX] . The input must be imperatively [LATEX]V_D_R_O_P[/LATEX] greater than the output.

So if you have a 7805, your output will be +5 volts. If the [LATEX]V_D_R_O_P[/LATEX] = 2 volts, then the input must be at least 5+2=7 volts.

In the same way, if you have a 7812, your input will be +12 volts. If your [LATEX]V_D_R_O_P[/LATEX]=2 volts, then the input must be at least 12+2=14 volts.

The regulator dissipates power proportional to that dropout voltage, hence some manufacturers sell what is called "low drop-out regulators" or LDO.

https://en.wikipedia.org/wiki/Low-dropout_regulator

 

[bountyhunter]

You can "raise" a fixed regulator's voltage by sticking resistance in the ground leg but understand: ground pin current is NOT CONSTANT, it varies with load current. So the output voltage will vary with load current. You can also get into stability problems with putting resistance in the ground lead.

 

[Gobbledok]

I am wondering why I even purchased the 7805 now. What is it useful for if it can not accomplish this?

The 7805 will give you a regulated (non-adjustable) 5v out. A lot of electronic circuits run from 5v.

As has been mentioned, you need a 7812 to give you a regulated 12v out.

The standard circuit for a basic non-adjustable regulator:
View attachment 61284

 

[alec_t]

You could use a 7805 and replace R2 by a 7V (or thereabouts) zener diode.