How do I use voltage regulators

[kal.a]

Hi iGuys,

I wired two different circuits both with voltage regulators and not getting the results I was hoping for and looking for insight on what I'm doing wrong.

**broken link removed** is a 24VAC rectified to 24DC. The rectifier I'm using is GBU605 and the voltage regulator is a 78248T. I couldn't find either in LTSpice so I used whatever I could find to for illustration purposes.
I get 35.4VDC at the input of the voltage regulator and the output is exactly the same with no change. Also the regulator gets quite hot and I mean burning hot. I can see that the maximum input voltage in the datasheet is 35VDC is that the reason?


The **broken link removed** is a 0-10 volt simulator. I had one made for a 24VD supply with a 14K resistor and a 10K POT which worked fine but it was as inaccurate as the input voltage was so I thought I could make one with a voltage regulator to get a steady input and therefore a closer accuracy of 0-10VDC out of the POT.
What I get out of the 7815CT regulator is 15.06VDC without the 10K POT in the circuit but as so as I add the POT the voltage out of the regulator drops to 14.66 with the POT at maximum resistance of 10K. I also do not get the 0-10VDC I expected at output of what is a voltage divider between 5000K resistors(3.3K+2.7K) and the 10K POT. I only get 8.96VDC. Where am I going wrong?

Thanks
Kal

 

[crutschow]

A picture is worth a thousand words.
The usual request:
Post schematics of your circuits.

 

[audioguru]

The photo in the other forum shows an ON Semiconductor MC7824B, not ending with an 8. The datasheet shows that the B is where it was made. Its datasheet shows that its minimum input must be 27V or 28V, not the 24V you were feeding it.
Now its output voltage is the same as its input voltage so you destroyed it somehow. Maybe it is a fake one?

 

[kal.a]

crutschow, I posted hyperlinks of the schematics , just easier.

audioguru, You're right it's a B I thought it was an 8 but that explains why yesterday I had difficulty finding the datasheet. I have another one I will try it out on a bread board and see what happens.
What about the other circuit with the 10K POT, isn't the voltage regulator suppose to maintain its output to its specified voltage?

 

[kal.a]

I got the first one working audioguru, just like you said I burnt the regulator. I worked well with a new one.

The second circuit is still not functioning the way I would like it to. I'd like the voltage to remain stable.

 

[audioguru]

I didn't notice your schematics because I was distracted by the ads showing pretty girls with skimpy clothing.
The schematic with the 15V regulator missing important capacitors driving 3 series resistors is hopeless. Of course the voltage is not stable.

 

[kal.a]

I was racking my brain trying to figure out how you came up with 9.375V!!!. As it turns out I can't add I added 2700 added 3300 to make 5000!

Thanks for waking me up. I think I need a break after learning so much from you guys in such a short time.
Much appreciated.
Kal

 

[audioguru]

I was racking my brain trying to figure out how you cam up with 9.375V!!!. As it turns out I can't add I added 2700 added 3300 t be 5000!

I simply used correct arithmetic (2700 + 3300= 6000) and Ohm's Law. The total resistance is 2.7k = 3.3k + 10k= 16k. 15V/16K= 0.9375mA. The 10k resistor with a current of 0.9375mA in it has 9.375V across it if it has no load, if the 15V is exact and if the resistor values are exact.

 

[kal.a]

Hello Gentlemen,

One more question about voltage regulators if I may.

The datasheets of the Fairchild LM317 shows 0.1uF capacitor on the input and 1uF on the output and for LM78xx shows .01uF on the output and 1uF on the input. Why are the small vs larger capacitors reversed on them?

Thanks
Kal

 

[AnalogKid]

The two devices are significantly different in their internal circuits, and the different control loops have different stability requirements.

ak