Why a sine wave output of a voltage regulator?

[Steventyj]

I was in the lab today working on a circuit and it wasn't behaving as I expected. I diagnosed the problem as an irregular supply voltage from a 7805 voltage regulator. I am supplying it with 15 volts (also tried 12 volts) and getting 5 volts out. When the resistance of the circuit is low enough (does not have to be very low, 660 ohms for example) I start to get a sine wave along with the DC voltage. I realize that the regulator may start behaving irregularaly at higher current draws (lower resistances) but I am talking about milliamps. Any ideas?

Thanks,

Steven

 

[cbarn24050]

Hi, you need caps on the ip and op to get stability, try 470nF on the inp and 220nF on the outp. You could even try reading the datasheet.

 

[OperaHouse]

Brings back memories. Had a Action Pack position sensor module in a project that was 120V AC. Design then forced us to mount it remotely and we asked the factory to make a bunch without the transformer. The factory decided to also remove the input capacitor to the regulator. When we installed it we had this strong 1 mhz in all the signal lines. That regulator was singing like a canary. Also, in the lab they had built a FET ampliier for an accelerometer. I noticed they had a cap from the output to ground. I asked what that was for and they said it would oscillate without that. There was a length of coax on the FET's gate that looked like a tuned circuit. Internal capacitance of the FET was enough to start it. A 100 ohm resistor in series with the gate lead was enough to dampen it. Almost anything can become an oscillator!

 

[Steventyj]

Thanks for the advice guys. Although cbarn your comment about reading the datasheet was a bit of an assumption.

http://www.national.com/ds/LM/LM7512C.pdf

states that no external components are required.

 

[melone]

Page 4, Note 2: All characteristics are measured with capacitor across the input of 0.22uF, and a capacitor across the output of 0.1uF....

 

[Steventyj]

Alright I geuss I will have to start reading those more carefully. Thanks for the help guys, im in the lab now and its working.