×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Why a sine wave output of a voltage regulator?

Why a sine wave output of a voltage regulator?

Why a sine wave output of a voltage regulator?

(OP)
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

RE: Why a sine wave output of a voltage regulator?

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.

RE: Why a sine wave output of a voltage regulator?

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!

RE: Why a sine wave output of a voltage regulator?

(OP)
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.

RE: Why a sine wave output of a voltage regulator?

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....

RE: Why a sine wave output of a voltage regulator?

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources