> Have you tried it with the LM317 output disconnected from the rest of the > circuit to help locate where the problem actually lies? Just for interest and to possibly help others avoid spending an interesting time: Disconnecting the load as above MAY cause bad circuit operation, depending on what design decisions were made originally (or not). The LM317 has a MINIMUM load current of as much as 12 mA (typical =3D 3.5 m= A). Failure to provide the minimum load will make it work funny - oscillation or other depending on various factors. Ask me how I know :-). As the LM317 is usually used to produce a voltage above its basic value by using a divider twixt output, reference pin and ground. the normal solution to the need for minimum load current is to provide a suitably stiff divider ie high current drain. eg at 5V out to get at least 12 mA you need a load of at most 5/0.012 ~=3D 420 ohm. Base voltage output is 1.2V so the divider ratio must be (5/1.2 -1 0 =3D 3.= 1666:1 So resistors of 316 ohms and 100 ohms with total 416 which is < the 420 ohms upper limit. You'd be wanting to adjust to standard values. But also, as Olin suggests, before you start your design READ THE MANUAL. If the capacitor that you put in place is not in accordance with what the data sheet says then you should EXPECT it to not work properly. If you haven't read the manual and your circuit does not do what the manual says you should do then asking other people what is wrong is wasting your time and wasting theirs. When you don't say what capacitor you used on the output (or the input) or whether you used any capacitor at all then you are wasting your time and wasting other peoples time because these things MUST be known in order to answer your question properly. In your original query you said (or implied) that the LM317 was making a strange humming sound. This is extremely unlikely! LM317's do not hum - except perhaps just before they explode, but this usually means you have eg full voltage AC mains applied which we assume is not the case here. It is far more likely that something else that was related was humming. eg it could have been an input transformer or power supply. If something does hum, then telling us what is humming MAY help. When asking questions please try to think about what people need to know to answer your question well. Ideally when asking questions you should provide a circuit diagram which shows what you are doing that is relevant and a complete description of what you are doing. Russell --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .