08-04-2012, 11:28 PM
Progress: yes, Mark. I went back to that cct. using the LM317 and had a good look at my build (checking for wiring errors and poor layout): no problems found. I had a thorough look at it with the 'scope: no signs of oscillation. The one thing that did puzzle me however was the voltage on the control pin: it is fed from a potential divider, 240 ohms and 150 ohms, from Vout to 0v. With Vout at 2 volts, the control pin was at about 0.3 volt. To me, that implies that the control pin was taking a lot of current: methinks too much, and thus implying a defective LM317. I tries two other LM317s: same result. The PSU source was a bridge rect. across the 6.3 v.a.c. winding. (I am unsure about the history of these particular LM317 devices. 
I know that one of them I have used before. But . . . . )
At which point I stopped and changed tack, and began thinking about a simple linear regulator using discrete components. And that is what I now have working, albeit as the traditional 'lash-up', on the bench as I write this. It's a simple two-transistor affair and is good for 2v nominal @ > 1 amp with 20 mV peak-peak ripple. The regulation isn't heart-stopping: 2 volt at N/L falling to 1.9v @ 1 amp.: quite adequate for the purpose. Tomorrow I shall continue to get a crowbar cct. working, then put the whole issue (LT and HT sources) together and give an extended soak test. At that point I will then present the entire cct. here for a peer review: don't hold your breath!
Al. / April 8 //
I know that one of them I have used before. But . . . . )At which point I stopped and changed tack, and began thinking about a simple linear regulator using discrete components. And that is what I now have working, albeit as the traditional 'lash-up', on the bench as I write this. It's a simple two-transistor affair and is good for 2v nominal @ > 1 amp with 20 mV peak-peak ripple. The regulation isn't heart-stopping: 2 volt at N/L falling to 1.9v @ 1 amp.: quite adequate for the purpose. Tomorrow I shall continue to get a crowbar cct. working, then put the whole issue (LT and HT sources) together and give an extended soak test. At that point I will then present the entire cct. here for a peer review: don't hold your breath!
Al. / April 8 //
