On Sun, 3 Jan 1999 22:23:56 -0500 Sean Breheny writes: >Hi all, > >I just built a negative voltage regulator using a 741 op-amp, a >voltage >reference, and an external power transistor. Yes, I know that this is >quite an obsolete way of doing it, considering that there are ICs >available which require much fewer external components. It works quite >well, but it goes into oscillation if the capacitance on the utput is >around 0.1uF and you draw more than a couple mA. If the cap is much >smaller or much larger, it seems to work fine. Many regulators both commercial and homemade are unstable with zero or small values of capacitance at the output. Often the best solution is to add a "large" capacitor to guarantee a large minimum capacitance at the output, regardless of the load. The capacitor also increases the performance of the regulator under transient conditions. I am just wonderingwhy >it >would oscillate with such a specific capacitance there? Basically, it >is >just the transistor's base attached to the output of the op-amp and >the >emitter is the output of the regulator, and also is fed back to the >inverting input of the 741. It does that because adding an additional transistor increases the gain of the 741 and also causes more delay from input to output. The feedback is not fast enough to keep the output under control. It can get into a lot of math to figure out exactly what is happening, though there are simplified analytical methods to tell if a design will be stable or not. The simplified technique uses the concept of "phase margin". Many sophomore-level textbooks will present it. I like to not deal with math whenever possible, so my advice would be to add a large capacitor (10 uF). Stay at it, you're learning a lot. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]