Improved Pos/Neg Regulator Load “Problem”

I recently received an email from a reader, Anton, residing in Moscow, Russia. He had built a version of the Improved Positive/Negative Regulators from Audio Electronics, Issue 4, 2000. While his circuit was functional, he was concerned by the fact that when he applied a 360mA load, the output voltage dropped, an apparent loss of regulation. This sounded to me like the regulator didn’t have enough drive under load, so I suggested some things to check:

Is the D7 LED ON, under all conditions? If it goes out, it means the pass transistor is starved for current, and the output then falls. If you have a pass transistor with a Beta of 100, a 360mA load means the current source must provide 3.6mA. But note that this is close to what it can do with R19 (Fig. 1) at 249Ω. You may need to drop this resistor down some, if you really need 360mA of output. Note: there was a discussion on this very topic, on page 11 of the original article, under the topic “Change of Current Source Resistor” (i.e, R19). The upshot here is that the older R19 value of 100Ω provided for more current, so I suggested that Anton lower this resistor value, to see if it fixed the voltage dropping under heavy load. He later reported back changing R19 to 160Ω (just for test), and it worked correctly with the 360mA load.

I was glad that he got it working OK, and pointed out should drop that R19 value down some (if he does in fact need 360mA). I left that for him to determine. If you need much less current, then leave the value as it is.

The moral of the story is that the circuit is robust, but it does have fixed limits. So, if you find the output dropping unexpectedly, check the load current carefully, both on board and external.

Thanks for the report, Anton!