Hi!

I have an board labeled Dual PS LC1 that I'm having a small problem with. My input is 18VAC+18VAC and I'd like to regulate down to 2x15VDC. However, I run out of adjustment room in the 5K pot and am a few volts above my target. Looking at the datasheet, it seems that this exact supply should go from 1.25-25V with an input of > 28VDC. This circuit is very similar to the datasheet. So my question is, does the resistor in the CRC need to be recalculated, or do I need to lower the 240R resistor used to set the output voltage.

Thanks!

Russ:

Here's what I get:

Input is 18VAC+18VAC nominal. I measure under output load 20.8VAC+20.8VAC on the input to the board. I'm getting 1.50V and 1.54V drops across the resistor in the CRC supply. The lowest voltage I can get across the output is 22.4VDC and -21.4VDC. There is 24.6 VDC at the input to the CRC and ~23VDC on the output of the CRC. I'm using a pair of 3.3K resistors as load, which models my target current at the intended 15VDC output. On the LM317 I get 0.25 VDC on pin1, 23.2 VDC on pin2, and 22.4 VDC on pin 3, measured with reference to ground. On the LM337 I get 0.20VDC, 21.4VDC, and 23.0VDC on pins 1, 2 and 3 respectively.

I'm assuming the regulator has enough head room to operate. The heat sinks on the regulators get warm.

Any ideas what is happening?

Thanks!