Electronics > Beginners

Odd output from Elenco XP-15K Power Supply

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rf+tech:

What is the DC voltage across pins 1 and 2 of the LM317?

To be clear, place the DMM black lead on pin 1 and the red lead on pin 2.

This is expected to be 1.25 Volts DC, regardless of the potentiometer setting.

Also, several requests have been made for the DC voltage on LM317 pin 3. Please measure this, with the DMM black lead connected to P4 common/ground/minus terminal. And for all future measurements, unless requested otherwise.

tomherrick:
DC voltage across pins 1 and 2 of the LM317 is -0.10.
DC voltage across pin 4 and pin 3 is -0.83 @ 0V setting; -0.39 @ 15V setting.

rstofer:
Pin 3 to the black terminal should be 1.4 * 17.1V (assuming this is AC voltage between the blue and yellow or about 24VDC.  This is really high for a 15V power supply.  Make sure you measured 17.1VAC and check pin 3 as VDC.

One other thought:  The regulator takes between 3.5 and 10 mA of load current in order to regulate.  At very low voltages, say 0.1V, if your device needs 10 mA, you need a 10 Ohm load.  At very high voltage, say 15V, for 10 mA,, you need a 1500 Ohm load.  Notice that the load resistor varies widely over voltage.  You surely wouldn't want to use a 10 Ohm load on 15V as that would be 1.5 Amps.

I used 10 mA for those numbers because that is worst case.  Ordinarily I would expect your actual load to be adequate.

tomherrick:
Here are the measurements:

P2 to P1: 17.0VAC


P2 to P3: 17.0VAC


P4 output negative to LM317 pin3: -0.51VDC

rf+tech:
Now we're getting somewhere. The AC input side looks as expected and the focus is now from D1, D2, D3, D4 and to the right side of the schematic.

Please answer re is anything getting hot, especially the LM317.

Measure DC voltage from P4 output to D1 cathode (banded end) and from P4 output to D3 cathode.

Edit:

A second look at the back side seems to show a suspicious looking solder joint at P4, as noted in attached photo.

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