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I have this to try to get a regulated -15V DC:

schematic

simulate this circuit – Schematic created using CircuitLab

But my 7915 outputs a (positive) low voltage in the 1-3V region.

I've checked the input voltages and they are good. I've tried two 7915s and they operate the same way. I've tried a higher input voltage and get the same behavior.

Datasheet excerpt: Datasheet excerpt

What am I doing wrong?

Update: pics (the 7915 is the reg on the right) Breadboard overhead

Closeup

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    \$\begingroup\$ Strange that you measured positive voltage. Do you have a picture of your setup? Have you triple checked pinout? What's the voltage at the input of the regulator? \$\endgroup\$ – Andrés Sep 30 '17 at 10:00
  • \$\begingroup\$ –38VDC at the input when powered by 24V AC; -18V when powered by 12V AC \$\endgroup\$ – buildsucceeded Sep 30 '17 at 10:10
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    \$\begingroup\$ Check datasheet for minimum load requirement. \$\endgroup\$ – replete Sep 30 '17 at 10:12
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    \$\begingroup\$ @next-hack: I was addressing the failed measurement in general, whether he was measuring with the leads reversed is up to him to say not for us to guess. Minimum load requirement is a typical attribute of voltage regulators and varies from device to device. If the device is insufficiently loaded, it won't regulate. He hasn't linked the datasheet for his device so we can't say for sure. Implying that the internal resistors act as a sufficient load is nonsense. Read some datasheets. You are assuming all 7915s of all generations from all manufacturers are equal in this regard. That is untrue. \$\endgroup\$ – replete Sep 30 '17 at 11:41
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    \$\begingroup\$ Don't insult people, it only makes you look bad. The OP posted a schematic without a load so I posted a comment - not an answer - about the load's importance. That should not be controversial. You are jumping to conclusions about how the OP made his measurements, just wait for clarification. You need to understand that an insufficiently loaded regulator can oscillate. Don't look for the words "minimum load" in the datasheet, look for the minimum current at which the device is specified. An insufficient load means the device is being used out of spec. \$\endgroup\$ – replete Sep 30 '17 at 12:00
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It's likely you're measuring the voltage between the yellow and blue wires, on the bottom right part of the board.

However, the closeup of your 7915 shows that the blue wire is the central terminal of the 7915, which, unlike 7815, it's the input and not the ground.

Therefore you're measuring \$V_{OUT}-V_{IN}\$, which is about \$-15\ V-(-18\ V) = 3\ V\$.

You should measure the voltage between the yellow (output) and the red wires (ground) that are connected to the 7915.

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    \$\begingroup\$ Absolutely correct (and facepalm on my part) — measuring from the actual ground, and not the input, gives -14.98V! Thank you for your help! \$\endgroup\$ – buildsucceeded Sep 30 '17 at 12:10

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