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In a 3-terminal power supply like this, are there individual regulators behind the positive and negative terminals? For example a LM317 on the positive and a LM337 on the negative?

If not, what kind of circuitry drives a floating output like this, in general?

Side question, probably related: If you can connect either terminal to ground, why doesn't the connected terminal dump current into ground?

I want to build or buy a bench power supply, and I'm having trouble understanding how one control set can drive positive or negative voltage.

enter image description here

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  • \$\begingroup\$ In a digitally controlled supply you have a MPU reading your settings and comparing them to actual output. Dacs and look-up tables allow it to stay extremely close to what you want. This is much more sophisticated than analog stuff. \$\endgroup\$
    – user105652
    Mar 23 '19 at 2:58
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This type of power supply only has one regulator placing a settable voltage between the + and - (red and black) terminals.

The green terminal ties into the safety GND of the power cord and hopefully to the earth grounded network in your building.

In operation of the power supply you have the choice of a fully isolated output or you can tie one or the other of the + or - terminals to the green terminal if you want to have the supply output referenced to the safety ground.

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  • \$\begingroup\$ I guess I don't understand the point of negative regulators in general. If the only difference between positive and negative on that supply is hooking up the outputs "backwards", I don't understand why you can't do so with the outputs of a 7805 for -5V. \$\endgroup\$
    – Michael McDonald
    Mar 23 '19 at 3:59
  • \$\begingroup\$ You need a 7905 of you don't have two isolated channels. With two isolated channels, the "ground" of the negative channel is connected to the negative output and the regulator is connected to the circuit ground. This is fine for a bench supply, but it doesn't work very well if you have, say, a center tapped transformer. In that case, you want to use the center tap ss ground and then build symmetrical positive and negative regulators. \$\endgroup\$
    – alex.forencich
    Mar 23 '19 at 4:31
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Supplies look like this

schematic

simulate this circuit – Schematic created using CircuitLab

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