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I have the following transformer: 220 to Dual 24 Volt 50Hz Transformer

And the following power cable: 3 Prong Power Cable with Ground Pin Asia-Pacific

This is for a dual-polarity power supply for some audio circuits. How should I connect the ground pin to this circuit? Can the transformer I have be used to build this? LM317/LM337 Dual Polarity Power Supply Thanks in advance!

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Your transformer should be bolted to the chassis of your project and connect ground from our mains to the chassis. I you are using a plastic project box, it will likely be best to connect to a bolt you are using to mount the transformer to the plastic box. That way, if you ever fry your transformer and you develop a short to the transformer chassis, you'll immediately pop your circuit breaker (and hopefully the fuse in your project).

Yes, your transformer is fine, the two mains wires connect to the two red wires of your transformer. The three wire bundle will become your input to the power supply - two blue to the bridge rectifier and yellow is the circuit "ground". I would NOT connect circuit ground of your project to your Mains Ground until you run some tests and make sure everything is ok. After that, it really depends what you plan t use it for whether you want to connect to ground. F you are just learning to use an oscilloscope, I would not connect to ground until you understand how your scope works and the ground reference of your scope.

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    \$\begingroup\$ If he doesn't connect the secondary centre tap to ground, his grounding relies on accidental grounding of audio connectors. \$\endgroup\$
    – Ian Bland
    Commented Jan 24, 2022 at 4:43
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    \$\begingroup\$ If you use a 12VAC transformer, with rectification, it will be about +/-16 volts unregulated. This is a good voltage for op amps. Dealing with +/-24v dc means dealing with large voltage drops and heat as you regulate voltages down for projects that need less voltage. Voltage will sag below 16v under moderate loads but not for simple op amp circuitry. \$\endgroup\$ Commented Jan 24, 2022 at 4:58
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    \$\begingroup\$ That's good advice regarding separate power supplies for sure. That makes sense. Thanks for the advice! \$\endgroup\$ Commented Jan 24, 2022 at 5:06
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    \$\begingroup\$ Look at page 13 on this Texas Instruments doc. ti.com/lit/ds/snvs778e/… \$\endgroup\$ Commented Jan 24, 2022 at 5:23
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    \$\begingroup\$ Note: this type of power supply gets quite hot when it is set to output low voltages. And the more current at low voltage, the hotter those regulators will get. \$\endgroup\$ Commented Jan 24, 2022 at 5:24
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You never want to conduct line frequency rectifier current through PE ground as this adds conductive noise. Using the transformer for high impedance and insulation from high voltage transients gives the audio better common mode noise rejection ratios (CMRR).

Grounding is used for noise reduction when it stays at the same voltage by not forcing noise current thru it but rather providing a low impedance reference to high impedance stray noise ingress thus attenuation occurs in shielded audio lines or low impedance speaker drivers.

There are occasions where RF can create audio noise in preamps so a capacitor can suppress this to the audio power 0V whether that is PE grounded or not.

Thus when using PE ground for computer audio, realize the PC power supply uses CM chokes and lots of filtering so that there is minimal audio band noise current on PE ground. Expect all peripherals to be floating and then rely on the host only for PE ground to suppress radiated noise thru the network or shielded cables.

When multiple source and destinations use PE grounds, you can get the "ground loop" noise due to the difference in ground voltage and current that caused that.

The term "ground" simply means 0V locally whereever you define that reference point.

Safety or Protective Earth (PE) ground is 0V in the earth and close enough elsewhere to be safe. So with interconnections you don't want to add noise current when audio is interconnected. The current should only be the intended audio.

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If you want your audio circuits ground referenced- you probably do- the centre tap of the secondary (output side) connects to Earth, as does your safety Earth in the mains lead. The primary goes across Live and Neutral.

I've just noticed your capacitors C9 and C10 are shorted to ground!

edit following the discussions, I would suggest a "bog standard" linear power supply design, this one turns up first on my search and is the kind of thing

https://www.eleccircuit.com/power-supply-regulator-15v-15v-1a-by-ic-7815-7915/

It's a fixed 15-0-15. For 12-0-12 just use 7812/7912 regulators. They're dirt cheap and have automatic output shorting protection, which is good working with experimental circuits.

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  • \$\begingroup\$ Yes, I'm not sure why those capacitors are there specifically. I am building this from a schematic that I found elsewhere. \$\endgroup\$ Commented Jan 24, 2022 at 5:05
  • \$\begingroup\$ Looking at the datasheet, there are no capacitors shown in that position in the reference circuit and you can probably do without. They are presumably intended to smooth the potentiometer output but cant because they're shorted. The pot is shown as a reference current not voltage. Take a look at the datasheet, I'd follow that reference application circuit myself. \$\endgroup\$
    – Ian Bland
    Commented Jan 24, 2022 at 5:12
  • \$\begingroup\$ Thanks, I'm going to rework the think. Thanks for that reference circuit that you sent me. It looks like there are some other goodies there, also. May I ask, with a max current output of greater than or equal to 1amp, why is the linked circuit fused at .5 amp on the primary side?) Shouldn't it be fused at 1 amp? \$\endgroup\$ Commented Jan 25, 2022 at 6:49
  • \$\begingroup\$ Also, does the LMxxx short circuiting protection take care of fusing on the +/- 15 V outputs or should those also be fused? \$\endgroup\$ Commented Jan 25, 2022 at 6:51
  • \$\begingroup\$ @DavidGraham The 0.5A fuse is to protect the circuit from emitting smoke if the transformer primary shorts out. The current draw is actually much lower than that (remember, when you step down the voltage on the secondary, you proportionally reduce the current on the primary) but there is some inrush current when the transformer starts. The fuse is basically just short circuit protection. The LMxxx's output protection will protect everything downstream from shorts. You wouldn't normally bother fusing on the secondary side of a low power circuit like this. \$\endgroup\$
    – Ian Bland
    Commented Jan 25, 2022 at 10:33

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