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Old 10th January 2010, 08:07 PM   #1
gary h is offline gary h  United States
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Default repurpose wall wart a good idea?

Hi all,

I'm building a regulated supply for an active crossover circuit. The crossover uses 12 op2134 chips and will want +/- 15V at less than 100ma.

In the spirit of reusing, reducing, and pure cheapness I was wondering if I could employ one of the several wall wart ac/dc adaptors I have laying around. I have one of those Mac computer supplies that gives 24V, could this be used to get +/- 12V for a regulator circuit? If this is advised, how would I get the +/- 12V from the 24V and keep ground at 0.

Of course the other option is to purchase a $10 transformer and build one like Rod Elliott has on his (very informative) website.

What would the resourceful/stingy DIYers do?
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Old 10th January 2010, 08:15 PM   #2
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If you want to use the laptop adaptor and the supply ground to connect to your split 0V then that requires a transformer.
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Old 10th January 2010, 08:42 PM   #3
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The lazy way is just to divide the 24v in two with two identical resistors, and put a fat capacitor (1000uF or bigger) across each resistor to minimise the ground impedance. The centre point of the resistors becomes the new ground.

A better way is to buffer the new ground point with a power op-amp, like in the diagram. For 100mA you dont need anything as beefy as an L165, an L272 will do.
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Old 10th January 2010, 08:54 PM   #4
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That was going to be my suggestion, but said he wants to ground the 0V line. Gary, are you sure the power supply is already connected to mains earth?
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Last edited by richie00boy; 10th January 2010 at 08:56 PM.
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Old 10th January 2010, 10:28 PM   #5
gary h is offline gary h  United States
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This site has impressed on me the importance of safety grounding. Every piece of audio equipment I have built has a star ground to which signal ground, PS and transformer, and earth ground from my mains are connected.

This supply is not connected to safety earth but yes, I am grounding the 0V line.

The crossover is the Siegfied Linkwitz 3 way 24 dB/oct
Click the image to open in full size.

I also have the input buffers done
Click the image to open in full size.

so maybe Steerpike's first suggestion would work? This is a voltage divider, right? Wouldn't the C R pair work for this with the 0V rail going to ground?

I appreciate the help, thanks,

gary
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Old 11th January 2010, 03:21 AM   #6
gary h is offline gary h  United States
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Rereading the replies above I realize my misunderstanding; creating a voltage divider would put ground at 12V, - rail at 0V and + rail at 24V. Is this correct?

Perhaps purchasing the transformer is the best idea. I'll save the Mad Max stuff for after the apocalypse.
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Old 11th January 2010, 06:29 AM   #7
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Laptop power supplies, and other such "wall wart" devices, are designed to be safe without requiring grounding of the output. Use the op-amp voltage divider circuit given above. You'll end up with the 2 wires from the power supply at +12 and -12 volts, and the centre tap of the voltage divider at 0V.

In fact, connecting one side of the power supply (or, in fact, the signal ground as a whole) to the mains ground is actually less safe than keeping them separate, especially where you cannot guarantee a mains (safety) ground connection from the case to the building earth.

Consider 2 failure scenarios: One where the mains comes in contact with the case, and one where it comes in contact with the circuitry.
Also consider whether the device has a 2-wire power cord, or 3-wire cord and grounded wall socket.

Mains to case, case grounded: fuse blows. Safe.
Mains to case, no case ground: Dangerous.

Mains to circuit, signal ground to case: Same as above cases.
Mains to circuit, signal isolated from case: (relatively) safe.

Even in the last case above, the amount of "live" metal that you can come in contact with will likely be limited to fingertips on signal connectors, whereas a whole-hand grasp of a metal case would be much more dangerous.

As a general rule, if I have to connect signal and mains ground I do it at only one point, in the power amp, and make sure the amp has a 3-wire cord and grounded power outlet. All other devices in the chain either have 2-wire power cords with double-insulated class power supplies, or 3-wire cords to grounded sockets.
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Old 11th January 2010, 05:51 PM   #8
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If your laptop PSU is not earthed then you are OK. To make things easier you might like to check out the DC splitter project on my website - it was designed for exactly your need.
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Old 11th January 2010, 06:55 PM   #9
gary h is offline gary h  United States
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Thanks for the help.

gary
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