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Old 12-31-2011, 04:01 PM #1
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Default LM317 Current too high

I have built a DC power supply using a variation of the Tread schematic based on an LM317T regulator. Voltages are perfect but the current level on the output side seems way too high. From the transformer I read a current output of 3 amps and 12.36 volts. On the output side I am reading about 2.3 Amps. I was under the impression that the LM317 would regulate the current down to 1.5, and my bridge (radio shack 276-1152) is also rated at 1500ma. Is there a practical explanation for this or somewhere I should focus on? I have attached the pcb layout for review.
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Old 12-31-2011, 06:53 PM #2
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Default Re: LM317 Current too high

The LM317 is rated at 1500mA max current. That doesn't mean it regulates current to 1500mA (unless you tell it to). There are tolerances to every IC, and if you've heatsinked it, higher than 1500mA will be possible. If you want to lock it at 1500mA, set it up in a "DDL" style driver, and use a 0.83 ohm current set resistor.
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Old 12-31-2011, 07:00 PM #3
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Default Re: LM317 Current too high

Thank you for the sanity check. I planned to build a variable current circuit at the end of this one but got concerned when I was double checking my work.
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Old 12-31-2011, 11:19 PM #4
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Default Re: LM317 Current too high

The schematic is too small on my monitor . . .

but it looks like D2 may allow too much current.

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