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Regulated power supply
Regulated power supply
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Old 3rd November 2006, 10:52 PM   #1
akunec is offline akunec  Canada
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Location: Alberta
Default Regulated power supply

Hey guys,

In this pic, i dont understand the diodes across the regulators. Since they allow current to flow from a higher voltage to a lower voltage in only one direction, wouldnt they essentially bypass the regulator?

Also, those diodes across the rails to the ground. Wouldnt those short out the rails to ground?


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Old 3rd November 2006, 11:13 PM   #2
jaudio is offline jaudio
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http://focus.ti.com/lit/ds/symlink/lm317.pdf page 7

Maybe the last diodes are zeners?
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Old 4th November 2006, 01:09 AM   #3
I_Forgot is offline I_Forgot  United States
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In both cases the diodes are normally reverse biased, so they will not short the output to the input or just short the output. The input to output connected diodes are there to protect the regulator from charge stored in large capacitances on the output side of the regulator. The diodes wired from output to ground are there to protect the regulator from current surges that might occur if you connect/disconnect or switch on/off an inductive load (such as a motor, relay, or solenoid).

If you have neither inductive loads that are switched or connected on the output side of the regulator, nor large capacitances on the output side of the regulator, you don't really need either diode. On the other hand, using the diodes is "cheap insurance" so it is sort of silly not to use them. Four diodes will set you back about $0.20.

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Old 5th November 2006, 10:27 AM   #4
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
read thread
start at post 4475 and read all about correct grounding of regulators.
The schematic you have drawn is electrically correct but the wiring is NOT done that way. It leads to noisy/buzzy/hummy speaker output.

BTW. the protection diodes are correct. To save the reg if power goes down. It is all explained in the datasheet.
regards Andrew T.
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