Help with LM317L Regulator circuit

Discussion in 'The Projects Forum' started by dk-info, Mar 9, 2009.

1. dk-info Thread Starter New Member

Mar 9, 2009
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Attached is a PDF file with the schematic of the circuit I am trying to build.

The purpose of using a LM317 and not just a LM78L05 is I need 3.3V if the display is from a Nokia 1100 phone, 5V if it is a generic Text LCD.

My calculations show a 1k/3k divider should provide 5.0V output for a nominal 9V input.

With the circuit as illustrated, I get 6.74V no load (VCC to GND), and less than a half a volt under 100 mA load (50 Ω VCC to GND). I tried another LM317L, with the same results. With 8.98V input, the feedback voltage is 6.043V. The circuit Ohm's out at 1004(R9)/3001(R8) Ohms. With an O'Scope, the voltage is steady as a rock, no ripple.

With the 1k/3k feedback divider I believe I am well within the > 50μA feedback current requirement;

I can't understand what I am doing wrong. Any help would be greatly appreciated, even if it is an admonition to "RTFM" as long as the requisite "FM" is given.

Cheers,
David

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2. dk-info Thread Starter New Member

Mar 9, 2009
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Here is a picture of the PCB I fabricated for this circuit. The regulator is an ST LM317LZ TO92 regulator. The resistor divider R9/R8 visible.

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3. SgtWookie Expert

Jul 17, 2007
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The LM317L requires a minimum load of 5mA for guaranteed regulation.
Changing R9 to 240 Ohms would provide that load.
R8 would then need to be around 610 to 680 Ohms, depending on the individual regulator.

4. AlexR Well-Known Member

Jan 16, 2008
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I could be wrong but it looking at your photo, the LM317L seems to be in back to front.

5. SgtWookie Expert

Jul 17, 2007
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AlexR - yep; he forgot the "BOTTOM VIEW" note that needed to be on his schematic.

VIN is opposite of the ADJ pin; Vout in the center.

In a TO-92 case with the flat face towards you, pins down, the pins are (left to right):

Last edited: Mar 9, 2009
6. Audioguru New Member

Dec 20, 2007
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Why not look at the datasheet next time?

7. SgtWookie Expert

Jul 17, 2007
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Looking at it again, it appears that the datasheet WAS consulted, but a critical piece of information was left out: "BOTTOM VIEW".

LEDs D1 and D2 will each get only about 3mA current (assuming Vf=2v) due to R1 and R10 being so large. Dim!

Also, I'm not quite sure what R7 is feeding (LCD's or ?), but if there's a short via J2 pins 6, 7 or 8, VCC will get pulled to around 1v.

8. dk-info Thread Starter New Member

Mar 9, 2009
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0

Yes, that was it, I overlooked the "bottom view" note on the data sheet.

David