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DIAR 5th October 2004 07:21 PM

Overwhelming difficulties with regulators, please help!
I have two problems whitch are probably easy to solve.

1st problem LM317

I'm trying to replace CD player's LM7805 regulator with simple LM317 regulator circuit:

I used 240 ohms for R1 and 750 ohms for R2.
The pin order for LM7805 (in, out, gnd) and LM317 (adj, out, in) is different.
I connected the pins and resistors like this:

I tested the regulator with 1 kohm and 47 ohm load and I had a 9 volt battery as a powersupply. The voltage was first 5.8 and started getting higher. I had a 4,7 uF cap between putput and ground. No other caps.

2nd problem LM7808
I'm building a simple regulator for Tentlabs XO clock. I used this PCB:

Again the voltage was first over 5 V (should be 3.3 V) and started getting higher every second.

I've tried everything and searched half the internet for a solution. You people are pretty much my last hope. Please help !! :bawling:

chris ma 5th October 2004 07:25 PM

Normally I see a diode of some sort to prevent the regulator get into trouble in these sort of circuits.. can you post the entire schematic ?

DIAR 5th October 2004 07:49 PM

LM317 is going to be installed to replace this:

I add nothing else but the regulator with the two resistors (no diode). I haven't tried it yet in the cd player.

LM7808 is connected to that circuit. There is nothing else in it. PSU is going to be unregulared 16 V DC.

godzich 7th October 2004 08:48 AM


LM317 oscillates easily if you have capacitance at the output but not at the input. Connect a small ceramic 0.1uF capacitor fron the input pin to the same point where the voltage setting resistor is connected to ground - keep wires short. This should fix it is the problem is due to oscillation. Use an oscilloscope to verify. It might be that you already have there a cap - but I was not able to tell because your spelling putput spelling mistake ;)



DIAR 7th October 2004 12:54 PM

Countryman to the rescue.

I designed this module last night. There are 2000 uF caps in the power supply and 1000 uF after the regulator. Unfortunately I don't have an oscilloscope (I know it's a basic household appliance) but I might know a few people who might be able to help...

gmphadte 12th October 2004 04:34 AM

LM 317...
Refer to the datasheet on

You will find there is a cap to ground from adj pin.
Use the given formula and calculate the value of R2.

If you connect a larger cap at the output then the input, the output cap tends to supply voltage to the output of the regulator when the input voltage is switched off and damages the regulator

Every aspect and variants of the application exist at national website.

I suspect you are trying to replace 7805L with LM317T

Also fix a heatsink since your i/p voltage is quite high.

Gajanan Phadte

tekman 14th October 2004 07:20 AM

2 DIAR: To probelm #2: It's hard to read the circuit for me just by seeing the board, and no shcematic.
AFAIK the 7808, the mounting in the board should be the following: metal cooling fin should be facing the 100 uF caps (if the metal is facing to the "outer" border of the printed circuit board, IN and OUT are reversed forthe 7808 -- this might be the reason for the voltage change.

Let us know how things turn out, pls.

Circlotron 14th October 2004 09:59 AM

Re: LM 317...

Originally posted by gmphadte
You will find there is a cap to ground from adj pin.
Yes. :att'n: Across R2. 1uF is a good value to start with. It could indeed be oscillating.

DIAR 14th October 2004 11:43 AM

2 Attachment(s)
LM7808 schematic can be seen here:

I noticed the wrong pin order. I made the pcb but I haven't tested it yet.

I assembled LM317 regulator but I haven't tested it yet.

The PCB layout is attached.

I really hope they work this time. I have burned several regulators already :dead:

tekman 14th October 2004 11:58 AM


thank you for the website link.

Well, I can't understand why they do so much efforts to propose this circuit for genrating the Supply voltage for the XO:

Datasheet says 5 V +/- 5 %.

So you can simply use 1 7805 regulator, 100uF at input, 10 uF at output -- voila - 5 V with 3 parts.

The original circuit is much more effort, and leaves you with a little bit lower noise level on the +5V rail -- but that's not relvant for the XO.

BTW: If you "burn" the regulator IC's than it is very very shure that there is a problem with your pinout on the board. If I run into this, I let a freind look at the board -- people that are not involved in the project see faults in secounds where I fail for days not seeing these.

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