LM317T 6V, 4.5V and 3V out. I need help with 3V

Discussion in 'The Projects Forum' started by Fenris, Jun 28, 2009.

1. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi all

I have just put together a circuit to drop 12VDC to 6V, 4.5V and 3V. The veroboard comprises 3 identical circuits one for each voltage needed. Now the 6V and 4.5V are OK. But the 3V is not in the range of the existing design. I can get down to 4.45V.

Now I have read the data sheet and although I 'get' the principle I am up against my old enemy mr maths. I can't do the sums to work out what values need changing to get the 3V out. Could someone help me on this please. I attach my vero circuit and the schematic I am working from.

regards

Fenris

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2. Jony130 AAC Fanatic!

Feb 17, 2009
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Change R1 to 100Ω or calculate the R1+R3 value with equation

$R1+R3=R2*\frac{Uwy-1.25V }{1.25V+I_{ADJ}*R2}$

Or
$R1+R3=R2*(\frac{Uwy }{1.25V}-1)$

Last edited: Jun 28, 2009
3. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Tried that and I can only adjust down into the 9V area now. I really do have trouble with the maths part so the equation, although I know what it's for, makes no sense to me. Think of it in terms of dyslexia with numbers.

regards

Fenris

4. mik3 Senior Member

Feb 4, 2008
4,846
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The LM317 needs at least 10mA at its output to regulate properly. Change R2 to 120R as to always draw 10mA (1.25V/120) from the output or connect a LED via a current limiting resistor to the output to draw 10mA.

Apr 5, 2008
15,797
2,384
Hello,

The "print drawing" is confusing.
The in and out are exchanged.

Greetings,
Bertus

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6. SgtWookie Expert

Jul 17, 2007
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It took me a minute to understand what Bertus was saying.

Fenris, on your Veroboard layout, you have mixed up the input and output pins of the LM317 regulators.

As you have them on the board, the lower terminal is ADJ, the middle terminal is OUT, and the upper terminal is IN.

As for getting down to 3v, replace the resistor R1 (560 Ohms) show in the original schematic with a piece of wire.
You should then be able to adjust the output down to 3v using R3 set to around 305 Ohms, +- 20 Ohms.

Last edited: Jun 28, 2009
7. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi Guys

Thanks for all the input. It is much appreciated. Ah yes the pin outs are wrong. I am using program called lochmaster to draw up the vero pattern and it is their library component. It can be corrected but not removed from the library. I forgot to amend it for my drawing especially as I suffered a momentary confusion because it didn't match a single regulator circuit I had built previously.

I will change to a wire link now. Thanks again all.

regards

Fenris

8. Fenris Thread Starter Active Member

Oct 21, 2007
288
2

Thanks for your help I now have 6V, 4.5V and 3V outputs I just need to fit a small single piece heat sink (there will be insulation between each LM317) to it now to deal with any heat. The unit is destined to be fitted into a full size scale replica of the new series Davros Skirt from Dr Who that a fellow member of the project dalek forum has built. The unit will be driving 3 torch bulbs, a string of LED's a laser module and a sound chip with phrases spoken by Davros during his brief appearance in the new series.

Thanks again everyone. I really appreciate your patience and time.

regards

Fenris

9. SgtWookie Expert

Jul 17, 2007
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Glad you got it sorted, Fenris
Mind that the OUT terminal is also connected to the tab. A short anywhere will mean higher voltages than expected.
OK, now you've mentioned three different applications which actually require very different supplies.

What is the voltage and current (or voltage and WATTAGE) rating of the torch bulbs?

LEDs need to be regulated by current, not voltage.

The sound chip will need to be regulated by voltage. It's amplifier will likely need much higher voltage.

10. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi there

OK the amplifier and 3 12V/3W bulbs go direct to the 12V battery.
The voice circuit comes out of a 12" davros toy and is powered by 4 AA batteries = 6V
He has an Alan Bradley Switch which has a green light. It simply says 6V.
The LED's. These are a string of 'fairy lights' from a mini xmas tree totalling 12 LED's in total. He currently uses 2 AA batteries to power them = 3V

Not the fullest specs in the world but this is all the info he was able to furnish me with.

regards

Fenris

11. SgtWookie Expert

Jul 17, 2007
22,183
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So far, so good.
This might be a problem. The 'laser pod' may be dependent upon the internal resistance of the batteries to maintain the correct voltage across the laser diode. If the voltage is even a bit too high, the laser will die very suddenly. This really needs current regulation rather than voltage regulation.
This is another somewhat dicey proposition. LEDs require current regulation. A very small change in voltage will cause a large change in the current through the LEDs.

12. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi Sgt

Right the guy says the laser pod has a mains adapter as well so he is going to let me know what specs are on it V/A wise.

The LED's presumably I need to know if the are in parallel or series on the string and then I can work out what resistor needs to go inline to deal with the current?

regards

Fenris

13. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi there

Ok heres the spec of the mains adapter for the laser pod.

INPUT:240V ~ 50Hz 35mA
OUTPUT: 4.5V ------- 300mA

Now the LED's are 12 off in parallel. The battery holder is transparent and there is no sign of a resistor in line to deal with the current. I presume as you say, and I have seen elsewhere, that the LED's are relying on the internal resistance of the battery's.

regards

Fenris

14. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Just a little bump

regards

Fenris

15. SgtWookie Expert

Jul 17, 2007
22,183
1,728
Sorry Fenris, I've been busy and distracted.
Any chance that the actual voltage and current could be measured while the laser is operating? Note that you'll need to be careful measuring such current with a meter, it's easy to blow the meters' internal fuse if you accidentally try to measure current across the voltage supply. I have a precision 1 Ohm 50W resistor that I use for current measurements.

Some lasers have built-in regulation. Some require external regulation. If the mains adapter is just a "wall wart" type supply, the regulation is probably internal to the laser.

You could make a 270mA constant current supply from an LM317 using a 4.7 Ohm 1 Watt resistor from the OUT to the ADJ terminal, and taking the laser's current from the ADJ terminal.

Well, then you'd need to measure the current on those, too. Use a couple of fresh alkaline cells.

A 10 Ohm resistor from the OUT to the ADJ terminal would give you roughly 125mA output current, or about 10.4mA per LED (if they were perfectly matched).

16. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi Sgt

I appreciate you are busy and the time you spare Thanks for looking at this.

Right I will get in touch with the chap and see if he can make, or has a local friend, the measurements.

Right then I have looked up the LM317 as a constant current supply here is the amended Vero board layout. The component count just took a bit of a dive . I have understood you correctly I hope. I will wait on the guys reply before implementing any changes. Thanks again for your help!

regards

Fenris

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17. SgtWookie Expert

Jul 17, 2007
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Fenris,
There are some jumpers that are hidden by the heat sinks of the regulators, so I can't be certain where they terminate unless you reveal them.

C2 and C3 shouldn't really be necessary as long as C1 is a decent size; say 220uF or so.

18. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi Sgt

Here's the layout with the wire links moved into view and C2 and 3 have been removed. Thanks again for your help.

regards

Fenris

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19. SgtWookie Expert

Jul 17, 2007
22,183
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The jumpers look OK.
R4 = 4.7 Ohms 1W, instead of 4.5v the output should be labeled 270mA.
R6 = 10 Ohms, instead of 3v the output should be labeled 125mA.

20. Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi Sgt

I will relabel the 2 relevant portions of the circuit and also explain very carefully to the guy this is for that he mustn't attach anything other than what it has been designed for to it. I think I need to have a look at this current over voltage supply and get a better understanding of the why's and wherefore's.
Thanks again for your time it is much appreciated

regards

Fenris