# voltage regulator LM-317T

Discussion in 'General Electronics Chat' started by parisathena, May 6, 2013.

1. ### parisathena Thread Starter New Member

Feb 22, 2011
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I am experimenting with this voltage regulator and as far as I know, or maybe not know the input voltage shall be positive. In my experiments I want to use this regulator to vary the speed of a motor and to reverse its rotation. The circuit I build is the standard circuit in the datasheet and I apply 0 to +12V and 0 to -12V and regulator and its working even with negative input voltage. I am missing something?

2. ### crutschow Expert

Mar 14, 2008
21,338
6,116
Likely. The LM317 only works with a positive polarity. It can regulate a positive voltage but not a negative.

Post your circuit diagram.

3. ### parisathena Thread Starter New Member

Feb 22, 2011
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Please open with http://expresssch.findmysoft.com/.

I am puzzled too. I had the circuit controlling a 12V motor that was turning a load of 20lbs for 5hrs with -12V and no problem; I am an electrical engineer, PE and I through all my books out, I cannot figure it out. I was planning to use the LM-337 to build a negative voltage regulator and I wire the LM-317 reverse by accident and it was working.

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4. ### ScottWang Moderator

Aug 23, 2012
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You can using Relay, Bjt, Mosfet to drive the Motor.
If you have LM317 and LM337 then you can do as these ways:

Hi → NPN Bjt(-) ← PNP Bjt(+) → Motor → GND
Lo ← PNP Bjt(+) → NPN BJt(-) ← Motor ← GND

Hi → NPN Bjt(-) ← P ch Fet(+) → Motor → GND
Lo ← PNP Bjt(+) → N ch Fet(-) ← Motor ← GND

5. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
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All I see in your link is a 9.7mB download I do not want.
Please confirm your circuit here at AAC.

In particular what is the regulator connected across? +12 and -12 / or 0 and +12 / and then 0 and -12?

Further what is the motor load connected across?

0 and +12 or what?

6. ### Wendy Moderator

Mar 24, 2008
21,528
2,973
It is easy to misuse linear regulators. For example, this will not work well if you expect the lower voltage regulator to sink current. They can't, they are not designed that way.

I cover the specifics as to why not in this thread...

Creating a Virtual Power Supply Ground

BTW: Local hosting is preferred, as is showing schematics directly. Viruses are getting nastier all the time.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
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Post your schematic here as a .png or .gif file, so we can see it.

8. ### parisathena Thread Starter New Member

Feb 22, 2011
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This circuit worked with -12V input controlling a motor with 20lbs load for 10hrs and still going. Can anybody explain why is working?

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9. ### ScottWang Moderator

Aug 23, 2012
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Normally when the input voltages are -12V and 0V, then it should be using Lm337 to do the voltage regulation.

But when you using LM317, and the input voltage is independent from others, then you can connecting input voltage 0V to the Vin(pin 3), and -12V to the GND, the situation is similar like input +12V and 0V on the input pin of LM317.

10. ### kubeek Expert

Sep 20, 2005
5,452
1,036
The schematic shows that it should not work at all, as the switch disables the regulator in either position. Obviously that drawing is not the same as your real circuit.

11. ### studiot AAC Fanatic!

Nov 9, 2007
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Last edited: May 7, 2013
12. ### Ron H AAC Fanatic!

Apr 14, 2005
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682
Where does the negative terminal of your 12V supply connect? If you connect it to GND, then the 12V will be shorted when the switch is in either position.
As Kubeek suggested, post what you really have.

Attached is what I think you intended to draw.
It won't work.

• ###### power supply won't work.png
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Last edited: May 7, 2013

Feb 22, 2011
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14. ### parisathena Thread Starter New Member

Feb 22, 2011
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I did in a hurry and I left out what is obvious
Thank you for the help.

15. ### parisathena Thread Starter New Member

Feb 22, 2011
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Scott:
Can you explain "and the input voltage is independent from others" as I am still trying to comprehend it.

Thank you.

16. ### ScottWang Moderator

Aug 23, 2012
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1,021
I said the "independent from others", because the method as I mentioned that it is not a normal method, if the power is not independent then it may damage the regulator or load.

The normal input polarity:
+V → LM317 → +Vout
-V → - → - → - → GND

The method that I mentioned:
0V → LM317 → +Vout
-V → - → - → - → GND

From two comparation as above, you can see the outputs are the same, but the inputs are different, that's what I'm care about if it is connecting to other load then it may occur something bad.

17. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
Try it like this instead:

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18. ### parisathena Thread Starter New Member

Feb 22, 2011
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That is how I had build the circuit but I needed an additional ON/OFF switch at the input which it was adding cost and complexity, and therefore I was looking for a way to use just one switch.

Thank you.

19. ### SgtWookie Expert

Jul 17, 2007
22,201
1,806
Then use a 3PDT switch, which will likely be more expensive than a DPDT + SPDT switch. Use one of the three poles for the power on/off.

20. ### studiot AAC Fanatic!

Nov 9, 2007
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522
I think you should be very very careful using a reversing switch to change the polarity of output terminals to a supply capable of more than one amp.

A single wrong polarity error could cost more than the psu.

For the benefit of others reading this
Having such a switch is not recommended practice.

Why not simply leave the terminals floating and connect the appropriate way round as dictated by the circuit?

Further, you have drawn asd labelled your switch a double pole double throw switch.

You have also labelled it as centre off, which it is not if it is as drawn. A true centre off would obviate SgtWookie's comment about using three poles.