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Forum Index : Electronics : potmeter
| Author | Message | ||||
| irishron40 Senior Member  Joined: 22/09/2014 Location: IrelandPosts: 251 |
Hi. I'm a newbie to electronics. I build a simple voltage regulator based on lm317. The potmeter is 50 k. It seems very hard to preciseley controller volt output.I even tried putting a 5 and 10k resistor on potmeter outer lugs. Any suggestions very welcome. Thx. Ron |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Have you followed the application note? What voltage are you trying to get? What value resistor are you using from the output to the adjust pin? 50K seems an extremely high value. |
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| Pete Locke Senior Member Joined: 26/06/2013 Location: New ZealandPosts: 181 |
Yes, you have the wrong value pot. It should be 5k not 50k. No wonder it's touchy :-) http://www.ti.com/lit/ds/symlink/lm117.pdf This is the data sheet you should be using as a guide. Cheers Pete'. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
Between output and adjust i have 180 ohm resistor.my power supply is a 19.5 volt laptop charger which is already balanced. So I dont use capasitor on my veroboard. Just lm317t. A 170ohm capasitor and 50k pot.could I put a 5k resistor across outerlugs of potmeter? |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
That will make it difficult to adjust to some values, as it becomes very "non-linear". What do you mean by "balanced"? |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
I mean stable. Voltage not jumping due to capasitors in charger |
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| Pete Locke Senior Member Joined: 26/06/2013 Location: New ZealandPosts: 181 |
OK so you are feeding the LM317 with a laptop power supply. You should still use a de coupling capacitor at the input and output of the 317. A 5K resistor across the pot will sort of work, but as Davef mentioned above it will still be very tricky to set your end voltage, and possibly lead to instability issued in the regulator. What voltage are you trying to get? Remember that the 317 needs a minimum of 3 volts above what you are aiming for. |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
My laptop charger supplies 19.5 volt. I be happy to get anywhere from 2 to 17 volts.as I'm a beginner could you explane what size if capasitors I need and are the connected to neg and positive before and after lm317?. Thank you |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
I also noticed that potmeter doesntvdo anything in first quarter turn |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
The application notes Pete mentioned tell you what values to use, a 0.1uF on the input and a 1uF on the output will be fine. http://www.ti.com/lit/ds/symlink/lm117.pdf Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Pete Locke Senior Member Joined: 26/06/2013 Location: New ZealandPosts: 181 |
Gizmo has again put the link into the data sheet for the regulator. If you copy the diagram that is on it, you should be fine with your project. Keep all leads as short as possible, and fit the de coupling capacitors as close to the regulator I.C as you can. I find it's easier (if using vero board) to start with the I.C, then add the de coupling caps before anything else. Good luck! |
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| Pete Locke Senior Member Joined: 26/06/2013 Location: New ZealandPosts: 181 |
Again, it's the WRONG value pot to be using. As you say, you are new to the field of electronics. Start with what has been proven before you try and re-invent the wheel  |
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| irishron40 Senior Member Joined: 22/09/2014 Location: IrelandPosts: 251 |
I ordered right one. Thank you |
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