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Old 09-27-2015, 03:23 PM #1
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Default need help setting the current with lm317

Im in the process of making my first home-made laser.
I ordered the components to make a test circuit and driver online.
I connected 3 1 ohm resistors in series to a lm317 and when measuring the mv over the resistor in the test circuit a got about 380 mv, dropping to about 300. I guess adding a capacitor will make it more stable (i ordered a pack on dealextreme but it hasnt arrived yet). The problem I have though is the output should be higher with 3 ohm resistance right? Also with less resistors the measurement is about the same. Even if I remove the connection between the resistors, so there is no connection between the adj and Vout it seems the same.
Am I doing something absolutely wrong?
Please help me as I dont get this at all..


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Old 09-27-2015, 04:14 PM #2
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Default Re: need help setting the current with lm317

Could you post a picture of your setup?
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Old 09-27-2015, 04:20 PM #3
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Default Re: need help setting the current with lm317

Quote:
Originally Posted by Jonathane View Post
Im in the process of making my first home-made laser.
I ordered the components to make a test circuit and driver online.
I connected 3 1 ohm resistors in series to a lm317 and when measuring the mv over the resistor in the test circuit a got about 380 mv, dropping to about 300. I guess adding a capacitor will make it more stable (i ordered a pack on dealextreme but it hasnt arrived yet). The problem I have though is the output should be higher with 3 ohm resistance right? Also with less resistors the measurement is about the same. Even if I remove the connection between the resistors, so there is no connection between the adj and Vout it seems the same.
Am I doing something absolutely wrong?
Please help me as I dont get this at all..
Try this and post a picture of your setup if it still doesn't work. Make sure you have the pins correct.

Iout=1.25V/R
R=3Ohm
1.25V/3Ohm=0.417A=417mA~390mA

Last edited by Apocalypse; 09-27-2015 at 04:23 PM.
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Old 09-27-2015, 06:58 PM #4
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Default Re: need help setting the current with lm317

Thanks for the link!,
I looked at this drawing for the driver:
How to build a laser (general guide) : Step 3: Driver

This is what is looks like:


Looking at that link it kind of looks like the test load should be connected to the other side of the resistors, but if I try that out I measure even less mV, like 280.

Last edited by Jonathane; 09-27-2015 at 07:00 PM. Reason: fix image link
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Old 09-28-2015, 05:34 AM #5
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Default Re: need help setting the current with lm317

Quote:
Originally Posted by Jonathane View Post
Thanks for the link!,
I looked at this drawing for the driver:
How to build a laser (general guide) : Step 3: Driver

This is what is looks like:


Looking at that link it kind of looks like the test load should be connected to the other side of the resistors, but if I try that out I measure even less mV, like 280.
I can't see what's going on in the picture, it's tiny.

Last edited by Apocalypse; 09-28-2015 at 05:34 AM.
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Old 09-28-2015, 09:38 AM #6
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Default Re: need help setting the current with lm317

http://s17.postimg.org/nzb2igldb/pho...8_11_35_47.jpg for larger size
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Old 09-29-2015, 02:54 AM #7
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Default Re: need help setting the current with lm317

Quote:
Originally Posted by Jonathane View Post
Am I doing something absolutely wrong?
Yes, you're using a 9V battery. And possibly one of the inferior carbon zinc "heavy duty" ones at that. Alkaline 9V batteries aren't reliable for much over a hundred mA or two, and carbon zinc batteries are probably only reliable for <50mA.

You can confirm this by measuring the battery voltage when you load it down. You'll see the voltage drop substantially.

Switch to something with more current capability.
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Old 09-29-2015, 08:01 PM #8
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Default Re: need help setting the current with lm317

Quote:
Originally Posted by Cyparagon View Post
Yes, you're using a 9V battery. And possibly one of the inferior carbon zinc "heavy duty" ones at that. Alkaline 9V batteries aren't reliable for much over a hundred mA or two, and carbon zinc batteries are probably only reliable for <50mA.

You can confirm this by measuring the battery voltage when you load it down. You'll see the voltage drop substantially.

Switch to something with more current capability.
Thanks for the help, what power source would you guys recommend? Something like 4 AAA batteries?

I tried connecting 3 of those using a container that i got from a flashlight, but I measured like 15mV so I guess 4.5V is not enough.

I dont have anything that can contain 4 to try that out though..
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Old 09-29-2015, 09:05 PM #9
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Default Re: need help setting the current with lm317

put 6 AA's in series. that should do it for you.

since youre making a test load, you want diodes, not resistors... see pic. you put in enough diodes to get close
to the voltage drop of the LD you want to actually use. each of these diodes has a .7v drop. so the 6 of them drop 4.2v.
your lm317 has a 1.25 v drop. ideally you want 1.5 v higher than both drops combined.

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Last edited by chipdouglas; 09-29-2015 at 09:12 PM.
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Old 09-29-2015, 10:28 PM #10
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Default Re: need help setting the current with lm317

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Originally Posted by chipdouglas View Post
put 6 AA's in series. that should do it for you.

since youre making a test load, you want diodes, not resistors... see pic. you put in enough diodes to get close
to the voltage drop of the LD you want to actually use. each of these diodes has a .7v drop. so the 6 of them drop 4.2v.
your lm317 has a 1.25 v drop. ideally you want 1.5 v higher than both drops combined.
Thanks for the info, very useful
Found that picture too though in a thread about how to make a dummy load, mine contains 4 1N4004 diodes. I have a LPC826 diode.
The 3 resistors I was talking about and you can see in the picture are part of the driver.
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Old 09-29-2015, 11:01 PM #11
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Default Re: need help setting the current with lm317

As a side question, I found old 2 capacitors in my house, one is 10μF and the other 100μF. I assume a higher capacitance can handle higher voltage spikes, but how do I know how much is enough, and is it bad/possible to have too much?
I can find very little information about selecting the capacitor, everyone just says use this or that.
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Old 09-30-2015, 04:08 AM #12
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Default Re: need help setting the current with lm317

The cap is to prevent oscillations, not voltage spikes. A 0.1uF ceramic cap on the output will work. The value isn't critical. One of your 10uF caps will also work.
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Old 10-15-2015, 09:58 AM #13
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Default Re: need help setting the current with lm317

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Originally Posted by Cyparagon View Post
The cap is to prevent oscillations, not voltage spikes. A 0.1uF ceramic cap on the output will work. The value isn't critical. One of your 10uF caps will also work.
Thanks!

I ordered 2 battery cases for 3 AA batteries and hooked them up.
Now I measured about 500mV on the resistor, which seems to be too much for the 3 ohms of resistance.. also when I add a resistor I meaure something like 450 mV.
I decided to try it out with the diode, it lit up, the lm317 got quite hot and the diode too, though the beam from the diode didn't feel hot at all.
Same for the resistors that were put between the lm317 pins, they did not get warm at all.
Does anyone here know what my problem is?

Last edited by Jonathane; 10-15-2015 at 01:44 PM.
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