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Old 06-18-2011, 01:28 AM #1
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Unhappy Bad LM317T?

I am trying to build my own DDL based on the LM317 circuit shown on this site. I ordered some LM317s from eBay but as I had everything but those, picked up a LM317T from RadioShack today.

I bread-board build the circuit but the output voltage read too high on a volt-meter.

With 6v applied to vin, vout reads 5.45v and adj reads 5v. no matter what resisters I bridge adj and vout with, vout stays at 5.45v. Am I doing something wrong or could this actually be a bad LM?

Thanks.



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Old 06-18-2011, 01:54 AM #2
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Default Re: Bad LM317T?

Assuming you're using this as a laser driver:

It is a current source, not a voltage source. It tries to push the required current through. If you measure the voltage with no load, the regulator will shoot to max trying to push the current through.

You MUST put a load on it for it to work. Measure the current, not the voltage. The voltage depends on the load, but the current will be constant.
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Old 06-18-2011, 02:17 AM #3
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Default Re: Bad LM317T?

Your LM317 is fine but as Cyparagon said you need a load on it. Build an appropriate test load and you'll get the current (plus or minus a few mA) you want.
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Old 06-18-2011, 03:36 AM #4
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Default Re: Bad LM317T?

And if you don't have a test load, you can put your DMM on the 10A mode (Most have it), and just short out the circuit through your DMM. That will tell you the absolute max your driver will give you. However, don't do it for too long as it may overheat
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Old 06-18-2011, 04:40 AM #5
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Default Re: Bad LM317T?

It's constant current. The "absolute max" current and the average current will be the same thing.

If it overheats with your DMM, it should probably have a heat sink anyway.
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Old 06-18-2011, 09:37 PM #6
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Default Re: Bad LM317T?

The part I was waiting on to finish my test load did not come today...

The o-like drive I purchased did though!

To kill time until the missing part arrives I decided to make use of one of the diodes I ordered as well as the o-like driver. Got the diode out of the heat sink (without messing it up) and connected it to the driver. I ran everything off of a power supply as I have not yet selected a host.

I don't think this is too bad for my first go...

Bad LM317T?-laser1.jpg

Bad LM317T?-laser3.jpg

Bad LM317T?-laser2.jpg

Bad LM317T?-laser4.jpg

Oh and yes, I was wearing safety glasses.
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