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Old 11-18-2011, 08:59 AM #1
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Default 0.1uF cap on DDL with LM317 (or similar) really necessary?

Hi,
Just curious as to why a 0.1uF cap before the LMxxx DDLs used for lasers are felt to be necessary? Given that almost always battery supplies are used and there really is no ripple issues with batteries...why the need for the cap? I am pretty sure I understand why the cap is needed under normal applications but I don't see why it's needed here. Anyone try to build a DDL without it?

I ask because I am trying to stuff a LM1084 into a tiny space in a tiny host and I have just enough room for the sense resistor and that's about it...adding this cap will just take up too much room unless I go with all SMD (which I have none on the shelf at the moment).

Please enlighten me...show me I'm wrong (or right)....

Thanks,
Jeff


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Old 11-18-2011, 09:34 AM #2
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Default Re: 0.1uF cap on DDL with LM317 (or similar) really necessary?

The cap is just for input decoupling/filtering, you're perfectly fine leaving it off. In some rare occasions, the LM317 can go into oscillations, but you'd have to meet a few requirements for that to happen first.

All you really need is the resistor and the LM317, that's all. A diode if you want some reverse polarity protection, which isn't usually a bad idea.
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Old 11-18-2011, 10:29 AM #3
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Default Re: 0.1uF cap on DDL with LM317 (or similar) really necessary?

You should only need the cap if you are going to be making a lot of
electrical noise around you laser, so no using it around Tesla coils
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