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The spec's for the LM317 are with at least 3V between its input and its output.
There is a graph of its typical dropout voltage (when it has stopped regulating) that shows a dropout voltage of about 2V at room temperature at 1A but it could be as high as 3V.
@audioguru
Thank you so much this information. I have just read the datasheet. I understand something better. For 1A , 25C temperature , 16.2V unregulated voltage and 14.5V output my source is useless. Is it true ?
Only if your LM317 is typical or is worse. Do you like to gamble?
They are all different. Some are excellent and some are not so good. You don't know if yours is excellent or not. A poor but passing one needs to have its input voltage at least 2V or 3V more than its output.
Maybe a large company ordered all the most sensitive ones before you got yours. Then yours isn't very sensitive.
Maybe the latest production before you bought yours made all poor but passing ones. Then yours is poor.
When you draw a load there will be ripple which will make the situation even worse.
Audioguru,
What you're saying (about 3V) sounds reasonable but where on the datasheet does it say that? I agree that it mentions the typical values but it doesn't mention the worst case values.
When you draw a load there will be ripple which will make the situation even worse.
Audioguru,
What you're saying (about 3V) sounds reasonable but where on the datasheet does it say that? I agree that it mentions the typical values but it doesn't mention the worst case values.
Kalaman - you need to describe the source voltage in a little more detail. You said it's 16.2 volts unregulated. You need to consider the lowest voltage at any instant - which must include the effects of load, unregulation (so to speak) and ripple.
As an example - if you determine that at 1 amp your source can drop to 15 volts peak then it gives you a place to start. If the source is rectified AC you'll need to subtract the peak-to-peak ripple (unless you've measured the 15 volts as average or RMS). If for example the ripple is 1 volt p-to-p then the "bottom" will be at 14 volts.
Another thing to consider, if your source is rectified AC - will the primary input to the source be relatively constant - or could the primary vary a little. You might want to account for that as well.
It might barely work with an input voltage only 2.5V higher than its output voltage but many won't peform as well as specified.
Its spec's are guaranteed if the input is 3V or more higher than its output.
When it has dropped out then its output voltage has already dropped because it isn't regulating anymore.
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It was poor according to these.
