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Old 1st June 2004, 08:06 AM   #1
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Default regulation of 78XX and 79XX at low curent

just put together a +/- 15 V supply using 7815 and 7915 regs. Rectified and filtered supply to te regs is ~ 20V. With no load, the neg side runs at ~16.5 V, and the + side closer to 15 V. Noticed that the factory tests at 500 mA, so maybe they need a slight load in order to operate. I only had .25 W resitors available, so I hitched them across the supply legs, giving a load thru each reg. of 100 mA. Seemed to regulate better, but the test period was pretty short due to overloading the resistors. Any opinions. Sorry if it is a dipwad question! Thanks!
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Old 1st June 2004, 10:45 PM   #2
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Default may have it figured out

Just talked to someone who knows these things much better than me. He said that many brands of 78XX and 79XX regs need a small current through them to start regulating properly. He often uses a 1K resistor from output to gnd. I guess it doesn't really matter if the regs are actually in-circuit, being used, and flowing current, but it kind of threw me when I checked the voltage with no load.
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Old 3rd June 2004, 07:23 PM   #3
cpemma is offline cpemma  United Kingdom
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The NS datasheet (LM7805) quotes line regulation typically 9mV from 5mA-1.5A load, 4mV between 250mA-750mA, so you don't need much loading to settle things down. More noticeable in cmos logic, where quiescent current may be negligible.

317T is similar, in that if you use high value voltage-setting resistors, the zero-load regulation is poor. One fix is to add a led/series resistor which also shows it's on...
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Old 3rd June 2004, 07:34 PM   #4
dhaen is offline dhaen  Europe
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Default Be careful...

Negative regulators (79XX) have lousy low current regulation. Not all manufacturers' figures are the same for this parameter.
This has been discussed before, but I'm too lazy to search...
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Old 3rd June 2004, 11:08 PM   #5
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So a way to fix the problem of lousy negative regulators would be to just use positive regulators for both legs right? One from the positive rail (+) to ground (-), and the other from the ground (+) to the negative rail (-) right?
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