Post by thread:[OT]Linear V-Reg Weirdness

I just built a power supply that provides +6V, +5V, +3V, and -6V, using several linear regulators, 2 of which are 7806 and 7906 (for the +6 and -6). Sometimes it's fine, but sometimes when I turn it on the +6 supply doesn't appear, and hovers around -0.8V instead. I vaguely remember reading about this problem somewhere in the dim and distant past, but I can't for the life of me remember the trick to fix it. I tried putting asymmetric loads on the outputs, but that doesn't seem to do it. Can anyone shed some light on this for me? Dave Johnson

>I vaguely remember reading about this problem somewhere in the dim and >distant past, but I can't for the life of me remember the trick to fix >it. I tried putting asymmetric loads on the outputs, but that doesn't >seem to do it. Can anyone shed some light on this for me? Lot's of times it helps to do power sequencing to turn things on correctly. Andy \-----------------/ \ /---\ / \ | | / Andy Kunz \ /---\ / Montana Design /---------+ +---------\ http://www.montanadesign.com | / |----|___|----| \ | \/___| * |___\/ Go fast, turn right, and keep the wet side down!

Sometimes the 78/79 series regulators can be "pulled" into protect mode by a supply of the opposite polarity during turn-on. This will happen a lot using opamps, for instance, since the load is actually from the + voltage to the - voltage (rather than individually to ground). If that's the case for you, the fix is simple: Add a reverse-polarity protection diode (1N4002 or similar) from each supply output to ground. You will see this diode in most of the regulator data sheets (Morotola, National, etc.) and this is the reason it's there. >I vaguely remember reading about this problem somewhere in the dim and >distant past, but I can't for the life of me remember the trick to fix >it. I tried putting asymmetric loads on the outputs, but that doesn't >seem to do it. Can anyone shed some light on this for me?

If the regulators are more than a couple of inches from the power supply capacitors, there is a recommendation that you place a 1uF Tant. capacitor on the input and output of the regulator. These regulators will occasionally break into a high frequency oscillation that can give some rather screwy voltages. This may not be your problem - but it will not hurt to try. David V. Fansler Network Administrator AutoCyte, Inc. 336-222-9707 Ext. 261 spam_OUTdfanslerTakeThisOuTautocyte.com Now Showing! http://www.mindspring.com\~dfansler\ {Original Message removed}

>Sometimes the 78/79 series regulators can be "pulled" into protect >mode by a supply of the opposite polarity during turn-on. <snip> >If that's the case for you, the fix is simple: Add a reverse-polarity >protection diode (1N4002 or similar) from each supply output to >ground. Thanks a ton, that seemed to do the trick beautifully. I KNEW there was a simple solution... Dave