Hi Jeff, >In general I don't like raising the ground on 7xxx series fixed regulators by adding diodes to what would normally be the ground lead.
You're right, the diode trick is not recommended. National recommends putting a voltage divider across the output of the regulator and connecting the ground lead to the tap. See: http://www.national.com/ds/LM/LM340.pdf. 73, Bob, WA9FBO -----Original Message----- From: Jeff DePolo <[email protected]> To: [email protected] Sent: Sat, Feb 27, 2010 9:32 am Subject: RE: [Repeater-Builder] Re: Micor UHF Repeater Base 9.6 vdc current requirement? > In a station, the 9.6 volt circuitry *could* be run from 12 volts, as > long as its regulated. I'm not suggesting someone does that, because > the circuits were optimized for 9.6 volts, but I'm trying to > make a point. Point taken, but to add to your cautionary note, there are some circuits that will not be happy at 12V. I remember trying to run a Micor PL decoder off 13V and it did some strange things (i.e. didn't work right/reliabily). > If it were me, I'd just use a 9 or 10 volt three terminal regulator > (7809 or 7810) and skip the whole LM-317 or diodes in the ground leg > deal. It only serves to make the circuitry more involved , harder to > mount, and for NO advantage whatsoever. In general I don't like raising the ground on 7xxx series fixed regulators by adding diodes to what would normally be the ground lead. 7xxx series regulators can (and will) oscillate, which is why adding caps per the datasheets' recommendations isn't just a good idea, it's mandatory. My concern is that putting a diode in series with the ground lead results in the filter/bypass caps being at a different ground potential than the device, which may increase the chance of oscillation. --- Jeff WN3A
