>-----Original Message-----
>From: Charles Craft [mailto:chucksea@MINDSPRING.COM]
>Sent: 10 May 2004 07:32
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [EE:] Raising GND of linear reg for higher output
>
>
>I've used zeners and regular diodes in the past to raise the
>ground reference for a linear voltage regulator.
>
>When will this come back to bite me in the backside?
>
>Needed a 9v regulator but only had a 5v so used a zener? or a
>couple diodes to raise the ground a couple volts. Still
>working but wondered how bad a practice this is?
>

A 78xx regulator has a small current flowing out of the ground pin, that
varies with the current drawn from the output.  If you put a device with a
finite impedance in series with the regulators ground pin, the voltage at
the ground pin, and hence the voltage at the regulators output pin with vary
according to the current drawn (i.e. load regulation will be degraded).  A
simple resistor is by far the worst case, but low voltage zeners also have
significant dynamic resistance.

You may also care about temperature stability depending on the circuit,
which will also be degraded somewhat by "jacking up" a regulator like this.

Neither of these may be a problem, depending on your application,  but it's
something to be aware of.  Far better IMO, to buy a bunch of LM317T's and
benefit from the adjustable voltage output, lower quiescent current and
better regulation characteristics.

Regards

Mike




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