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Multiple regulator question... regs in series...
Multiple regulator question... regs in series...
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Old 20th September 2007, 06:09 PM   #1
Nordic is offline Nordic  South Africa
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Join Date: Sep 2005
Default Multiple regulator question... regs in series...

I would like to know if the following can work...

IT IS NOT A SCHEMATIC only a rough connection diagramme..

I have one transformer stepped down to 17.5V using lm337/17

next I have two buffers I would like to give local regulation with their own 78lxx/79lxx's...

Actualy it works, but I noticed the first regs were quite a bit hotter than just powering the buffers directly...

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Old 21st September 2007, 07:54 AM   #2
gootee is offline gootee  United States
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Join Date: Nov 2006
Location: Indiana
Hi Nordic,

Is it possible that they're oscillating?

Are they close together?

A schematic would be nice. Is there any resistance, besides the conductors, between the 317/337 outputs and the other regs' inputs? Assuming there's not much current, it might not hurt to try a very small R after each 317/337.
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Old 22nd September 2007, 02:26 PM   #3
theChris is offline theChris
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Join Date: Dec 2001
Location: SIUE, Illinois, USA
whichever regulator has the highest voltage drop across it will run hottest. for example, one way to get 37VDC converted to 5VDC @ 1A would be to connect multiple series regulators in series. Normally, this would be 32W on one device, but using more then one regulator distributes losses to each device.

In such a case, a single power transistor capable of 32W dissipation could have been used, controlled by the 317/337
if only it could be used for good, not evil...
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Old 22nd September 2007, 02:35 PM   #4
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
there is nothing wrong with using cascaded regulators.

Each one should be considered on it's own and use the correct input and output caps, have sufficient no load current and heatsinked to dissipate the appropriate power.

The first obviously passes the same current as the total of the two later ones.
regards Andrew T.
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