[time-nuts] Answers to regulator choices comments

[Perry Sandeen]

Wrote: If you are in the US (maybe elsewhere) you can
request two free samples.
 
That might work for projects on and two, but what
about projects three and onward?
 
 
Wrote: I suppose that the same philosophy
[paralleled regulators] would apply to getting more power with a 7805 farm.
 
It does however with the general output rating of
1 to 1.5 amps for each regulator it wouldn’t be very practical.  It’s much cheaper and simpler to use ones in
the 3 amp plus range.
 
Wrote: A regulator needs to be specifically
designed for parallel operation. If it's not designed that way you will have a
very hard time with it.
 
I’ve never heard of any three terminal regulator
designed for parallel operation.  I believe
that all three terminal regulators use a pass transistor.  When one uses them in parallel they need a
slight resistance added to each pass transistor to prevent current hogging just
as one had to do when paralleling power transistors in other high amperage
circuits. 
 
Wrote: It may be a bit more complicated than
that.  You need some way to share the
load and you also need to make sure things are stable.
 
The TI/National data sheet doesn't show anything
about paralleling regulators.  The AD
data sheet shows 2 ft of #18 wire between each regulator and the load.
 
I'm not enough of an analog guru to reverse
engineer that setup and figure out the stability constraints and transfer them
to 78xx type devices.
 
 
I stand corrected about the LM 1084 showing
paralleled regulators.  However the data
sheet says it is pin compatible with the LM 317.  So we get to the paralleled regulator
circuits by a bit of a circuitous route. If we go to National Semiconductor Linear
Brief 51 March 1981 titled “Add Kelvin Sensing and
Parallel Capability To 3-Terminal Regulators” it
shows how to parallel two or more three terminal LM 338 regulators.
 
The stability problem is solved for us by the
Nation Semiconductor engineers. The 2 ft. of #18 wire for each regulator
provides the load balancing resistance needed.  One could use an ordinary resistor instead if it had the value of 30
mili-ohms.
 
The operation of all 3 terminal regulators are the
same.  The internal circuitry looks at
the relationship between the output voltage and the *ground* terminal.  As the data sheets show, if we change that
relationship with resistor combinations we can manipulate the output voltage to
our needs.
 
For most low voltage applications one can usually
find a three terminal regulator that will fit the current needs,
 
My original point was that the LM 1084 [$14]
IMNSHO is very expensive for what it does. By paralleling two far cheaper of
the LM 338 family one gets a larger ampacity of 10 amps instead of 7.5 amps for
$3 to $5 instead depending on one’s scrounging abilities.
 
In the end you pays your money and you make your
choices.
 
Regards,
 
Perrier
 


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