[time-nuts] Answers to regulator choices comments

Bob Camp lists at rtty.us
Sun Jun 30 12:38:57 UTC 2013


Hi

If you want the regulators to share the load equally when paralleled, they need to be a bit different than a +/-5% LM317

Bob

On Jun 30, 2013, at 12:19 AM, Perry Sandeen <sandeenpa at yahoo.com> wrote:

> 
> 
> 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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