Regulator voltage

[JOddino]

John,
I'll try to explain it this way.  Think of the regulator as a variable
resistor that automatically changes its resistance to cause the voltage to
drop from whatever the input is, say 6.1 volts, to whatever the regulator is
set to, say 5.7 volts.  The resistance this takes is dependent on the
current.  For example, at one amp the resistance would need to be .4 volts
divided by one amp equals .4 ohms.  At 5 amps it would need to be .08 ohms.
If the input voltage is 7.9 volts and the load is one amp the resistance
would need to be 2.2 ohms.  It is just like an automatic valve in a water
line that adjusts for upsteam pressure and water flow rate.

The chip I use has a minimum resistance of .05 ohms so it is the low input
voltage we are worried about.  If we want the regulation to hold at 5.7 at
one amp, the input voltage must be more than 5.75.  (.05 ohms x 1 amp = .05
volts).  At five amps it must be 5.95 volts ( .05x5=.25).  This drop is very
important if the pack drops real low to say 5.5 volts.  We'd like the output
to be as close to 5.5 as possible.
I should point out that you might have trouble duplicating these numbers
because of voltage drops in the wires and connectors.  The R/C connectors
are not designed for 5 amps and will have a significant drop at those kind
of currents.
Many voltage regulator ICs have as much as 1.5 volts drop at high currents
and therefore are not suitable with 5 cell packs but you can see they might
work okay with Li-Ion or six cell packs.
If this isn't clear don't feel bad about asking again.
Jim


----- Original Message ----- 
From: "john tarpinian" <jtarpinian at yahoo.com>
To: <discussion at nsrca.org>
Sent: Friday, February 27, 2004 8:05 AM
Subject: Re: Regulator voltage


> Jim,
>
> At what current level does this 0.2 v drop occur?
>
>
>
> --- JOddino <JOddino at socal.rr.com> wrote:
> > Here is the thought process:
> > You would like the highest voltage your servos can
> > handle.  Some can take
> > six volts, some should be limited to 5.1 or 5.2, the
> > value of a fully
> > charged four cell pack.
> > You would like to have good regulation with the pack
> > discharged to 5.9
> > volts.  This allows for good regulation over most of
> > the battery's discharge
> > voltage range.
> > You want a regulator that has a low drop out voltage
> > (low difference between
> > input voltage and regulated output) at the rated
> > current.
> >
> > The Jaccio regulator has a dropout voltage on the
> > order of .2 volts, so the
> > recommended output with a 5 cell NiMH pack is 5.7
> > volts, if the servos can
> > take it.
> >
> > Many of the regulators out there have a higher
> > dropout voltage and are fine
> > with Li-Ion packs where you would not operate below
> > 7.0 volts but are not
> > recommended with 5 cell NiCd or NiMH packs.
> > Jim
> >
> >
> > ----- Original Message ----- 
> > From: "john tarpinian" <jtarpinian at yahoo.com>
> > To: <discussion at nsrca.org>
> > Sent: Thursday, February 26, 2004 8:01 AM
> > Subject: Regulator voltage
> >
> >
> > >
> > > I'm planning to use 5 digital servos and a 5 cell
> > NiMH
> > > pack with a regulator/perfect switch.  What
> > voltage
> > > regulator should I puchase? Is 5.6v too high,
> > > resulting in a lack of regulation at times.>
> > > >
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