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From: John De Armond
Subject: Re: Voltage Stabilization Problem
Date: Tue, 14 Sep 1999 05:06:07 EDT

"Piotto, Bryan (EXCHANGE:CAR:CF11)" wrote:

> Some Definitions:

As long as we're defining things, let's get 'em right.

> Uninterruptable Power Supplies (UPS) - These give standby power in the case of
> power outages for short periods of time. They kick in to supply power to your
> appliance when the line voltage drops below some fixed value (~90 VAC) and
> provide a fixed output (~115 VAC). They do not change the line voltage even if
> it varies from 90 - 125 VAC. These won't do anything for darkroom applications.


Standby power supply.  Provides power to the load from a battery
during a power failure.  Consists typically of a 60 hz inverter, a
battery and a low capacity battery charger.  Under ordinary
conditions, line power is passed through unchanged, perhaps with
some surge suppression and filtering.  The inverter is off.  When
power begins to fail, circuitry detects the deviation from the sine
wave, starts the inverter and transfers the load to the inverter. 
This normally happens with computer standby power supplies within
half a cycle.  When the line power comes back up and is stable, the
load is transferred back to the line and the inverter stopped.  This
type would be applicable to enlarging provided: 

*) the equipment can withstand up to a cycle of power interruption. 
The exposure on the enlarger should not be affected by the brief
flicker but electronic timers might not like the glitch.  

*) The inverter is adjusted to output the same voltage as the line. 
Or is capable of adapting to slowly varying line voltage.  Otherwise
the lamp intensity changes and the exposure is affected.

Uninteruptable power supply - Load is constantly powered by an
inverter operating at battery voltage (12 volts for small units to
250 volts or more for utility UPSs) A large line-operated power
supply is connected to the battery and inverter and is capable of
supplying enough DC to run the inverter at its full rated output
plus charge the battery.  When the line power fails, nothing happens
except that the power supply stops supplying DC.  The battery picks
up the load.  When the line comes back, the power supply picks up
the DC load from the battery and simultaneously begins charging the
battery.  Some more sophisticated UPSs include a separate fast
charger that will bring the battery up faster than the line-operated
power supply would.  Surges, dips, dropouts are all eliminated by
the process of converting to DC and then back to AC.  These devices
are perfect for the darkroom but are expensive, large and heavy.

> Line Conditioners - These filter out any spikes or high frequency noise from the
> line voltage but they won't stabilize voltage fluctuations.
> Voltage Stabilizers (sometimes called Power Conditioners) - These provide a
> constant output voltage (~120 VAC +/-1%) for input (line) fluctuations from 132
> - 96 VAC. These work well for darkroom applications but they don't come cheap.
> You will need 250 - 500 VA of capacity which could run you $300 - $800.

Two different kinds of voltage stabilizers: Ferroresonant and
switched tap.  Ferroresonant stabilizers (Sola is the most well
known brand) react instantly and are as good as a UPS for short
transients.  Switched tap transformer-based stabilizers (Tripplite
computer stabilizers with the multi-LED panel on the front) use a
voltage sensing circuit and a series of relays to change the taps on
a conventional transformer.  They do a good job of stabilizing the
voltage over the long term but would work poorly with an enlarger
because the response time is slow.  This will let the lamp intensity
vary for a second or more.

Another solution, and a cheap one at that, is to fit the enlarger
with a 12 volt lamp and run it from a battery backed by a regulated
power supply.  Small halogen bulbs are available up to about 150
watts.  larger bulbs are available for 24 volts.  This is the
approach I used on my copy stand when I lived in Atlanta where Cobb
EMC was nice enough to chug the power several times a day.


From: John De Armond
Subject: Re: Voltage Stabilization Problem
Date: Wed, 15 Sep 1999 06:24:34 EDT

"Piotto, Bryan (EXCHANGE:CAR:CF11)" wrote:
> Neon John wrote:
> >
> > "Piotto, Bryan (EXCHANGE:CAR:CF11)" wrote:
> >
> > >
> > > Some Definitions:
> >
> > As long as we're defining things, let's get 'em right.
> >
> I fail to see where I conveyed incorrect information. The industry typically
> groups both types of supplies under the UPS category.

As an engineer who has designed UPS and standby systems, I'll assure
you that the terms are not used interchangeably.  If you ask for a
UPS expecting to get a SPS, you'll be quite surprised at the price,
size and weight of the supplied device.

> > This type would be applicable to enlarging provided: ...
> >
> No. They are totally inappropriate for darkroom work simply because they pass
> line voltage over a wide range. A 20% voltage variance from one enlarger
> exposure period to the next makes it very difficult to nail down a subtle tonal
> value.

Again, you know not what you speak.  While what you say is probably
true for the $79 standby supplies sold at Wal-mart, that is
certainly not the case for higher grade standby power supplies. 
Example.  The 1 KVA Emerson standby power supply that powers
critical instruments in my lab has an internally adjustable line
voltage deviation setpoint.  I have it set to 2%.  Anytime the line
voltage varies outside that range, the supply switches to battery
power until the line returns to normal.  This unit replaced the UPS
I used previously because it does the job close enough to the UPS
without the associated cost.  Because of the double conversion from
AC to DC to AC again, UPSs tend to be power hogs.  I should note
that this Emerson has the capability to operate as a UPS with the
addition of an external 24 volt power supply.  Cheap standby power
supplies cannot do this because of thermal management issues.


From: John De Armond
Subject: Re: Voltage Stabilization Problem
Date: Thu, 16 Sep 1999 05:06:50 EDT

John Coggins wrote:
> What equipment do you have that requires a 2% voltage tolerance? And
> what do you use when the batteries fail from overuse?

An HP rubidium frequency standard, a Measurements Systems signal
generator, and a Cushman service monitor are the three RF-related
instruments that are the most sensitive to line voltage.  The high
voltage power supply on my gamma spectroscopy system is also pretty
particular about line voltage.  

The two Delco Voyager deep discharge batteries that run this SPS
have been in service since 92 and are still trucking right along.  I
take very good care of them including keeping them clean and
administering quarterly load testing and equalization charges
(controlled overcharging to equalize the state of charge on each

The SPS has an event counter on it.  Over the long term, the thing
kicks in about 3 times a day. TVA power is pretty stable. The
transfer is all solid-state and the 60 hz oscillator in the SPS is
on and phase-locked to the powerline so the transfer is pretty much
a non-event other than the whine of the PWM 60 sine wave


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