Univolt
II & WFCO Converter size comparison
I
like Airstream’s original equipment Univolt. But I also appear to like doing things
the hard way like lighting pilot lights & winding clocks. Sure, the modern day power converters do
a fine job of powering 12 volt circuits & keeping the battery properly
charged. But I enjoy having a
“say” as to when & how much my battery is charged. And the Univolt’s hum is not THAT
loud…
My
Univolt was conveniently located under the bathroom sink. While reinstalling it during the refurb, the opportunity was taken to install a simple,
external ON/OFF switch. When
camping on shore power, the Univolt was turned off unless an electric motor was
running, or a lot of lights were on.
Between the switch, and the original analog battery minder, I never had
any battery problems.
But
alas, after our first two seasons in the Airstream, I found that my wife did
not share my enthusiasm for constantly monitoring the electrical system. So, for Christmas, I asked for, and
received a WFCO, 35 amp, Converter/Charger from bestconverter.com to replace
the Univolt. Charging system topics
include:
How the New Power Supply/Battery Charger Was Chosen
While I did not mind keeping an eye on my Univolt, my
mind was made up long ago that if or when it was replaced, the new unit would
be one capable of operating unattended by anyone (even me). Face it, batteries & battery
chargers have been around a long time, and keeping a battery both charged, and healthy should have been a hurdle
cleared long ago. Not necessarily
in 1967, but certainly within the last decade since microprocessor controlled
circuits are in just about everything nowadays.
Casually
monitoring news groups during the refurbishment, the Intellipower with Charge Wizard appeared to be the best choice. I was, however, surprised that it still
had an override button if operator intervention was desired. Randy, the proprietor of bestconverter.com,
is a contributing member at Airstream Forums, and judging by his answers to a
variety of 12 vdc questions, appeared to be quite familiar with the goals of
successful, self-sufficient trailer operation.
Around
the time Univolt replacement was seriously considered, Randy started a thread
recommending the WFCO Converter/Charger
explaining that, although the Intellipower is a fine unit & he would still continue to
carry it, the WFCO unit was proving to be a better choice. So I hit his site, studied the two
units’ operations, and concluded for myself that the WFCO unit was indeed
a better choice.
But
which size? My Univolt was rated to
deliver up to 30 amps. The WFCO
unit is sold in five categories ranging from 35 to 75 amps output current. Is more always better? Making the small jump from my 30 amp
unit to the closest equivalent unit offered was no big deal. But, since I had the opportunity to
dramatically upgrade, would more output current do anything good for me? What would more power hurt?
My
Overlander’s DC power grid, from it’s single battery forward,
consists of 8 AWG wire running through 40 amp fuses powering three, 20 amp
circuits protected by self-resetting breakers. The Univolt, as it was in a remote
location, was connected to the system (between the battery fuses & circuit
breakers) by approximately 15 feet of 8 AWG wire.
Good
design dictates that branch circuits should not pull in excess 85% of their
rated capability. Applying this to
what I saw means that the distribution grid is capable of safely handling 51
amps. With the Univolt delivering
30 amps, the battery makes up the difference if/when more power is needed.
So was the Univolt meeting my present power needs? How much power was projected to
be needed? At a campground with
electrical hookups (a must for us at present), we seldom have more than a
couple of light bulbs on, and prefer to run the AC powered air conditioner in
lieu of vent fans. While the TV/VCR
combo & DVD player will run on 12 vdc, normal operation finds them plugged
in to AC.
The
AM/FM radio, sound system, and CB radio are hardwired into the DC grid, while
the cell phone is plugged in as required.
None of these devices pull that much current though. A laptop computer may one day make
it’s way to the kitchen table, but neither it nor possible, future
rechargeable kid’s toys will pull significant power. All-in-all, the amount of DC power used
on a typical camping has been well within the means of the 30 amp Univolt, and
I could not see an appreciable increase happening anytime soon.
A
high output charger would have it’s benefits charging a bank of batteries
after boondocking. At present, though, we do no boondocking. In
fact, the primary reason I have a battery in the Overlander at all is to power
the brakes in case of breakaway.
Other than running the water pump occasionally while on the way to a
campground when a public restroom is not convenient, the battery is not really
needed. While the battery did
supply filtering for the Univolt, a bank of capacitors did the same thing for
me during the refurb.
There is a realistic possibility,
though, of the Boyz & me boondocking in the
future, and the thought of upgrading for this occasion crossed my mind. Although the present wiring between the
charger & distribution point was not rated for the task, it could be
upgraded to carry more current.
But
I am uncomfortable with letting one big charger maintain two or more
batteries. Charging is fine,
but unless the two or more batteries are in the exact same condition, I do not
feel the charger’s microprocessor will necessarily be able to determine
the best maintenance voltage to deliver to more than one battery. An example would be two batteries wired
in parallel, and one cell of one battery shorts out. Since this would lower the overall
voltage output, the microprocessor would think the batteries need charging, and
boost it’s output. Unless
detected soon enough, damage to the good battery could result.
Let’s
say the charger could act accordingly hooked to multiple batteries. My Overlander carries it’s one
battery inside the trailer underneath the street-side bed. Although there is room for another
battery, for a variety of reasons there are no plans to fill that available
space with a battery. The plan will
be to carry an additional battery(ies) in the tow
vehicle, and connect them through the trailer’s electrical
umbilical. When not camping, the
battery would sit in my shop alongside the batteries for my other toys, and be
maintained with a trickle charger.
With this scheme a bigger charger would not benefit my camping
activities.
But
to make sure nothing had been overlooked, I contacted Randy and outlined the
above. He agreed that the 35 amp
WFCO unit was adequate for my present needs. With his confirmation of suitability,
the WFCO model WF-9835 was asked for, and received as a Christmas gift from my
in-laws.
In
early Overlander production runs, Airstream had the Univolt mounted near the
battery. Probably in response to
customer complaints about hum while trying to sleep, it was moved to underneath
the bathroom sink. Due to the
Univolt’s size & dead-space-for-ventilation requirements, it pretty
much dominated the space as far as storing anything like towels.
Since
the WFCO unit is extremely quiet, thought was given to locating the WFCO unit
next to the battery, but a warning label on it’s cover advised of
internal components that could spark on occasion – Not a good thing if
the battery was busy off-gassing hydrogen.
Deciding
to keep the new charger under the bathroom sink, the only modification
necessary was to add a 120 vac outlet.
The Univolt had been hard-wired whereas the WFCO unit came with a cord
attached. No big deal there other
than purchasing a Cu/Al receptacle
to work with my Overlander’s aluminum wiring. The small size & fan-cooled
operation of the new unit allowed for much better placement, and freed space
for storage of bathroom items.
Worth mentioning is how much lighter the WFCO unit is in that there is
now less cantilevered weight to stress the trailer aft of the axles.
So
far so good; The WFCO charger appears
to be working as advertised. This
page will be updated should anything change. [back to top]
A
Univolt is an amazingly simple device: A big transformer to step down the
voltage, diodes to convert AC to DC, and depending on the version, either
capacitors to tune the circuit or a Silicon Controlled Rectifier to…do
something. The Univolt schematics I
have seen do not regulate the voltage they deliver. They were designed to
output one single voltage with as much current as available.
My
Overlander came with a Univolt II
manufactured by Newmark Products Inc. The left-hand schematic above is my
interpretation of what was revealed after the case was cracked open. Not understanding what was in front of
me, the schematic was shared with three different electrical engineers, and
posted on a circuits forum. Nobody
seems to understand what the SCR is doing for the circuit especially since
neither D3 nor D4 are Zener diodes.
The “13.5 vdc @30 amps” declaration is what is stickered on
the case. Although all the
individual components tested good, i.e. the device is operating as designed,
the device’s output was more like 14.2 vdc even under load. If you have insight into the
circuit’s design goal or would like more as-measured values, please drop
me an email,
and I can provide more details.
[Update 01/23/10: Reader Cliff B advised
me, “FYI diode D-3 in your Univolt II (Sencor
LMZX-13A) is a 13V zener diode @5% tolerence it is regulating diode that sets the reference
voltage to SCR1 (D-4 is a reverse voltage protection diode for the SCR's
gate).”]
Newmark Products Inc. went out of business so
long ago that Google has never even heard of them. As a consequence, it is unknown to me
how long the Univolt II was used.
To be quite honest, the Univolt II’s circuit surprised me as,
after researching a lot of Airstream material, I was expecting to find the ferro-resonant circuit depicted above. The ferro-resonant
circuit is cool because it will continue to deliver full current, with no
adverse affect to the transformer, even
if the output terminals are shorted together. 
BTW, ferro-resonant
basically means there is a capacitor (C1) hung off a tap on the output windings
to tune the circuit. This type of
power supply can operate without C2 & C3 as they are there for noise
suppression.
Some
people appear to think that a battery must be present for the circuit to be
properly tuned. This is
incorrect. The battery serves to
filter, and smooth the Univolt’s output, but is not required for operation.
However, your 12 volt lights will be dim, and your radio will have a lot
of static in the absence of filtering & smoothing. During the winter refurb,
I used to keep a bank of old electrolytic capacitors out of my toy box wired up
to act as a “battery simulator”. The capacitors did all the filtering
& smoothing, and allowed me to keep the battery in plain view in my Shop so
I didn’t forget to check it every so often.
Update 01/11/13: Reader Art L wrote:
Tom,
I
own an early-80's-vintage Newmark
Model DCV 40 R-A-C Power Converter and originally ran across your page
a couple of years ago while looking for Newmark
service literature.
Although they look somewhat similar, my Newmark is
rather different. But your Univolt II was about as close as
anything I could find. Since then, I've had several failures and replaced both
transistors and the SCR series resistor but I never could find a schematic or
any component information.
I finally decided to reverse-engineer a schematic and identify the parts so I
could make sure it was operating correctly -- particularly since I had to
select replacements for the unmarked transistors! The good news is that it now
works great and smoothly proportions the battery charge right down to about 10
mA.
If you happen to have (or know the
whereabouts of) a converter like mine, that would be great since I could use
some help confirming the identity of several parts with missing and smudged
markings…
Art L
Art went on to include:
As
you can see in the photo, the project's not quite complete yet. One remaining
issue is the SCR resistor: this is one of the values I could learn from another
operating converter.
The old resistor burned out, taking the markings with it. As best I could
determine from measuring the burned-out pieces was 2 ohms. I ordered a 2 ohm
replacement before I started analyzing the circuit but that hardly sounds right
at this point. (2 ohms & 5 amps doesn't leave much
for the battery!)
The test resistor I used is 0.5 ohm and gave me a maximum charge of only 3.3 to
3.6 amps into a fully discharged battery. I have some 0.25 ohm resistors on
hand that should give something closer to the 5 amps the label specs.
During the charging process, I saw some oscillation in the gate signal. A
small, 0.1μF, capacitor upstream stopped it.
My
'67 owner's manual says that
the Univolt has a 100% shut off and that it is impossible to overcharge the
battery. I will give Airstream the benefit of the doubt, and assume that
some Univolts had a circuit breaker which would trip
to effect "100% shutoff". The Univolts I
have seen have no "smarts" in that they could care less if your battery
is boiling, simmering, or even there.
A Univolt’s job was to supply roughly 13.5 vdc with as much
current as possible. I put it that
way because, the voltage will drop as the load on the Univolt increases. If you are actively camping and have a bunch
of lights & fan motors going, the voltage difference between the battery
& the Univolt will be small. In fact, the battery may even help the Univolt
out if you have A LOT of stuff on.
A
battery is charged by applying a DC voltage to the battery that is higher than
the voltage the battery currently has. The higher the voltage difference, the
greater the charge rate will be. I believe a fully charged battery will read
12.9 vdc on a volt meter.
Charging
a battery is an electro-chemical reaction. As the battery charges, tiny bubbles
of hydrogen gas & water vapor form on the plates, and eventually bubble
off. These bubbles eventually escape from even sealed batteries. The higher the
charge voltage, the more bubbles that are formed. It is this bubbling that is
commonly referred to as boiling although it does not occur the same way as
water boiling in a tea kettle.
But,
with no lights or anything but the Univolt on, the battery will be subjected to
the maximum voltage the Univolt can deliver. It will attempt to charge. After a
while, the water will bubble off. [back to top]