Why Two Fuse Panels?

[dbj216]

The vintage Airstreams advertise "self containment". They were designed for 12v lighting, furnace blowers and water pumps.


115v circuits were added for luxury items like air conditioners and TVs. Airstream was one of the first to convert 115volts to 12volts so the same lights, furnace blower and water pump function the same no matter what the power source.


My 66 Trade Wind 12v wiring diagram in the manual shows the "univolt" having 3 wires from it, positive, negative and ground. It now has a 1999 Magneteck converter that has an internal fuse panel with the DC circuits coming from the converter to the original Airstream 12v fuse panel (which has three fuses, 4 circuit breakers, and 5 12v circuits to the rest of the trailer. These wires go to the original fuse panel.


Why? I assume the converter is a power source no different from the battery. EITHER converter OR battery. Two power sources for one 12v system.


New converters don't have a built in fuse panel. I'm told I need to add one. It appears I will have a 12v circuit protected at the converter with a fuse and then protected three feet later at the Airstream 12v fuse panel.


What is the design purpose of two fuse panels?


David

[wahoonc]

Originally Posted by dbj216

The vintage Airstreams advertise "self containment". They were designed for 12v lighting, furnace blowers and water pumps.


115v circuits were added for luxury items like air conditioners and TVs. Airstream was one of the first to convert 115volts to 12volts so the same lights, furnace blower and water pump function the same no matter what the power source.


My 66 Trade Wind 12v wiring diagram in the manual shows the "univolt" having 3 wires from it, positive, negative and ground. It now has a 1999 Magneteck converter that has an internal fuse panel with the DC circuits coming from the converter to the original Airstream 12v fuse panel (which has three fuses, 4 circuit breakers, and 5 12v circuits to the rest of the trailer. These wires go to the original fuse panel.


Why? I assume the converter is a power source no different from the battery. EITHER converter OR battery. Two power sources for one 12v system.


New converters don't have a built in fuse panel. I'm told I need to add one. It appears I will have a 12v circuit protected at the converter with a fuse and then protected three feet later at the Airstream 12v fuse panel.


What is the design purpose of two fuse panels?


David

Right there is part of the answer. Univolts HAD to have a battery in the circuit to operate properly. FWIW until I worked on an Airstream the only other place I have ever seen a fuse on BOTH sides of a DC circuit had been in a power plant control system.

Aaron

[HowieE]

Not sure where you got that diagram. The convertor and the battery should both be on the same side of the fuses leading to the loads.

It is not the battery OR the convertor. It is the battery AND the convertor.

The convertor is not meant to be a standalone source but rather a charging source for the batteries. Think of it as the gas pump at the gas station and the gas tank in your car. You would not pore gas directly into the carburetor from the gas pump if you were parked at the station.

[dbj216]

Hi Howie,


The diagram I attached to my post came from the 1966 Airstream Owners Manual. The manual states it is applicable to various size trailers. One manual fits all!


I often run my converter without my battery. It delivers about 13 volts to the system by my voltmeter. Some folks add a switch to the battery so they can disconnect while on long term shore power so the battery doesn't get overcharged.


I think of the converter as more than just a battery charger. I just don't know what the intent of the converter is in modern campers, but I think of it as a power source, not a battery charger.


I'm better at mechanical and hydraulic things than electrics. So I am trying to understand so I can wire my new converter properly. I read in this forum about Airstreamers having difficulty wiring their new converters. I figure if I understand how it works I will install the new one with less difficulty.


Thanks,


David

[GeocamperAS]

If I understand it correctly,
#1. The converter/charger does both converts 120ac down to 12dc to run all your 12v stuff as well as charges the battery. A good converter/charger will step down the charge to a trickle charge to maintain the battery so you would not have to worry about boiling them if you don't want to switch the off.
#2. The converters built-in fuses act as a main to protect the converter, while your external fuses are for the individual circuits. Much like your home breaker box has a main breaker with individual breakers.

[dbj216]

I learned a little bit more today. I got my trusty voltmeter and went on a exploratory trip around the fuse panel.


There are three wires coming from my old Magnetek converter: Red (battery charge voltage), blue (powers 12v circuits) and white (ground). I also have the original Airstream "battery tester" and it is connected to the converter with 18ga wires.


The converter must have two "taps" off its transformer. One is 14 volts (red wire) for charging the battery, and the second is 12 volts (blue wire) for powering the 12v circuits inside the trailer.


The old Airstream battery tester has a scale that says "green = good battery voltage" and more green = "battery charging". I never noticed the little "charging" letters under the far right green bar. Makes sense. This "battery tester" is nothing more than a voltmeter with a green bar scale, 12 volts for good battery, 14 volts for charging voltage.


So we can power our Airstream with EITHER the converter OR the battery. Both are legit 12v power sources. The battery can't push 12 volts out when it is receiving 14 volts in. So it suckles away on 14 volts until it's cooked. (Old fashioned converter technology.) I've been working on my old Trade Wind all winter with shore power to the converter and the battery in the basement.


I better understand the Airstream wiring diagram in my OP above. If I look closely, I see the converter red wire is connected down stream of the battery fuse. Follow the red wire from the battery, then follow the red wire from the converter. This is the "ah ha" moment for me. The battery pushes down with 12v, and the converter pushes up with 14v.


I have not yet taken my old converter apart. I do not know if there is a fuse panel inside. If there is, then it protects the red and green wire from overload due to converter transformer malfunction. Don't want a fire in the two wires before the trailer fuse panel (which is only 4 feet away).


So I feel a little more knowledgeable. I will not have to rewire the Airstream fuse panel. I'll see what the new converter installation instructions say when it comes.


Please correct me if my converter understanding is wrong.


David

[Kyle401]

You have fairly well described the operation principles of your Magnetek. I have attached a block diagram and schematic of the Magnetek converter. The 3200 series can function as a 12V power supply without a battery in the circuit.

The only internal fuse is a AC slow blow fuse shown as F1 on the schematic. If it blows your converter won't work, but you will still have 12V if the battery is charged. Since yours is working, the fuse is good.

If your converter was installed similar to the schematic in your first post, you won't need an additional fuse panel. I would expect that you have a few more 12V circuits since your trailer is 8 years newer than the '66 schematic.

[dbj216]

Thanks for the information on the Magnetek Series 3200 converter. My new Power Max Boondocker converter is here. It did not include a comprehensive installation instruction. I think the two positive wires out of the old style converters confuses folks like me. My new converter only has a red positive and ground wire. The electronics inside the new converter take care of safely charging the battery while supplying the 12v circuits. I don't think I can run shore power to 12v without the battery in the trailer anymore.


I think a lot of confusion would go away if Power Max provided a good installation diagram and some description about the extra positive wire coming out of the old style converters.


David

[Kyle401]

The Powermax certainly doesn't come with comprehensive documentation. On the other hand, it would be hard for them to predict what every installation may require.

The Magnetek required separate circuits for the 12V load and the battery since the load circuit was not controlled to prevent rapid overcharging of the battery. Since your new converter doesn't have this limitation, you can connect both your positive lines to the positive output from the converter. You will want to ensure that they are properly fused.

The Powermax should be able to function as a 12V power supply without a battery in the circuit.

[Kyle401]

This thread has some pictures of a similar install. The Progressive Dynamics and your Boondocker should install the same way.

[dbj216]

I purchased this fuse panel from VTS thinking it would be an upgrade from the old Airstream "home built" one. The literature says it will handle up to 9 12v circuits and has 3 positive side connector lugs. It is small, but so is my new Power Max Boondocker. My old Trade Wind has 3 12v circuits.

Boy, did I have a hard time installing this fuse panel. 12v circuits are 10 gauge, and the battery wires are 8 gauge. These are hefty wire sizes for high currents, low voltage circuits. Along with the three 12v circuits, I have the converter positive and negative wires, the battery positive and negative wires, the two vehicle charging wire, and the trailer chassis ground wire.

I struggled to get the wires in the enclosure. I struggled to get the terminal screws tight. The screws have a square drive with a slot cut across the top. They won't handle the torque to securely clamp a 10 gauge wire.

There were only 4 ground terminals in the negative buss. Four! How in the heck is a guy going to jam two 10 gauge wires into one of these lugs and clamp them both tight? Again, the screws were not up to the task at hand in my view.

Then the three positive lugs have a socket head set screw. This is a better choice, but back to the tool box for more tools. I think all the fasteners should be the same specification.

The cover does not have a pull handle on it. A new guy will spend some time figuring out how to get the fuse panel door open. And the edges of the panel door are sharp from the shearing process. Open the door, cut your finger.

I was less than impressed with this fuse panel. I'm no electrician, so I'm looking for advice on where I might be wrong.

Thanks,

David

Pictures of the old and the new below.