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Losing The Cord: Converting From AC To DC

Having a long-running show like the one for which I work, you’re constantly looking to improve equipment and processes. For us, changing large, flying scenic pieces from wired to wireless is one of those improvements. Over the years, I have, with the help of others, redesigned several scenic pieces to DC power and wireless DMX.

One great advantage to making an AC powered device DC is managing and reducing logistics in scene changes. Having to patch and unpatch set pieces during a busy scene change goes away. Another is that flying pieces are no longer hindered by power cables that need to be paged in and out with the item as it moves about.

One item we redesigned years ago required an electrician to drop a socapex end from the grid into the basement before the start of the act. Since we are in the round, this was done in full view of the entire audience.

An electrician in the basement then needed to receive the soca, patch it into the set piece, and test it while the riggers were connecting the clew plate to the piece—all in a few seconds. Making the piece wireless eliminated many of these steps as well as the ugly cable.

Here are some things to look for when redesigning a large prop or set piece. When redesigning something from AC to DC, one of the biggest things to be aware of is your current consumption. This will determine the capacity needed from your batteries. Be aware that if you are converting from a 120 volt AC system to a 12V DC system, your current is going to increase significantly. In a recent retrofit, we counted up 298 lamps at 4.2 watts each. These were 12V lamps powered by step-down transformers. The load drawn from the dimmers, the primary (120V) side of the transformer, was a little over 10 amps. On the secondary (12V) side of the transformer, the current was over 100A. Some quick math using the power law (Watts = Volts x Amps) will help you figure this out. Don’t underestimate what your load is going to be when dropping your voltage down to DC levels!

Once you’ve figured out the load requirements, you need to ask, for how long does the piece need to run? Batteries are rated by amp/hours. This is an indicator of how long a battery can supply power at a given load. A 7A/h battery, for example, can supply 7A of current for 1 hour. That is when the battery is new. Give yourself some wiggle room when specifying batteries. In our current project, we decided to go with three deep cycle marine batteries each rated at 55A/h. That should be able to power the 100A piece for over an hour, which is much longer than the act for which it is used. We’ve broken the load into seven controllable circuits. The loads are balanced and distributed between the three batteries. The reason we decided to keep three separate power systems was that, in case of battery failure, we would still have 2/3 of the piece working. Also, cabling wire gauges can remain manageable when you break the loads up.

AC devices get a little tricky when you try to convert them to wireless. You’ll need to add an inverter in between the battery and the device to turn the DC back into AC. Inverters are rated by wattage so be sure to get one with the capacity you need to power whatever devices you need. Also remember that if you are inverting up from 12V DC to 120V AC, your load on the 12V side will increase. That 3A tv monitor at 120V will draw 30A on the 12V side of the inverter.

There are times when a UPS can be deployed in a wireless application. The UPS works as the inverter and will automatically switch from AC to DC when moving the piece from storage into its set position. The UPS does this without interrupting the power going to the devices. This is great for those times you are using equipment like computers that could be corrupted if they suddenly lost power without a proper shutdown. Often, the UPS will need to be supplemented with larger capacity batteries. Your average $500 UPS may only give you a few minutes of power as they are designed for giving a computer time to shutdown gracefully in the event of a power failure. Be sure you know what the UPS battery voltage is. Often UPS units will use 24, 36, or even 72 volts on the DC side of the circuit.

Once you change the battery capacity of the UPS, you’ll also have to specify and use a larger battery charging circuit. The charger built into the UPS will not have the ability to charge these larger batteries. Also, when using a UPS, make sure it has the ability to handle the current load your piece needs.

Finally, there is the question of control. How are we going to control our set piece? This could be as simple as a manual switch controlled by a performer, relays actuated by a wireless remote device, or 12V dimmers controlled by some kind of wireless DMX setup.

There are many wireless DMX systems out there, and they all have their positive and negative aspects. If possible, try to get a rental or demo setup in your room to see how well the system works. I’ve had plenty of conversations with other techs over the years who swear by equipment that didn’t work well for me and hate equipment that I have found flawless. There are a lot of factors when dealing with anything wireless, and commiting to a particular system can lock you in. My best advice is try before you buy!

Changing a powered set piece or prop to wireless can have many advantages. It could help smooth out scene changes and get rid of unsightly electrical cable. Just remember to check your loads and design your power system accordingly so that you know you will have the power you need on stage and not get left in the dark.