Having enough power is at the core of having a reliable and responsive focus pulling experience. About half of the calls I get here at ARRI are from a user experiencing an issue related to power, so I am writing this article to offer some tips and tricks about how to properly power ARRI Lens Control Systems.
As you probably know, years ago we revolutionized lens motors and motor controllers with the intuitive LBUS daisy chaining system that we jointly developed with cmotion. This LBUS system provided bi-directional communication and power over a single cable, and can support multiple power supplies providing power at different voltages across the daisy-chain.
It’s safe to say LBUS has changed the way many focus pullers configure and build out their cameras. It allows for super compact builds with elegant cabling and the single LBUS port with daisy-chain support removes the need for multiple motor connectors on a camera body, resulting in smaller cameras.
So let’s get into it – the first thing to understand is the power draw of motors varies greatly based on the type of motor being used and of course also the torque setting on the motor. For example, the cforce plus motor has the ability to draw nearly twice as much power as the cforce mini motor when it’s set to torque level 4. That means that if you are driving x1 cforce plus motor and x2 cforce mini motors it’s almost the same as running x4 cforce mini motors. As a more extreme example, if you are running x3 cforce plus motors the system is capable of using as much power as x5 cforce mini motors! As you can see, these setups vary greatly when it comes to power consumption so you can’t expect to use the exact same power configuration all the time. The cforce mini RF motor uses a bit more power than the cforce mini motor due to its built-in radio module.
The ALEXA Mini was designed to provide enough power for x3 cforce mini motors at max torque through the LBUS port on the new PL mount. However, soon after the introduction of the ALEXA Mini (and its unexpected success as an A-Camera!) we had customers requesting a more powerful motor, so we delivered with the cforce plus. But with that bigger motor and more power draw potential we began to advise customers that they needed to not only power their motors from the lens mount side, but also from the end of the motor chain via the free LBUS port on the last motor.
For this purpose we offer an incredible variety of powering cables for the LBUS ecosystem. Let’s start off with the basics: we have an LBUS to RS cable 0.8M/2.6ft (K2.007659) and two different LBUS to D-Tap cables: a 0.8M/2.6Ft (K2.0006758) and 1.2M/4ft (K2.0006757). We also have an LBUS to XLR-4 pin solution (K2.0006760) for when you are working with block-type batteries.
Newer to our cable offerings are two products that allow you to inject power into the middle of an LBUS daisy-chain, instead of at the end. These are great for reducing the amount of cabling in your rig or if you are working with an LBUS device that does not have a second LBUS port for adding extra power, like the cmotion pan-bar zoom. These new Y-shaped cables are known as LBUS Star cables.
The LBUS Star (K2.0023823) features two female LBUS connectors and a male RS cable. It’s designed to plug into a power source, such one of the many RS sockets on an ALEXA LF, and then connect to standard LBUS cables which plug into your motors.
The LBUS Star 2 (K2.0039875) features two male LBUS connectors which you can use to daisy chain two lens motors, and then a female RS connector for injecting power. The female RS socket allows you to use one of the many RS-IN power cables that we have – you might be familiar with them from the UMC-4 or our Wireless Video System. This Star 2 cable is particularly useful for the ALEXA LF if you’re running cforce motors via the LCS to LBUS cable, as the LCS interface cannot provide enough power on its own, and you can make a nice loom with the LCS to LBUS cable and the Star 2 + RS – RS/PWR IN cable (K2.0001637) as I’ve done here:
Here’s a diagram with an example of how you could expand on this setup with the LBUS Star 2 cable:
Another consideration when using a motor controller like the UMC-4 and cforce motors via the LCS to LBUS cable (K2.0007318) is that the pan bar zoom would normally make it impossible to add extra power to the system, as it only has one LBUS port. The LBUS Star cable fixes this issue and allows you to inject the necessary extra power.
Now one thing I want to mention about using genuine ARRI cables is that you will notice that these genuine cables have two ferrite cores, one on each end of the cable. This is to prevent electromagnetic interference being introduced into the system by sources such as a very strong radio signal or cables lying on top of one another, so please only use ARRI cables.
Has a cforce motor suddenly requested a calibration even though “nothing” seems to have changed? Well chances are the system has experienced a voltage drop below the minimum voltage required by LBUS devices: 12V. This voltage drop would reset the motor to its “power up” state and trigger a request for calibration. We find this often occurs when an onboard battery drops below 13.2V, and is one of the reason the default voltage warning for ALEXA Mini and Mini LF is set to 13.5V. Yes, our cameras can run at lower voltages than that but if the camera is only getting 13.2V the power it can provide to accessories like LBUS devices is diminished, especially LBUS devices further down the daisy chain. It’s better for the batteries too if you don’t let them run completely out. Even better still would be running the system on 24V as the current requirements are halved. Bring on B-Mount!
Hopefully these insights will help you ensure you system is “juiced up” with the power it needs!
Let us know in the comments if you’ve got any other tips about powering the LBUS system.