Powering your rig with USB-Power Delivery
Most astronomical equipment, such as cameras, mounts, focusers etc. require 12VDC to operate. This usually means the use of a collection of power bricks and/or wall adapters. When in the field, the use of power stations is often the way to go. Most of them have one or more 12VDC outputs. An extra voltage regulator may be required if such outputs are not regulated. The different cable management needs for both scenarios requires special attention when switching between them.
When putting together a portable travel rig, I was wondering if the use of USB Power Delivery (USB-PD) would simplify things. And spoiler alert, yes, it did. But navigating the USB-PD ecosystem was not so easy. This blog describes my findings in as far as they are relevant to the astrophotographer. Anyone who wants to explore the USB-PD path as well, may find some useful tips below to help in that journey.
What is USB Power Delivery?
USB is a well known standard for data exchange. For power, USB offered limited options until recently. The maximum power that could be delivered was 7.5W. Therefore only small devices could be ‘ bus-powered’. In astrophotography, planetary cameras and smaller light-panels are typical bus-powered devices.
But with the introduction of the USB-C connector, this all changed. The port was designed to support significantly more power. The standard by which that power was made available became known as USB Power Delivery, or USB-PD. USB-PD standard version 3.0 supports powers of up to 100W. Version 3.1 (2021) takes that even further and includes an extended power range up to 240W. USB-PD made it possible that most laptops these days are powered through standardised USB-PD instead of some sort of proprietary DC-adapter.
Unfortunately not every piece of equipment with a USB-C port can just draw the full 240W from a 240W charger. The power that is actually delivered depends on voltage and current being used. These are specifically defined in the USB-PD standard. The below table shows the various options available.
| Current | Voltage | Power rating | |
|---|---|---|---|
| Standard Power Range | 3A | 5V, 9V, 15V, 20V | 15-60W |
| 5A | 20V | >60W-100W | |
| Extended Power Range | 3A | 5V, 9V, 15V, 20V | 15-60W |
| 5A | 20V | >60W-100W | |
| 5A | 20V, 28V, 36V, 48V | >100W-240W |
So with that many options, which one will be used in any given case? That is determined in a process called ‘negotiating’. Negotiating takes place between the device that delivers the power and the device that uses the power, as soon as the cable is plugged in. Together the electronics on each side will determine which power level will be used for that connection. This does takes into account potential other connections as well that may draw from the same power source already.
Only USB-C ports labeled USB-PD will be able to do this. Regular USB-C ports don’t have the necessary electronics on board. If one side of the connection involves a regular USB-C port, no Power Delivery protocols will be engaged and power supply will default back to the standard 5V, 1.5A or 7.5W power. Also the cables need to be able to support USB-PD. A simple data cable with USB-C connectors on both ends will not automatically be able to use USB-PD.
USB Power Delivery to regular DC output
Unfortunately almost no astro-gear is equipped with a USB Power Delivery port. Instead, most utilise standard 12VDC input. And a DC input cannot negotiate with a USB-PD connector. Fortunately there is a solution to this problem. And that is called a trigger board.
A trigger board is plugged into the USB-PD port and has a regular two-pin DC output on the other end, to which wires can be soldered. The trigger board does the negotiations with the power source. Often the firmware on such trigger boards will allow some user input, such as manually selecting which voltage to select. Working with these open electronics boards may not be very practical for a telescope rig. But luckily there are cables that have a trigger board integrated in the cable itself. They are referred to as trigger power cables, or trigger cables.
A trigger cable has a USB-C connector on one end and a regular DC plug on the other end. The voltage output of these cables is typically fixed, so when ordering make sure to order the one that has the required voltage output. Also maximum currents and DC-plug dimensions will vary, so make sure you get the cable that fits your needs.
A trigger board like this negotiates with USB-PD and outputs the selected VDC.
A trigger cable has a USB-C connector on one end and a DC-plug delivering a fixed voltage on the other end. Electronics to negotiate the correct voltage is embedded in the cable.
Where is the 12V?
The USB-PD standard supports 5V, 9V, 15V and 20V power supply. But in astrophotography we often need 12V, so what happened with the 12V? Well, the standard does not require 12V. But manufacturers can opt to include 12V as one of the options. Some do, some don’t. This is something to look out for. Unfortunately many manufacturers don’t make it easy to check. Advertised numbers are typically in Watts. But what you want to look for is the list of voltages supported for a given PD output. This can be written on the device, or mentioned in the documentation. But some manufacturers don’t publish this information at all. The images below show the USB-PD specifications written on an Anker power bank (left) and a Satechi charger (right). Clearly the Anker power bank does not include a 12V option and can not be used to power our 12V equipment. While this is a specification of the device, my impression is that the in-/exclusion of 12V as an option is brand specific. Anker and Apple chargers and powerbanks typically don’t support 12V. While Satechi and UGreen chargers do support 12V. So far the only power bank that I have come across that supports 12V is the GoalZero Sherpa 100PD.
If the power source is not supporting 12V, plugging in a 12V trigger cable will negotiate to the next voltage level down from 12V. In most cases this is an output of 9V.
The USB-PD outputs on the Anker 737 power bank do not offer an option to output 12VDC
All four USB-PD outputs on this Satechi charger support an option for 12V/3A DC.
Rigging up using USB-PD
The rig that I wanted to try this on is a portable travel rig. The equipment on the OTA is powered via a PegasusAstro Powerbox Advance (PPBA), which requires 12V input. The mount is a RainbowAstro RST-135E strain wave drive. It requires 12-16V power. 12V for the mount can be delivered through the PPBA. But the mount can be quite sensitive to slight voltage drops below 12V, and some have reported better consistency when powering with 15V. The computer that runs it all is a Fitlet2 fanless miniPC. It has a very flexible 7-20V input power requirement. When out in the field I typically use a Ubiquiti Amplifi Router to create a local network. This router requires 9V input voltage delivered through a USB-C connection.
As power source the Satechi 165W USB-C 4-port PD GAN Charger was selected. This charger utilises the newer Gallium Nitride (GaN) technology to pack a lot of power into a relatively small package. Like all chargers, the maximum power of each connector changes depending on how many ports are in use. But even when using all four ports, the first two will still deliver 60W and 45W, more than enough for the maximum 12V at 3A (36W) we will be pulling from it.
The Satechi 165W USB-C 4-port PD GaN charger packs a lot of power into a small package. More than enough to power the whole rig.
The UBS-PD charger is connected to the tripod using a custom-made 3D-printed bracket, for tidy cable management.
For the Trigger Cables, three aspects need to be taken into account: output voltage, maximum current and dimensions of the DC-plug. On Amazon there is a wide selection of these cables available, but filtering on these three aspects is not always easy. After some exploring, I settled on the following three cables:
PPBA: SZRMCC 12VDC 90W/7.5A DC 5.5x2.5mm
RST-135E: WITRN 15VDC 75W/5A DC 5.5x2.5mm
Fitlet2: WITRN 15VDC 75W/5A DC 5.5x2.5mm
The PPBA requires a DC 5.5 x 2.1mm plug, a problem which was solved using an adapter. The Amplifi router was connected using a regular USB-PD cable. The Satechi PD charger was fitted to the tripod using a custom 3D-printed bracket to help with the cable management.
The results
Using the charger and trigger cables mentioned above, all equipment could be powered through USB-PD. The only cable left to deal with is the mains cable of the Satechi charger. When mains is available, the AC cable just plugs into a regular mains socket. For field use, a GoalZero Yeti 400 lithium has been my go-to power station. It has an inverter on board delivering 300W, 220V AC power. So the AC cable from the charger was just plugged into the 220V port of the Yeti 400. Using the 220V output of a power station is not the most power efficient solution if you only need 12V. The up- and down regulation of the voltage causes energy losses. Whether this is an issue or not will depend on the overall energy use in relation to the capacity of the Yeti 400. So let’s turn everything on and see how it works out.
First of all there is the PPBA. Camera cooling and dew heaters can draw quite some energy, and USB-PD at 12V is limited to 3A. With camera cooling and dew heaters on at 50%, the total current drawn fluctuated around 2A. That is well within the 3A limits of USB-PD and leaves some room to crank up cooling or dew heating if needed.
The Fitlet2 and mount only draw around 0.6A and 0.3A respectively, so well within all limits and no concern from any perspective. Using the 15V trigger cable gives a little bit more peace of mind that the power supply to the mount will be sufficient under all circumstances. The Amplifi has prove to be very tricky and does not work with many powerbanks, but powering through the Satechi worked well.
With the whole rig connected and turned on, the total power use was 50W. The total capacity of the Yeti is 400Wh. So the Yeti would have enough juice to support the rig for about 8 hours. For a regular field session, this is more than enough. In case more observation time is required, additional ~100Wh power banks could be brought along to have a bit more juice available. Also the Yeti could be charged from the car while connected to the rig. If sessions need to be routinely >8h, a bigger power station is also an option. The successor of the Yeti 400 lithium is the Yeti 500x, which is smaller, lighter and packs 25% more energy at 500Wh.
Overall the conclusion here is that powering the rig completely via USB-PD is possible and makes for a very simple and tidy cabling of the rig.
Compared to the regular option.
So the USB-PD setup worked and seemed like an elegant compact solution. But how would this compare to the regular setup with 12V outputs? Below are images of the same rig powered using the previous 12V-based solution and the new USB-PD solution.
To minimise wiring, the mount is now powered trough the PPBA. The Fitlet2 is powered through a second 12VDC cable plugged into the cigarette plug connector. Because the 12VDC output of the Yeti 400 is not regulated, a voltage regulator is included in the system. Powering the Amplifi is not possible with 12VDC, so a separate power bank is needed for that.
The total power drawn from the Yeti in this setup was 34W, significantly less than the USB-PD solution. Part of the difference (3.5W) is due to the Amplifi now having its own power bank. The rest of the difference (a little more than 10W) is energy lost because of the up- and down-regulation via 220VAC. At 34W, the total time to use this rig in the field is almost 12h, significantly more than the USB-PD option. When using this setup in the backyard where mains power is available, the best option is to keep the whole setup the same and keep the Yeti topped up during the session using the regular 60W power supply.
Traditional 12VDC powering, not connected to mains supply.
Traditional 12VDC powering, connected to mains supply.
USB-PD powering, not connected to mains supply.
USB-PD powering, connected to mains supply.
Conclusion
USB Power Delivery has great potential for astrophotography. Powering a full rig with USB-PD only is certainly possible. And when mains power is available, for a small rig like this, it is a simpler option overall. If the main use is in the field with very long sessions (>8h), the DC option is the more power efficient one.
Let’s hope that manufacturers will soon switch to USB-PD as a way to power their devices. But as long as that has not happened, using a USB-PD charger with several trigger cables forms an interesting alternative. Overall the setup is simpler and smaller, and switching between mains and battery use is easy. For field use, valuable battery life is lost when utilising the 110/220V AC output. But with 8h of use this may only be an issue in some situations.
For bigger rigs that may draw a lot more power, the 3A @ 12VDC limitation of the USB-PD standard may be an issue. But for any rig that remains within that 3A limit, using USB-PD is a viable alternative to regular 12VDC power sources.