Most frequent questions about Ratherboard:
Dear Ratherboard Community,
You have already sent us several messages on facebook with questions, possible use cases, suggestions and kind wishes, thank You for that!
We are now releasing a User Manual, you may download it from here: RB User Manual
The output current of the switching power supplies
Q: “The power requirement of the pi3 is quoted at 2.4A and it’s always better to run a power supply below it’s maximum output. How can the RB fulfill this requirement?”
A: The switching power supplies of the Ratherboard are designed using a 3.5A IC, with the other components also sized above 3Amps. The multifuse on the DC input has a holding current of 3Amps.
The video narration (in the main board’s intro video) calculates with worst cases. With only 7VDC input, and 80% efficiency the output power of the 5V and 3.3V PS should not exceed 16,8W, which means 2Amps on both.
With 12VDC input RB fulfills the 2.5A requirement easily.
Q: “How does the Ratherboard deal with possible failures of the input power?”
A: We have defined future extension board types with built-in LiPo battery charger and protection circuit. The battery enables uninterruptible operation, or provides supply voltage for the RPi to perform a proper shutdown.
Alternatively the user can install a supercapacitor or a battery based circuit (on the prototype area on one of the expansion boards) which can store enough charge until the Raspberry Pi shuts down properly. You can make your unique circuits on the prototyping area.
In one of our applications we use an external 12V battery.
Why we have chosen to use the Model B versions?
Q: “Great idea, but as it’s designed for industrial applications, wouldn’t using the compute module be a better fit for this?”
A: RB was specially designed for the Model B versions, our goal was to create a useful device for their users. The future of Ratherboard depends on the success of the RB community and the crowdfunding campaign.
Powering the RPi using it’s USB-connector
Q: “Why don’t you use the pin header 5V pin instead of the USB connector?”
A: We used the USB-cable-solution because the Raspberry Pi has built-in protection on it’s usb port but it hasn’t on the pin-header. Reading all of the feedbacks we decided to provide a jumper option for the users in the production version. This way everybody will have the option for using the 5V pin instead of the cable.
The thermal conductivity of Ratherboard’s enclosure
Lots of You have asked about Ratherboard’s thermal performance, so the question is:
Q: “Will the Raspberry Pi will overheat inside Ratherboard if it is exposed to direct sunlight?”
A: The short answer is that we did not experience overheating problems in our rooftop weather station application during last summer (despite the fact that in Budapest 30-35°C temperature during the day in the shadow is common).
The long answer is quite complicated, let’s see it below!
We have done a crude measurement to estimate the thermal conductivity of Ratherboard’s enclosure. We put a power resistor and an NTC temperature sensor inside the enclosure. These devices were connected to Ratherboard’s external connector header. The power resistor was connected to a bench power supply. The NTC resistance was measured with a multimeter. The ambient temperature was also measured. We noted down the measured values in every 5 minutes.
During the measurement the resistor dissipated 4.2W. After 55-65 minutes the internal temperature became constant (34°C), while the ambient temperature was 23°C. The temperature difference was 11°C and the thermal resistance was 2.63K/W.
After the first measurement we increased the voltage and the resistor dissipated 7.56W. After 55-65 minutes the internal temperature was constant again (42.3°C), while the ambient temperature remained at 23°C which gives a thermal resistance of 2.56K/W.
According to this measurement the thermal resistance of the enclosure is approximately 2.6K/W, which means that if you dissipate 1W inside the enclosure then the internal air temperature will be 2.6°C more than the ambient temperature.
Let’s calculate some more details!
According to the official Raspberry Pi FAQ the Raspberry Pi 3 model B consumes 1.2A at 5V (worst case scenario with lots of external USB devices connected), which gives 6W. Ratherboard’s other components power consumption is negligible compared to this value.
If we assume that Ratherboard’s switching power supply has 80% efficiency, then the total dissipated power inside the enclosure is 7.5W, which means that the temperature will be 20°C more than the ambient temperature.
Let’s say that the ambient temperature is 50°C because of direct sunlight, than the air temperature will be 70°C inside Ratherboard’s enclosure. According to the official Raspberry Pi FAQ the LAN controller is qualified to maximum 70°C temperature. The chip will have higher temperature than the air around it, so in this scenario the chip will probably have higher temperature than the qualification maximum value.
The above example showed that if the Raspberry Pi works very hard and lots of peripherals are connected to it then you should consider to install heatsinks on the Raspberry Pi and install Ratherboard in a shady area. Also note that the Raspberry Pi 3 model B typical bare-board current consumption is less than 500mA, which means only 3.2W dissipation, which means 8.3°C temperature difference.
So Ratherboard’s actual temperature performance is application dependent. For example if you have direct sunlight but lots of airflow then the heat transfer will be much better, hence the thermal resistance will be lower.
We did not experience overheating problems in our rooftop weather station application during last summer as mentioned above. The CPU load was not much (below 10%) and the Ratherboard consumed approximately 0.18A at 12V, which gives 2.2W dissipation. This value is the third of the worst case value (7.5W). We did not installed heatsinks on the Raspberry Pi 3 model B.
Thermal behavior has to be investigated in every electronics design, a Ratherboard application is no exception.