This article discusses environmental and design factors that can impact the thermal performance of OEM USB DAQ devices and helps you to ensure thermal limits are not breached.
“OEM USB DAQ devices” refers to the following products:
OEM USB DAQ devices allow system designers to integrate NI DAQ boards in a lower cost, easier to mount form factor into custom enclosures and systems. With these custom enclosures and systems come a wide variety of enclosure sizes and environmental conditions. It is important for the system integrator to understand the impact of their design choices to the ambient temperatures the OEM USB DAQ device experiences and ensure the device does not overheat.
Ambient temperature can be confusing to describe, depending on one’s interpretation. It is always a good idea to clarify what is meant. There are generally three types of ambient temperatures to be concerned with: (1) local ambient temperature very near the circuit board, (2) ambient temperatures inside an enclosure (away from the circuit board), and (3) ambient temperatures surrounding the enclosure. Self-heating caused by the DAQ device or other equipment in the system can impact ambient temperatures.
The image below illustrates the ambient temperatures to be concerned with when designing your system.
Ambient Temperature Locations
NI USB DAQ devices were originally designed for placement in small enclosures without a fan. Forced convection (oftentimes from using a fan) can help cooling, although airflow is not generally required for OEM USB DAQ products.
The physical space constraints of a system can impact the amount of self-heating around a circuit card or enclosure. The more restrictive the space, the more self-heating may increase the local ambient temperatures around the circuit card and inside the enclosure.
OEM USB DAQ devices may be mounted near heat-generating equipment or machinery. The heat generated by these external sources can directly affect the ambient temperature around the device or inside the enclosure. In addition, when installed in a suitable enclosure, these systems may also be deployed in outdoor applications where they may be exposed to the sun. The added effect from the solar radiation can significantly increase the ambient temperature inside the enclosure. Special considerations should be taken to reduce this impact, such as shading the system or choosing an enclosure that has appropriate radiation absorptivity, reflectivity, and emissivity properties.
Enclosures are typically a conductive metal, such as steel or aluminum, or an insulating material, such as plastic or fiberglass. Metals allow for a better ability to conduct heat away from the circuit board. An insulating enclosure can help thermally isolate the circuit board from the surrounding environment.
A smaller enclosure may create more self-heating around the circuit cards because of the restricted air space, which results in a higher temperature rise inside the enclosure compared to the air outside of the enclosure. A larger enclosure can reduce some of the self-heating effects.
Adding vents to the enclosure helps reduce the internal ambient temperature by allowing air to escape to the surrounding environment, thus reducing some of the self-heating effect. In natural convection environments, vents may only have a minimal impact on internal ambient temperatures.
Adding airflow to the system or enclosure by the use of fans or blowers can greatly improve the thermal performance of the system. Convection is improved, reducing component and surface temperatures, as well as exhausting hotter stagnant internal ambient air with cooler external air. With the addition of forced air, the ambient temperature measurements are a less reliable (and worst case) method of measuring thermal performance because the moving air does further cooling in addition to simply lowering the local ambient temperature.
Mounting a circuit board vertically helps with increasing natural convection and keeps hot spots to a minimum. Horizontal mounting can hinder the effect of natural convection.
The standard maximum ambient temperature for most USB DAQ (non-OEM) devices is 45 °C. This temperature refers to the external ambient temperature outside of the enclosure. Many components used on these products have a maximum local ambient temperature requirement of 70 °C. The self-heating caused by the product itself is oftentimes enough to raise the internal temperature of the enclosure close to 70 °C and is one of the reasons why the device has a lower rated temperature limit when compared to the limits of the components on the board. The OEM USB DAQ devices are not housed in the NI enclosures and the thermal limits can be changed to account for the unique environments they are designed into.
NI engineers have validated that, when in the standard NI enclosures, the USB DAQ devices have no components that overheat when subjected to this 70 °C local ambient limit. A local ambient of 65 °C is a good target for maximum air temperature when designing your system to ensure the component temperatures are never breached and have some buffer built in.
When testing your system for thermal compliance, ensure that several local ambient locations are measured and that none exceed this 65 °C limit when your system is placed in its maximum external temperature environment.
If you are testing at room temperature (or some other temperature less than the maximum external temperature), you may extrapolate your results linearly up to your maximum external ambient condition as a reasonable approximation of what it would be at that elevated temperature.
For example, if you measure a local ambient of 40 °C at an external temperature of 25 °C, there is a 15 °C temperature rise inside your enclosure. This temperature rise tends to be consistent across all temperatures, so your system could safely exist in an external environment temperature up to 50 °C before reaching the 65 °C local ambient limit.