Limiting heat rise in oscilloscopes

Any device carrying electrical current heats up to some degree. The reason, of course, is that electrical conductors and power sources aren’t ideal. Inherent in their nature is a certain amount of internal resistance or impedance. There is always impedance within a power source, for example. The impedance may be high as in a triple-A dry cell or low as in a utility generating plant.

An oscilloscope, being an electrical device, also generates a certain amount of heat. And sometimes this heat cannot be dissipated as fast as it is generated. As a consequence, there is a temperature rise within the enclosure. Devices and materials can be damaged or destroyed when their safe temperature limits are exceeded.

The move away from CRTs and tube electronics toward flat screens and ICs have profoundly affected how scopes generate heat. Modern scopes generate a small amount of inevitable I²R heat but don’t see the massive temperature rise produced by filaments needed to heat cathodes in tubes for electron emission, or by power ICs necessary to drive deflection coils on scope CRTs. For example, it is possible to find accounts of scopes made in the CRT era wherein the main horizontal deflection IC, operating normally, was too hot to touch.

The trend toward ever-more-compact circuit miniaturization has reduced the amount of heat that scope circuits generate. But this trend also has brought smaller enclosures. The reduced surface area of these smaller enclosures has made heat management critical. Additionally, most modern oscilloscopes have plastic rather than metal enclosures. This is another plus, but it does adversely affect heat dissipation, which remains an issue.

Though excessive heat is not a problem in most scope applications, it’s possible to run a quick Google search and find stories of scope overheating problems. In one case, BGA solder joints on the scope motherboard opened up as the unit heated up. In another case, scopes used to read out electrocardiograms overheated about once weekly because they were mounted inside a completely closed console. Some schools report instrument overheating problems because students lay books, back packs, and hoodies over the ventilation holes in the enclosure.

And some scope makers use the instrument case as part of the cooling mechanism. This practice can lead to problems if users aren’t cautious. One particular brand of PC-based oscilloscope uses its rear panel as a heat sink for the instrument’s 5-V regulator. It is normal for the rear panel to heat up to such a degree that it alarms users who aren’t familiar with the instrument’s ideosyncracies.

Big brand-name scope makers such as Keysight Technologies or Tektronix don’t usually field designs where thermal dissipation involves heating up the enclosure. These manufacturers tend to take more sophisticated approaches to cooling. For example, in the design of a calibration head for its Infiniium 9000 Q-Series oscilloscope (now discontinued), Keysight resorted to Ansys CFX computational fluid dynamics software for sorting out the design of a cooling system for a new electrical calibration head. The resulting cooling apparatus made the 60-GHz instrument viable.

Thermal management is also highly challenging in computer tablets because of the small size and component density involved. Each model is a case study in terms of cooling strategies. Rather than natural convection, forced convection is the norm in this tight form factor. Additionally, passive heat distribution techniques are employed, as well as other ingenious coping mechanisms.

An example is the practice of soldering onto PCBs where there are passive copper thermal conductors that facilitate the creation of a uniform thermal plane without dangerous hot spots. Oscilloscope engineers, performing teardowns, have studied tablet cooling solutions to see if they are relevant in the design of test instrumentation. The method is to disassemble the tablet and desolder components from the boards to ascertain the tablet’s architecture, heat flow, and the builders’ solutions to potential damaging heat rise.

There is another important consideration: How the user cares for the instrument. Obviously it is good to avoid destructive fault currents as from improper connection of the ground lead to a floating and referenced voltage, and avoiding trauma from rough handling.

New generation plastic-housed oscilloscopes are smaller than the old metal monsters, so heat dissipation may become problematic. This is especially true if the instruments are connected to voltages close to the high end of the safe range. If they are used intensively and the ambient temperature is not watched carefully, excess heat may cause temperature rise that will adversely impact the service life.

An oscilloscope should be powered down if it is not to be used for awhile. Because the memory is non-volatile, settings are preserved unless the machine is intentionally defaulted.

The instrument should be located carefully so as to avoid hot spots. It should not be shoved against an insulated wall but instead situated two or three inches back so that airflow is not impeded. The oscilloscope should not be operated while resting on a cushion but if possible elevated above the bench to permit airflow beneath it.

Rack mounting is a good choice because it allows excellent air circulation. Avoid direct sunlight on the instrument. It is always good to place a small fan near the oscilloscope, and putting a thermometer nearby is a good idea if high temperatures characterize the environment.

Electronic equipment is also vulnerable to damage from lightning. A strike anywhere near an outdoor power line or communication cable that supplies the building can create a brief high-voltage spike by means of inductive coupling. The intense electrical energy can infiltrate sensitive electronic equipment such as an oscilloscope.

Surge protectors including utility-owned devices and customer-owned protective devices downstream are generally effective. But if a severe thunderstorm appears imminent, unplugging the oscilloscope and disconnecting any data cabling is a wise move.