TDK’s industrial NTC sensors put mounting and response time in focus
Image: TDK Electronics
TDK’s M704 and M705 temperature sensor assemblies share the same nominal resistance and B value. Their behaviour at the point of measurement is quite different.
Both are specified at 10 kΩ at 25°C, with resistance tolerance of ±2% and a B25/100 value of 3,988 K, ±1%. The M704 provides a compact arrangement suitable for PCB integration and screw attachment. The M705 uses a metal tag with a ring lug, allowing it to be screwed directly to a surface.
- M704: −40°C to 125°C operating temperature, 3.5 mm lead spacing and screw attachment. Reported thermal time constant on a metal plate: approximately 34 seconds.
- M705: upper operating temperature of 180°C and a metal-tag ring lug. Reported thermal time constant on a metal plate: approximately 6.2 seconds.
The response figures deserve attention. A sensor’s resistance curve can be familiar and predictable while the assembly around it substantially changes how quickly a temperature excursion reaches the sensing element. Here, the reported time constants differ by more than a factor of five. They apply to the stated metal-plate conditions; contact pressure, interface material and the final mounting arrangement will affect the installed response.
There is also an important detail behind the M704’s 4,000 VAC figure: the datasheet specifies a test lasting one second. It is an insulation test condition, rather than a continuous operating voltage. TDK’s announcement gives 2,500 VAC for the M705.
Applications include power supplies, charging equipment and inverters. The M704 supports automated PCB assembly, while the M705 accommodates higher temperatures and offers the faster reported response of the two assemblies.
