Blog #2: Characterising Thermistors - What's In The Box?

In the last post, I mentioned my background and motivation for joining the element14 Experimenting with Thermistors Design Challenge. In this post, I'll be looking at the kit that arrived and the steps I took to prepare them for upcoming experiments.

Unboxing

It's always a good day to come home to a box on your doorstep, especially one with the Newark logo on it.

While it may have been a big box, it weighed nearly nothing. Crumpled paper was used to fill the space inside the box for shipping.

Taking this away revealed the plastic assortment box which contained the thermistors for this challenge.

Just one (minor) problem ... the thermistor's springy wires and the handling the package received during transit meant that many of them did not stay put. As a result, I now have a jumble of thermistors which need a bit of a closer look at. Also, interestingly, the packing sheet had a quantity of 10 written on it rather than the one of each type that I received. Perhaps that was an error and ten (total) was ordered for the whole challenge rather than for each participant. It is a bit of a shame we don't get more than one of each - it can be handy to have some spare in case things get destructive.

A Closer Look

Would you like a bouquet of thermistors? Don't mind if I do ;).

A selection of traditional "dipped bead" style epoxy-coated NTC thermistors with 100mm 28AWG insulated wires was included.

Something to note is that all of these thermistors share the same Beta value suggesting they are made of the same material system. Note that the Newark website has an error in its data claiming the Beta value is 3892K - the manufacturer's datasheet indicates they are 3982K. (Update - see comments, 3892K is confirmed). The prices listed with an asterisk (*) indicate special promotional pricing, all pricing is for single quantity. On the whole, the parts appear to belong to a single series - the cost of the this bunch would have been US$11.12* / US$23.76.

Of course, such thermistors are relatively traditional - but these "ring" type ones are not. A common problem is simply how one mounts a "round", shiny, slippery object to something flat - this seems like a rather elegant solution.

From the outside, it looks a bit like a shiny uninsulated ring crimp with a thermistor packed inside ...

... and indeed that's what it seems to be. It is potted inside the crimping area using some black coloured silicone, likely of the thermally conductive variety.

The finish is neat, with the silicone being confined to that area with no spillage outside. The other issue with regards to manufacturability is how to connect the thermistors to circuitry. As a connector company, Molex had an easy solution for that -

as they just crimped the other end to a latching Micro-Lock Plus connector.

Such connectors are quite small as they have a 1.25mm pitch. These connectors are recommended to mate with PCB headers 5055680231, 5055680251, 5055680271, 5055680281, 5055670231, 5055670251, 5055670271 and 5055670281. Unfortunately, as I don't have any of them to hand, I can't evaluate the connector itself even though it does look like it would fit the bill quite nicely. The latch on the connector is especially important where it is used in a high-vibration environment.

Of the ring thermistor type, three NTC units were provided:

These units had 300mm of cable provided, which is nice, but also differ in their Beta value. Perhaps the thermistors they use to put inside the ring are a different material type altogether, but there is a mix of two different Beta values in the kit. Now that I've mixed them up, identifying them could be a trouble ... but it would make for a good experiment. Why the 2% tolerance version costs more than the 1% tolerance version in regular pricing is a bit of a mystery, but we are in an era of supply-chain issues, so perhaps the pricing reflects this instead. The total value of these thermistors was US$7.76* / US$19.23, bringing the total kit of thermistors up to US$18.88* / US$42.99 excluding the storage case. Who would have thought a handful of thermistors would add up - the ring type definitely commands a price premium over the bare wire thermistors, but comparing the promotional pricing, the difference is surprisingly small when one thinks of the fact that additional wire, a ring crimp, thermal epoxy, connector and contacts are thrown in to create the ring thermistor product. The time-savings in terms of manufacturing are likely to make the ring thermistor a good choice where mounting to a flat surface is required.

Preparing for Use

In order to ready them for experiments, I needed an easy way to connect to the bare-wire thermistors as well as the ring-type thermistors. I decided to use the connector for which I had the most resources for - the humble "Dupont wire" 2.54mm pin header style connector which is popular amongst hobbyists and older electronics. By standardising on a single connection, this will streamline testing and swapping thermistors. Even though it won't be as good as Molex's Micro-Lock Plus, it should still do the job.

Half-an-hour of crimping later, I had each thermistor terminated into a two-pin connection, ready to go. Time now to sort out the mess of values ...

As my room is colder than 25c, the resistance readings will be higher. The bare wire thermistors were all sorted adequately, but the ring-type all have the same nominal resistance, with one having a wider tolerance and another having a different Beta value. This cannot be easily sorted out just by taking a single point measurement.

Conclusion

The thermistors arrived safely in a big box, just a bit jumbled up. The bead-type insulated-wire thermistors all appear to come from the same product series in a range of nominal resistances. All have a nominal Beta value of 3982K suggesting they belong to a single material system. The beads and wires came in a mixture of colours. The ring-type thermistors seem to be a little different - all provided units had a 10kohm nominal resistance but had differing Beta values of 3500K and 3800K. The ring type thermistors seem like an ideal solution to the problem of mounting a round bead to a flat surface, as they appear to be an uninsulated crimp ring style terminal with a thermistor thermal-epoxied into the wire entry section. The other end of the ring-type thermistors are pre-terminated in the Molex Micro-Lock Plus connector which is 1.27mm pitch with a latch. This connection appears to be very convenient and may be resistant to vibration - the added premium of a ring thermistor over a bare bead seems to be rather small based on the promotional pricing, especially when the additional wire, epoxy, ring, connector and contacts are considered. The manufacturing time saved is likely to make up for the any difference regardless. The total value of the thermistor kit adds up to US$18.88 on promotional pricing, or US$42.99 on regular pricing based on individual quantities at the time of publication.

Unfortunately, as I didn't have any compatible Molex connectors, I ended up terminating all of them in the hobbyist-standard 2.54mm Dupont wire style male connections. By using an LCR meter, I quickly determined which of the bead type thermistors were which based on resistance values. The ring type thermistors are a bit more of a mystery, requiring a bit more sleuthing.

Now that this is sorted, perhaps I can continue on with experiments ... but not before delving into a little bit more theory!

[[Characterising Thermistors Blog Index]]

• Blog #1: Characterising Thermistors - Introduction
• Blog #2: Characterising Thermistors - What's In The Box?
• Blog #3: Characterising Thermistors – A Quick Primer, Beta Value & Steinhart-Hart Coefficients
• Blog #4: Characterising Thermistors – An Inconvenient Truth, Taking Things to the Fifth Degree
• Blog #5: Characterising Thermistors – Measuring Resistance Is Not So Easy!
• Blog #6: Characterising Thermistors – Is Self-Heating a Problem or Not?
• Blog #7: Characterising Thermistors – Boiling, Freezing and Zapping the Truth Out of Them!
• Blog #8: Characterising Thermistors – Practically Running Multiple Thermistors
• Blog #9: Characterising Thermistors – Multi-T Results, Insulation R Redux, 5th Order Fits & Model Performance
• Blog #10: Characterising Thermistors – Multiple Thermistors on ESP8266
• Blog #11: Characterising Thermistors – Show Me Your Curves
• Blog #12: Characterising Thermistors – Sticking Rings on Tabs & Sinks, Absolutely Crushing It!
• Blog #13: Characterising Thermistors – Pulling Out, Overload, Response Time, Building a Flow Meter & Final Conclusion