Cheap Precision Resistor Mounting Scheme with Built-in Thermistor - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: March 07, 2025, 08:04:17 pm
content: We have some precision resistors that we wanted to create a mounting fixture for, but never got around to doing such!!Here's a simple mounting scheme we came up with that should work for most axial precision resistors that also has a Thermistor (10K 1%) attached (heat shrink). A simple 3D printed "Plug" with a "Standard" Banana Adapter allows the Thermistor leads to protrude from the top and bent over to create a measurement "Terminal" for small clip leads.After some brief testing today it seems the thermistor indicates the fixture precision resistor setup is 2~3C hotter than ambient due to the temperature of the DMM (KS34465A) Banana Sockets. Here Kelvin Clips allow the DUT to be ~ ambient, which might be an advantage.Anyway, not "Metrology Qualified" but good enough for our needs.As always YMMVBest

author: enut11
author_rank: Super Contributor
timestamp: March 08, 2025, 06:19:42 am
content: @mawyatt, great idea and nicely implemented. Thanks.enut11

author: trobbins
author_rank: Frequent Contributor
timestamp: March 08, 2025, 08:54:31 am
content: Would the preferred outcome be for the DUT resistor temperature to be as close to ambient as practical, and to be influenced by the DMM front panel terminal temperature to as little an extent as practical? This could be a secondary aim to measuring the DUT device temp as accurately as practical (using the thermistor).If so, then maybe the addition of a copper tab around the heatshrink assembly would lower the DUT temp difference to ambient, or perhaps soldering/crimping copper tabs to the end leads of the DUT could further suppress the temp diff between DUT and ambient, and may be a practical benefit. I guess the concern would be a temperature diff across the metallic junction of banana plug to DUT lead.The other method to manage temp could be to thermally box in the DUT and banana plug, so they are all iso-thermal to the DMM terminal temp - as is often done by those with such DMM banana terminal interfaces.

author: mawyatt
author_rank: Super Contributor
timestamp: March 08, 2025, 02:04:14 pm
content: Quote from: trobbins on March 08, 2025, 08:54:31 amNote that this is a 2 Wire Resistor configuration on the DMM, so DMM should be in 2 Wire configuration.We included the Resistor Probe terminals so one could measure the resistor away from the DMM at ambient with clip leads (Kelvin or not, as at higher resistances 4 Wire Kelvin aren't necessary), or can just use Banana Cables to the DMM, or directly connect clips to the Banana plugs.As mentioned the DMM Banana terminals are higher in temperature than ambient, thus the resistor is also at higher than ambient.The main concept was to produce something that is quick and easy to use without cables (like our AC and DC Voltage Reference), and provide some physical protection to the precision resistor. The thought to include a thermistor attached to the precision resistor with heat shrink tubing made this configuration even more attractive. We had considered using some crimp terminals (ferrules) over the precision resistor leads but decided against this because of the additional metal to metal interface created.Wrapping the Resistor and Thermistor in metal foil should improve the thermal connection and mass, as other techniques (inside metal tube) would also, but for our needs the simple heat shrink works OK when mounted on the Standard Dual Banana Adapter.Best

author: mawyatt
author_rank: Super Contributor
timestamp: March 08, 2025, 07:52:32 pm
content: We just did some measurements with the DMM6500 and SDM3065X. The DMM6500 Banana Terminals produced a temp reading on the Thermistor of 3~3.5C higher than ambient (~1C hotter than KS), while the SDM was under 2C.Recall the KS34465A actually measures the internal Banana Terminal Temp, but don't know if this is same for DMM6500 or SDM??Best