Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 37 years


• Article 29 - January, 2007


A Webb Systemizer III Sensor Update©


Clear back in the July/August, 1997 newsletter, I wrote a story about how to use a 30 K ohm sensor in place of the odd ball, and no longer available, sensor for the Webb Systemizer III solar controller. The scheme required a simple two-resistor network to be placed between the 30 K ohm sensor and the controller sensor input terminals. Back then, I wrote a small computer program in BASIC to help with the calculations and made some runs. These results gave values for the two resistors used for the network. The network/30 K ohm sensor combination resulted in a fairly close simulation for that odd ball Systemized III sensor. The two sensor curves were within +/- 6 degrees of each other over a temperature range of 68 176 degrees F. Now, with a little more time and Excel, I decided to go back to see if I could tweak those numbers a bit. Using Excel and its graphing capabilities made it much easier to see how different resistor combinations could narrow the temperature differences between the Systemizer III sensor and the 30 K ohm/resistor network. Here are the results. Using the two-resistor network shown in Figure 1 below narrows the temperature difference to +/- 5 degrees F over a temperature range of 40 180 degrees. Adding a third five cent resistor, as shown in Figure 2 below, gives a +/- 3 degree F difference over a 45 180 degree range.




                                                                   Figure 1                                         Figure 2


I get my parts for repairing controllers from Digi-Key ( They add a $5 fee for orders under $25, so an order for only a few resistors would get rather pricey. But if you want to try this scheme out, let me know and I'll order some resistors for you next time I place an order for parts. Here are the resistor values and Digi-Key part numbers FYI.


Fig. 1, R1 = 43,200 ohms, part # 43.2KXBK-ND
Fig. 1, R2 = 4220 ohms, part # 4.22KXBK-ND

Fig. 2, R1 = 43,200 ohms, part # 43.2KXBK-ND
Fig. 2, R2 = 3830 ohms, part # 3.83KXBK-ND
Fig. 2, R3 = 698 ohms, part # 698KXBK-ND


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