How can I use resisters to replace a Thermister?

[RonRock]

I have a pool heater that I have to replace the electronic control board often. I have now replaced the board 5 times. Each board shows the same issue when it needs replaced. I have tested all of the control switches all test good. The thermister is one item that is not basically an open or closed switch. As I'm sure everyone here knows the resistance value of the thermister changes with temperature change.

The one I need help with has three wires, ST-COM-SR. The manual shows to test Resistance based on the water temp. So Lets say that the water is at 60*F. The resistance between ST -COM is 15.31(K), resistance between SR-COM is also 15.31(K) within 10% of each other is good.

My thermister tests good with my VOM. But I still think that it may be bad. It is a fairly expensive part to just guess needs replacement. So I would like to make a "test" tool with two resisters. Seems simple enough to do use one side of the resisters together for COM then the other side for ST and SR.

But I am not savey enough with electronics to know if this will work or not to"trick the board into "thinking" that it is a working thermister. I also don't know how to choose a resister to give me the correct value of 15.31.

I will replace either the board or thermister after my test. But at this point it is hard to belive that the boards keep going bad with the same symptoms.

Any advice? TIA
Ron

 

[rjenkinsgb]

The SIPs are just single in line resistor arrays. They are unlikely to have failed.

Has the thermistor part ever been replaced?

The input and analog circuit on those boards looks quite simple.

You could draw the component layout several times larger on paper and add the connecting PCB tracks, to work out the full diagram for the thermistor input circuit.

The 8 pin ICs are 2903 dual comparators, interchangeable with LM393 or similar:

https://docs.rs-online.com/3da4/0900766b80e6f7fe.pdf

https://www.ti.com/lit/ds/symlink/lm393.pdf?ts=1662991950917&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM393

A bit more info - a wiring diagram for that board, found in a Certikin poool heater manual:

https://manualzz.com/doc/60405096/certikin-mb055-operating--installation-and-servicing-inst...

 

[RonRock]

Thanks again.

No I have never replaced the thermister. Been a while since I checked, but it seems that it was a $100.00 part. Probably more these days. A bit expensive to replace on a guess. Not that I won't if it seems like a good idea by someone who knows more than I do.

 

[rjenkinsgb]

Get a pack of these, attach some wires and try them in pairs to get matched ones:

10 PC CANTHERM THERMISTOR NTC 10KOHM 3470K BEAD MF52A2103J3470 | eBay

Manufacturer: Cantherm. B 25/50: 3470K. Resistance in Ohms @ 25°C: 10k. Color: Black,Silver. ROHS Compliant: Yes. Lead Free: Yes. Resistance Tolerance: ±5%. We will work with you to until you are satisfied.

www.ebay.com

Then epoxy two similar ones together so the temperatures stay the same & try warming & cooling them to see if the controller then switches on & off correctly?

Or a couple of these pre-wires "waterproof" ? ones that claim 1% tolerance:

NTC Thermistor Temperature Sensor Probe 10K 1% 3950 Waterproof 1M Length US | eBay

NTC Thermistor Temperature Sensor 10K 1% 3950 with Waterproof Probe. Type: NTC 10k ±1% 3950. Size of probe: 4 x 20 mm. Probe insulation: >100 MOhm. Stainless steel sheath and waterproof. 12V (8-23V) In To 3.3V Out 3A 10W DC-DC Waterproof Converter Module.

www.ebay.com

 

[Reloadron]

Get a pack of these, attach some wires and try them in pairs to get matched ones:

10 PC CANTHERM THERMISTOR NTC 10KOHM 3470K BEAD MF52A2103J3470 | eBay

Manufacturer: Cantherm. B 25/50: 3470K. Resistance in Ohms @ 25°C: 10k. Color: Black,Silver. ROHS Compliant: Yes. Lead Free: Yes. Resistance Tolerance: ±5%. We will work with you to until you are satisfied.

www.ebay.com

Then epoxy two similar ones together so the temperatures stay the same & try warming & cooling them to see if the controller then switches on & off correctly?

Or a couple of these pre-wires "waterproof" ? ones that claim 1% tolerance:

NTC Thermistor Temperature Sensor Probe 10K 1% 3950 Waterproof 1M Length US | eBay

NTC Thermistor Temperature Sensor 10K 1% 3950 with Waterproof Probe. Type: NTC 10k ±1% 3950. Size of probe: 4 x 20 mm. Probe insulation: >100 MOhm. Stainless steel sheath and waterproof. 12V (8-23V) In To 3.3V Out 3A 10W DC-DC Waterproof Converter Module.

www.ebay.com

I would just buy a few of the 1% flavors and epoxy two together. Tie one lead from each together leaving you a three wire configuration. That should work just fine for a small investment.

Ron

 

[alec_t]

..... and if the new thermistors provide a miraculous cure you could embed them in epoxy using a mould matched to the physical size/shape of the existing sensor.

 

[Tony Stewart]

I thought the 8 pin IC's were dual Op Amps 7032D

 

[rjenkinsgb]

I thought the 8 pin IC's were dual Op Amps 7032D

From photos of them, the upper numbers are the type and the lower a date code?

JRC2903D - Google Search

 

[alec_t]

It may be a trick of the light, but IC U1 and diode D14 (near the relay) both look cooked? Check the board for any dark patches or frizzled paint on components, suggesting over-heating. If any found, then I would suspect some supply voltage regulation has failed.