Add a resister in series. To go from a nominal 1000 ohm to 20,000 ohm you add 19,000ohm resistance.

I bought a multi set so I could mess about with stuff.

BOJACK Resistor Assortment Kit 1 Ohm - 1M Ohm 1/4W Carbon Film Resistor Assortment Resistor Kit (25 Values 1000 Pcs) from Amazon around £11.

Posted by: @john-connett

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I am considering changing my solar thermal controller. The existing controller uses a Pt1000 sensor in a roof mounted solar collector. My preferred choice of replacement expects input from an NTC 20 kΩ sensor. Can anyone suggest an affordable converter?

It would probably be possible to produce something using a PT1000 RTD Temperature Sensor Amplifier, DS3502 I2C Digital 10K Potentiometer and a Raspberry Pi Peco or similar if I possessed the necessary making skills.

However, I would prefer a ready made device that wouldn’t frighten a heating installer!

Any suggestions?

Probably the easiest solution would be to see if you can find a NTC 20k sensor that is housed in a probe with the same physical dimensions as the PT 1000. Have a look on the TC Direct website.

Posted by: @derek-m

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Posted by: @john-connett

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I am considering changing my solar thermal controller. The existing controller uses a Pt1000 sensor in a roof mounted solar collector. My preferred choice of replacement expects input from an NTC 20 kΩ sensor. Can anyone suggest an affordable converter?

It would probably be possible to produce something using a PT1000 RTD Temperature Sensor Amplifier, DS3502 I2C Digital 10K Potentiometer and a Raspberry Pi Peco or similar if I possessed the necessary making skills.

However, I would prefer a ready made device that wouldn’t frighten a heating installer!

Any suggestions?

Probably the easiest solution would be to see if you can find a NTC 20k sensor that is housed in a probe with the same physical dimensions as the PT 1000. Have a look on the TC Direct website.

 

Probably should have mentioned that the solar collectors are on a second floor sloping roof. Front of the house is almost on the footpath. Access to swap the sensors could involve scaffolding and a permit to erect the scaffolding on the footpath. I was looking for a solution that could be applied at the solar controller end of the connection.

@john-connett

It may be possible to build a simple electronic circuit that could emulate a NTC 20k thermister from a PT 1000 Resistance Temperature Detector (RTD) input.

The first thing to note is that the PT 1000 has a positive temperature co-efficient, in that it measures 1000 ohms at 0C, and its value increases with temperature in a quite linear fashion.

NTC means that this thermistor has a Negative Temperature Co-efficient (NTC), so its reasistance reduces with increase in temperature. When I have time, possibly later this evening, I will give the problem some thought.

The Pt1000 Resistance Table is straightforward. NTC Thermistors, Resistance/Temperature Conversion is more complex. I know it should measure 20 kΩ at 25°C but I'm not sure which of the 17 curves to apply? My background is software engineering so my first thoughts are read the input temperature; convert it to the NTC 20 kΩ resistance using a formula or lookup table; output the resistance. An electrical or HVAC engineer may well have a better approach!

@john-connett

The software part is simple, but how do you output actual resistance?

I assume that both the old and new controllers are passing a small current through the sensor and measuring the volt drop, and hence calculating the temperature. Can you measure this volt drop at various temperatures?

What make and model are the old and new controllers?

Over what temperature range do the sensors operate and what control function do they perform.

Posted by: @derek-m

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@john-connett

The software part is simple, but how do you output actual resistance?

My guess would be a Digital potentiometer. Might need a logarithmic one to cover the NTC 20 kΩ resistance range? I'm not an electrical engineer so this might be nonsense.

Posted by: @derek-m

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I assume that both the old and new controllers are passing a small current through the sensor and measuring the volt drop, and hence calculating the temperature. Can you measure this volt drop at various temperatures?

Sounds reasonable. I have a fairly limited electrical toolkit, not even a digital multimeter.

Posted by: @derek-m

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What make and model are the old and new controllers?

Old: Sonnenkraft SKSC2
New: Viessmann Solar EM-S1 extension (proposed).

Posted by: @derek-m

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Over what temperature range do the sensors operate and what control function do they perform.

There is a resistance against temperature graph on page 16 of the Viessmann Installation and service instructions manual (linked above) which has a range from 0 °C to 140 °C which seems reasonable for solar collectors. Curve A is for the NTC 20 kΩ sensor.

The test procedure is: Disconnect the plug of the relevant temperature sensor from the extension. Check the sensor resistance. I assume a multimeter would be used to check the resistance (by passing a small current through the sensor).

@john-connett

Hi John.

It would appear that there are several possible solutions. You could go down the digital route using something like a digital rheostat, though I suspect that will entail having a computer running continuously to perform the required calculations.

There is the possibility of an analogue solution, using electronic components to achieve the same effect.

Do you have a preference?

Posted by: @derek-m

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@john-connett

Hi John.

It would appear that there are several possible solutions. You could go down the digital route using something like a digital rheostat, though I suspect that will entail having a computer running continuously to perform the required calculations.

There is the possibility of an analogue solution, using electronic components to achieve the same effect.

Do you have a preference?

Requirements are for reliability and acceptability to heating installers / maintainers. Ideally, a ready made device with safety approvals (may not exist). Unfortunately, the acceptable solution may be to change the sensor on the roof ...

As an aside, I found the Ohmigo® TTL which is a precision digital controllable resistor capable of generating a resistance ranging from 68.5 ohm up to 900,000,000 ohm with superior accuracy, overall better than 1%.  That should be good enough for simulating an NTC 20 kΩ (and a wide range of other sensors). Only does part of the job but demonstrates that part is achievable. Originally found the Ohm On WiFi version.

@john-connett

What is the problem with the old controller?

Posted by: @john-connett

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The Pt1000 Resistance Table is straightforward. NTC Thermistors, Resistance/Temperature Conversion is more complex. I know it should measure 20 kΩ at 25°C but I'm not sure which of the 17 curves to apply?

Using the curve in the Viessmann guide you can see that the resistance at 140°C is about 0.5. So Rt/R25 is 0.025. You can then look at your 17 curves and see what the best match is. You could just estimate an approximate equation to match the Viessmann curve instead of trying to emulate an actual device. Or ask Viessmann what device it is?