Heat pump and unknown thermistor - Page 1
Forum section: electronics / repair

author: ita4ever
author_rank: Contributor
timestamp: May 09, 2025, 07:39:52 am
content: Hi all,I'm with a faulty inverter board of a Rotex (Daikin?) heat pump and I'm trying to identify a thermistor which is broken.The inverter-board is inside the outdoor unit.I asked for help from an Italian forum focused on energy (hoping for someone who having a spare board to compare with) and at a generic Italian electronics newsgroup but actually without luck.The question:having to drive a 4-way valve like this: reversing_valve.png(from YouTube video: 4EdpmCAdb3Q)which uses this actuator (Z521 DC120V): Z521.jpegwhich PTC/NTC thermistor makes more sense to put in series?The wiring diagram is this one and the valve to drive is Y1R (on the right, under PCB2)https://www.daikin.es/content/dam/MDM/TechnicalDrawings/HEATING/EN/ERLQ006-008CV3_wir1_3TW60816-2_EN.pdfThe voltage between DC_N2 and DC_P2 that I read at a random time is about 330VDC and the resistance of the actuator winding about 700 Ohm.About 280 Ohm @ 20C instead the resistance of what remains of the body of the original thermistor (not identifiable by any marking).RTH1.jpegWith a test in the freezer its resistance increases up to 330 Ohm and therefore would suggest an NTC.But, contrarily, other Daikin machines with quite similar schematics have a PTC there:https://www.daikin.es/content/dam/MDM/TechnicalDrawings/Split/4MWXM-A9/EN/4MWXM-A9_wir3_3D135807_EN.pdfFrom what I understand Z521 is a small linear actuator where the central core moves inside the winding when the latter is energized.This movement is then used to directly send pressurize gas, either at the right or at the left, inside the main cylinder of the valve yielding to the movement of the internal piston (which will allow the refrigeration cycle to be reversed).Therefore, unlike a "static" inductor, here the current will be greater for t=0 and then will decrease.Is this correct?If so, then the NTC would be a good choice because it would limit the initial current inrush, preventing the fuse from tripping.I'll really appreciate any advice, thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: May 09, 2025, 09:54:17 am
content: The stated resistance and voltage give an initial current of under half an amp, solenoids are inductive so the current ramps up from zero, and FU2 feeding the solenoid and the fan motor is T3.15A so I doubt the inrush current is an issue. A NTC just doesn't make sense. However its desirable to reduce the holding current of the solenoid so its coil doesn't overheat, and a PTC would be a simple way to do that.Edit: corrected fuse rating typo.

author: ita4ever
author_rank: Contributor
timestamp: May 09, 2025, 01:01:36 pm
content: So I have to assume that my measurements on broken thermistor aren't significant. May be, it's broken.Could be a good approach to start with a 250R T25 PTC?Thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: May 09, 2025, 01:52:17 pm
content: See if you can find a datasheet for the valve/solenoid. If you are lucky you may get the recommended pull-in current and holding current so you can work out what thermistor characteristics you need.

author: ita4ever
author_rank: Contributor
timestamp: May 09, 2025, 03:00:54 pm
content: I didn't find it.But I see it's sold as spare part so I'll try to remove it from the valve and calculate the engage and hold currents experimentally.The situation is now clearer to me, thanks.