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Hello All,
I bought a Russell Hobbs RH50FF144 firdge freezer which is just outside it’s warranty. I have a few questions and I apologies for the long post but if anyone can help with any answers I’d be grateful.
It recently stopped working and after some internet investigation I concluded that it was likely to be a failure of the PTC Relay attached to the compressor – the thermistor disk inside had broken. The code on the resistor is QP2 and they’re rated at 15 ohms.
Spectrum Brands (ProductCare???) who own the Russell Hobbs brand have written to the Chinese manufacturers but they seem to be having trouble identifying the part (which seems odd, but there you go). I’ve sent photos and details and am awaiting a reply.
So, I ordered up a few replacements from Hong Kong suppliers which were about £2-£3 each and with these the fridge restarts and works but they seem to fail again very quickly. The average life of the three I’ve tried is about a week; their working lives were 1.5 days, 4 days and the best of them was just over 2 weeks.
So, I have some questions if anybody is able to help me.
1) The replacements I’ve used are very cheap and bought from ebay and amazon – might the problem be due to poor thermistor disk quality? What experiences might others have had with them?
2) Might the failure of four relays, including the original, be a symptom of some other problem – what else might make them crack if not poor quality? As they’re rated for 240v I’m guessing there’s no voltage transformer in the fridge which might have failed, but that’s just a guess. Also I can’t see a capacitor anywhere near the compressor, though that doesn’t rule out there being one as I’m not an expert. Do fridge/freezers always have capacitors or anything else that might be faulty?
3) Would anyone know of a source for decent quality PTC thermistors ?
4) Finally, I’ve taken to scavenging PTC thermistors from fridges people are putting outside their houses for collection. I’m running the fridge now on one I scavenged so my thinking is that as it’s working it must have worked for some time in the original fridge and is likely to be a manufacturer part so has been tested in action without failing before. It’s only day 2 so my fingers are crossed!
However yesterday I got a second PTC thermistor but this was rated at 22 ohms. What I was wondering is that although the original was rated at 15 ohms – if I were to try the 22 ohm one and the fridge started up using it would there be any problem in keeping this as a spare and using it if need be.
I’m not an electrician but I think that Ohm’s law might apply here as follows:
15 Ohm original PTC thermistor
V = IR where V is 220 volts and R = 15; therefore ampage through the starter coil would be 220/15 = 14.66amps maximum, reducing as the temperature of the PTC thermisor rises and the resistance increases.
22 Ohm original PTC thermistor
V = IR where V is 220 volts and R = 22; therefore ampage through the starter coil would be 220/22 = 10amps maximum, reducing as the temperature of the PTC thermistor rises and the resistance increases.
So, provided the compressor actually started with the 22 ohm scavenged PTC thermistor my thinking is that’s fine as all it means is a lower but still adequate ampage is put through the starter coil but I’m wondering if this might cause any fire risks etc. Would anyone know?
Many thanks if anyone can help with answers to any of these questions.
Paul QQQ
By the sounds of it and, with the repeated failure and without looking at the machine, best guess is the compressor is shot and the relay/overload is doing as it should do, failing to prevent any further danger/damage.
Occam’s razor, normally the simplest answer is the correct one.
K.
Update on the above … I’ve scavenged a PTC resistor from a fly-tipped fridge and now all is working well again, so on this case it doesn’t appear to have been the compressor … although that’s what the so-called ‘engineer’ diagnosed it as.
I think it’s slightly different – if you buy replacement parts cheaply on-line from China maybe you’re getting the parts from batches that have failed the ‘manufacturer quality’ quality test i.e. the best batches go to manufacturers as it would be useless to them if the average life of their fridges was between 1.5 days and two weeks! Perhaps the batches that fail these tests are sold off individually on ebay for a few quid etc…and you’re just taking pot luck on what you get.
A second question on PTC resistors –
From my research it appears that the resistance increases as the heat rises, and the heat rises once you put a current through them on the fridge. However I’ve now tested three different PTC resistors and for each of them the resistance drops when I heated them with a hair dryer which seems to have been the opposite of what they ought to do!
Has anyone any idea why this is? According to internet articles a PTC resistor’s resistance should increase when it’s hot. My results show the opposites.
I’m confused!
https://en.wikipedia.org/wiki/Thermistor
😉
- With NTC thermistors, resistance decreases as temperature rises. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter.
- With PTC thermistors, resistance increases as temperature rises. PTC thermistors are commonly installed in series with a circuit, and used to protect against overcurrent conditions, as resettable fuses.
K.
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[TD]The compressor starts by resistive start and induction run, as the current flows through the PTC , it heats up increasing its internal resistance until it will not allow more current to flow , disengaging the start winding. The start winding is isolated and will not engage further as a trickle of current to the start winding will maintain heat and high resistance during the operation of the compressor.When the unit stops the PTC MUST!!! cool down to allow current to flow again on next start-up. If it trys to start to early the internal overload will activate in the compressor.
So it really is not designed for frequent starts and requires very low starting torque. [/TD]
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[/TABLE]Hello Tony, R,
Thanks for your reply – that’s my understanding of how PTCs work too – but interestingly I’ve tested my few remaining Chinese dodgy ones that haven’t been used, plus four or five others that I’ve scavenged from old fridges that are being dumped by connecting up the OHM meter, measuring the resistance then heating them with the hair dryer to warm up the PTC while watching the resistance reading – and what I’ve found in every case is that the resistance decreases when they get warmer, so they seem to be acting like an NCT rather than a PCT. Some of them do say PTC on the casing but have this characteristic even so.
This isn’t what I expected at all :o. Might anyone know the reason?
Thanks again,
Paul QQQHave same sort of problem. Russell Hobbs fridge RHTTLF1SS compressor resistances 38.3 + 16.0 + 53.9 so just outside the 0.5 ohms check. This fridge has now blown 2 Chinese 15 ohm start resistors as well as the original one. It won’t run with the thermostate leads connected so it’s either a bad compressor or dreadful start resistors. Perhaps different (better quality) ceramic start resistors are the answer. The new Ebay ones lasted about a week.
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