Tumble drier fun...

[Mark Hennessy]

Hi,

Last week, I said this:

The power supply in my tumble drier blew up yesterday. I blame you entirely

That uses a LNK304PN. Looks like an 8 pin DIL IC at a glance, but has 1 pin missing to provide some clearance around the drain of the internal MOSFET. These are known for going "bang" in a big way, and so it is with mine. Luckily, replacements are only a pound.

The inrush-limiting resistor is a fusible wire-wound type, and that had done its job reasonably spectacularly. Farnell sell replacements - 1738521 is close enough. Saves money, I suppose, but I prefer a fuse followed by a conventional surge-proof resistor - that's one of the main reasons why they still make carbon-comp resistors. Of course, anything bigger would have an NTC or similar for inrush protection.

And while it's open, I might as well change the caps. Only 6 or so. The two "big" ones (4u7, 400V) would have been a bit stressed by the sudden short. Same for the rectifier diodes - just 1N4007. Let's hope the Atmel microcontroller survived...

This one is fed from a half-wave rectifier rather than the usual bridge. Not sure if it was a mains surge, excess humidity, or just "natural causes". There is a MOV across the input for surge protection.

Busy week, so not sure when I'll get the chance to fit the replacement components, but fingers crossed. Replacement controller modules are over £100. Cheaper than a new machine, but this one is a condenser, so replacement won't be cheap. Like the idea of the heat-pump types, but they're still rather expensive.

So, a new IC, fusible resistor and choke, plus several caps later, it's working again. Hooray!

However...

[Mark Hennessy]

This machine has always been a bit iffy. And my numerous web searches failed to bring any enlightenment, so I thought I'd record it all here, just in case it might help someone else.

The machine in question is a Whirlpool AWZ8578. An ugly brute - not my choice.

We inherited it from my wife's sister. She bought it because she needed a condensing unit, given that there's no easy way to vent it externally in her house. But it didn't work very well for her so, assuming that *all* condensing driers were the same, she replaced it with a conventional vented one. I dread to think what state her house must be in, but never mind...

At first, it seemed like they hadn't read the instructions properly, as it seemed fine to us. It has various sensors built in, and it's just a case of telling it you'd like it "cupboard dry", or whatever. They apparently were using the fixed time programs - who knows why? Either way, our existing tumble drier was 20 years old and had a failed heater, so I wasn't going to complain...

Over the years, I'd notice that sometimes it would take a lot longer than usual to dry. The clothes inside would be cold and damp, so the heater wasn't heating. Power-cycling it would usually get it going again, and I'd forget about it...

When it went "bang" last weekend, I had to take it apart for the first time and start reverse-engineering it. I secretly hoped that the heater issue would be solved by a new batch of PSU capacitors, but upon reassembly yesterday, the heating was even more shy than before. So, time to really get in there...

[Mark Hennessy]

Naturally, there are no schematics available for these machines. So producing one was the first objective.

The mains comes in and goes straight to the main controller. From there, wires go off to the heaters, motor, condensate lift pump, door sensor, etc. And there is a second control box. This is powered from the mains (post main power switch), but that's just about all I could establish at this point. It didn't seem important. A visual examination (plus a few in-situ tests) failed to raise alarm bells, so back together it went.

Using a plug-top power meter, I was able to confirm that the heater wasn't running. Well, not quite. Shortly after starting a program, the heaters do indeed come on. After a delay - perhaps 10 to 20 seconds - they go off, and don't come on again at all.

I took the back off and removed the heater assembly. There are 2 heaters - each one a kW - and there are 3 thermal breakers. There's one per heater, plus a third for the common return of both heaters - presumably a failsafe. And there's a 4th object that looks exactly like a thermal switch, but is actually an NTC thermistor that goes back to the main control box (NTC2).

By (very!) careful probing, I was able to confirm that the heaters were being switched off by the relays in the controller - not by the thermal cutouts on the heater assembly. So, they are under software control rather than some safety cut-out - which means that "garbage in, garbage out" must be the principle to consider...

[Mark Hennessy]

What other sensors does this machine have? Not much, in all honesty. There's another NTC - this time it's in the outlet from the drum, so it's sampling the temperature of the moist air that has just passed through the clothes. I expected to find some sort of humidity sensor in this location, but no such luck. Hmmm...

Apart from the door sensor and the float switch that tells the micro to turn on the condensate lift pump (and pause the program and light the red "empty me" LED when it's full), I couldn't find anything else. So given a lack of a logical plan, I started taking the machine apart a bit more. The lower front panel came away fairly easily, and I took the opportunity to clean up the airways properly. But nothing really doing...

Then I stuck my head inside the drum. Using a torch, I don't know what I was expecting to see, but I noted that there is a section - just inside the door - that is separate from the other plastic mouldings behind the front panel. Wonder why? This section of plastic has two stainless steel strips, but these look for all the world like they're simply there to have less friction than the plastic - but I wasn't wholly convinced!

So upon undoing 3 screws, off it came, complete with 3 wires attached via "faston" connectors to these metal strips!

For a picture, the item in question is here: http://www.amazon.co.uk/Bracket-Sensor-W...B00GCFRYJM

Now, this was the missing link in the puzzle. With no obvious humidity sensor, how was the machine telling when clothes were dry? OK, perhaps you can get so much from the temperature sensors, and presumably these change as the drying proceeds, but now we have something that is clearly able to detect when wet clothes are touching the front of the drum.

As far as the reverse-engineering had gone, I still hadn't worked out what the second control box was doing. As mentioned, it gets mains. There's only 2 other connectors - each with a pair of wires. One pair went to the main controller, and was the transistor side of an opto-coupler, which obviously finds its way into a port of the micro. But the other pair disappeared down the front of the machine, along with the wiring to the motor, and my working theory was that this might have been some sort of feedback from the motor (pulse counting, perhaps?), simply because I had no better ideas! But no - these unidentified wires went to these metal contacts inside the drum.

This makes sense. The main control box is actually live - the PSU doesn't isolate. But the second control box has a tiny mains transformer (plus the opto-isolator to abstract it from the main control circuit). So someone can safely touch the metal strips just inside the door with no issues. I didn't reverse-engineer the whole circuit, but that fits.

[Mark Hennessy]

Anyway, when I removed this "sensor bracket", it was absolutely full of lint...

Also, I found that they'd gone to some effort to insulate one of the two metal strips from the surroundings. Look at the Amazon photo and you'll see a couple of extra bits of plastic - these clip in place to do just that. Only, the smaller of the two was initially missing. I'm glad I didn't get too enthusiastic with the hoover, because I found it buried in the lint.

So perhaps it wasn't installed properly to begin with? Perhaps lint was able to get in there from the off, which is why sister-in-law got such poor results?

So after a clean-up, I tried to reinstall this cover, only it kept popping out. Nothing a bit of super-glue can't fix! The larger cover fits more positively, and this can't be glued because you won't be able to remove the electrical connections... But it's actually secured by the machine itself when the bracket is replaced in the machine

With this done, the machine appears to be working well again. I did a quick test with a wet towel and a damp (not wet) duvet cover. This time, the heaters stayed on for several minutes before going off and staying off. As the towel was still wet, I tried Denim rather than Cottons, and this resulted in the heater staying on for nearly all of the cycle. Well, I could see that the heater was cycling on and off; as there was such a small load, I assume that was under software control...

A full load worked well, with the heat being on for pretty much all the cycle again. The condensate reservoir was emptied at the beginning and was over half full by the end, so that's pretty positive. Let's hope it stays that way.

[Mark Hennessy]

So, if anyone has a modern Whirlpool that seems to work, but is taking far too long to dry because the heaters aren't working, check the "sensor bracket" just inside the door. You just have to open the door and look down - the two metal strips will be obvious. Undo 3 Torx screws (T-15) and carefully withdraw the assembly, disconnecting the wires as you go (unplug the mains first!). Don't lose the small plastic cover in the lint, like I nearly did!

Despite numerous searches, this didn't come up. Even searching with the benefit of hindsight hasn't helped.

This machine has so-called "6th Sense", which presumably refers to how it can tell when the clothes are dry. Lesser machines won't have this, so I presume the equivalent "sensor bracket" has no metal strips. Also, the secondary control box that connects to the bars is clearly something that could be omitted on cheaper machines, saving quite a bit of money.

At this stage, I'm actually unsure about the failure mode. Wet clothes should produce a conductive path between the bars, which goes away when dry, you'd think. So what about the lint? If anything, I'd expect the lint to provide extra conductance, so the heaters should stay for longer? Clearly not. I don't think the lint was providing a path to mains earth, but you never know. Perhaps the clothes themselves are providing a path to mains earth via the drum? Or perhaps all this is just coincidence, and maybe the machine will be back to its old tricks in a few days time? Let's hope not!

What's interesting is that I've seen replacement sensor brackets of a different design, that lack the 2 extra bits of plastic that you can see in the Amazon picture. Doesn't seem like a good idea to me, unless the design has changed to prevent lint getting behind the bracket. Mine was absolutely stuffed full - amazing really.

Anyway, for the sake of a tenner, I've fixed the controller (that costs well over £150 to replace), and for the sake of a few hours, I might have got to the bottom of the long-standing poor drying/no heating problem. Most "normal" folk would have just bought a new machine and be done with it - which saves the stress and inconvenience of trying to repair it - but at least I've saved it from landfill for a while yet. The machine is about 7 years old, so it would have been crazy to have thrown it away. That said, just in case, I did start short-listing suitable replacements - but I'm pleased that I've not had to find a few hundred pounds to buy one just yet...

Sorry for the long story - hopefully breaking it up into separate posts makes it less of a chore. And I hope the search engines are able to make sense of it, because I suspect that other machines must have similar problems.

Cheers,

Mark

[Radio Fixer]

Always nice to get a fix Mark regardless of saving money: its just another small triumpth. Once we had a utility room the tumble drier went as my lady likes the clothes to go out, says they are nicer for having real sea air through them. Then they go on a drying rack in the util' to finish off. Means pegging out, pegging in and racking but I don't argue (just do as I'm told) and get on with it. Sometimes, if I'm busy, she does it so I get a let off and often she does the racking

If it were me I would just have a washer and drier combo and job done.

Gary

[ppppenguin]

I've never had a drier but then for many years I've had a utility area with drying lines. No pegs needed cos it's indoors. If I need something dry in a hurry that's no problem when the heating's on - just bung it on a radiator. On a warm day it can dry outside but there's been a handful of times over the years on middling sorts of days that I've had a problem.

I can see the benefits of a dryer for a family in a house without space for a utlity area. Though of course you pay for it in your leccy bills.

The combo washer/dryers are a good idea but they can't usually dry a full washload which slightly defeats the object. Theya re also rather complex and hence more prone to failure.