Lead-free solder is such a PITA (rant/whinge)

Missed that bit. Around 1600 ohms.

Have you looked into another valve company ?

I retired a few years ago from a large company and we had hundreds of valves similar to that and very few of them failed, especially the coils. Most did operate on 120 volts.

Not sure how the valves are in your oven, but maybe you could try 2 of the 120 volt coils in series.

Assuming 240VAC, that's: P = E^2 / R = 240Vrms^2 / 1600 = 36 watts That's way too high dissipation for a solenoid valve. Are you sure those are 240VAC solenoids and some other AC voltage? Are the solenoids run by 240VAC or some other voltage?

Also, could you check the eBay listing number? I want to see the coil specs. Nothing found:

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Jeff I think you are mixing up the AC and DC power formulas.

I am sure the impedance of the coil at 60 Hz is more than the DC resistance.

That would make the wattage less than what you have calculated using a DC equation.

A quick check of the Goyen catalog shows they draw about .05 to .07 Amps depending on the coil at 240 volts. That is around 10 to 16 watts. The DC resistance of the coils was not given. The coils on many of the valves can be changed to several voltages.

The DC resistance averages around 1600 ohms. According to my calcs there are around 4700 turns in the coil, so at 50Hz the impedance incorporates a significant reactance. The coil (obviously) has a ferrous valve plunger in its core.

They are marked 240VAC 50Hz (= our supply here in Oz) and the one I attacked with the Dremel is marked 5W.

No

Of course at 120V the wire would be twice the diameter. If it is corrosive flux at work, it would take longer to eat it away. If it is failing Pbfree solder bonds, who knows.

I am considering that. Meanwhile SWMBO is considering a replacement oven (which as you'd all know, means a complete new kitchen)

Oops. Y'er right. Big rush -> No time -> No brain -> No excuse.

Ok, that's better and less than the 5A fusing current for #34 awg.

10 - 16 watts might get the coil quite warm if energized continuously. Probably not enough to melt solder, but certainly will add a few degrees to the heat from the furnace.

I don't think it's overheating from the furnace or the warm coil. It will get hot, but not enough to melt solder. Since various other solenoids have failed in the same manner, I don't think the failures are caused by some kind of soldering defect. The long time that it takes to fail might be the copper wire slowly dissolving in the solder. However, my limited experience indicates that most of the damage occurs immediately during soldering, not many months later. With a 5A fusing current and the rather large inductance of the solenoid, I don't see a high current "surge" fusing the #34 wire.

Offhand, I was thinking something else might be happening here. Something like a sharp edge on the solenoid terminal slowly cutting its way through the copper wire every time the solenoid is energized. It might be 50 Hz vibration work hardening of the wire causing embrittlement. However, these are unlikely to have also occurred in the other replacement solenoids, that also failed. It might be the difference in coefficient of thermal expansion between the epoxy and the copper wire. Usually, some paper or tape wrapping will provide room to expand, but again it would be an amazing coincidence if the same problem appeared in a competetors solenoid.

That leaves external influences, such as line voltage glitches and surges. Get your Dranetz power quality monitor/logger back, borrow something, or just setup a data logger on the AC line.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Sorry, that should be oven, not furnace (in multiple places).

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

e solenoids run by 240VAC or some other voltage? "

Ummm, where is the "L" in your equation. If it is 1600 ohms DC resistance i t is likely to have considerable inductance. There is also usually some typ e of metal core which takes it even higher.

That reminds me of a time I threw together a thermostatic control for a win dow unit air conditioner in a hydroponic grow room. I put the box together with a little transformer and a relay, a DC operated relay. Come to find ou t once it started it would not stop. I used an electronic thermostat so the re was no worries about leveling it or anything, plus he liked the ability to set the timer. Turns out the SOB used a small triac and with DC it would never shut off so I had to start over. Wasn't so bad, just get a different relay and eliminate the rectifiers and filter.

But, the DC relay would not run on AC, and that had to be because of the in ductance. I wound up using one for an air conditioning condensing unit, you know, like in back of the house. Typical 24 VAC job, which is what the the rmostat was designed for. Put a long ass wire on it too because he wated to be able to place it wherever. he had something like 4,000 watts worth of l ights and the thermostat has to be a bit away from them, and also away from the window unit.

In the end it worked but I had learned the hard way you just can't intercha nge relay with DC and AC coils. And damn, 1600 ohms is alot for just about any wire. Much more than that and it almost ain't wire !

I just posted something to that effect not having seen your post. I know the guy is smart and I guess he just had a blonde moment.

WE had hundreds of that brand where I worked and some were pulsed every

30 seconds or so and some were almost always on. Hardly any problems with the coils. The ones that were on most of the time ran hot enough that you would almost burn your hand. Some of them were where the room temperature was over 100 deg F. The plant made polyester and the process required about 300 deg C of heat. I said room, but those areas were not occupied except to check on the equipment from time to time.

Most problems were either the rubber disk wearing out or the plunger enlarging due to all the beating they take or the spring wearing out. The plunger is enclosed so no way for it to touch the coil form. While it could be something in the coil, I would think that as many that he changed out it must be something external.

One other thing, is the coil all the way seated on where the plunger is ? If not it could be drawing enough current to burn out the wire. We often left the coil hooked up to the conduit and just changed out the mechanical part. Mainly did that to keep from having to go to another floor and find the power source for the coil. Found that we needed to stick a large screwdriver or other item in the core of the coil or a fuse would blow or the coil would burn out.

Not that smart yesterday. I was on hold with a customer trying to walk them through a messy problem. I'm easily distracted by paying work. I also have a problem with some meds. In the morning, I take a sustained release vasodilator, which initially causes a rather radical drop in blood pressure. When that happens, the brain goes offline or is reduced to operating at the speed of a snail for a while. I just took a pill for breakfast and am posting drivel while waiting for my ISP's support people to call me back, something that has been rumored to happen at geological intervals.

In addition, the OP reverse engineered the oven wiring and produced a schematic of sorts. Generally quite good but a few mistakes. One was showing an SCR keying the solenoids instead of a Triac. If it were an SCR, it would rectify the AC going to the solenoids. That would produce some DC on the solenoids which would raise the dissipation. However, I didn't catch that at the time and was still concentrating on the DC part of the puzzle. Somewhere along the line, I simply forgot about the inductance.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I meant outside of the oven enclosure. Bypass the valves with plumbing in the enclosure and then put the valves somwhere else in the gas supply line. Eric

A triac can also rectify, if the gate has become insensitive (or too responsive to noise). If you use the triac to run an AC relay, and the relay to activate the valve, the valve is immune to rectification (and if the relay burns up, it's cheap to replace).

Or, just replace the triac on suspicion

Even a small amount of rectification in the triac circuit will walk the magnetic core toward saturation and remove inductive back-EMF. Next event: the coil burns up.

Yeah those pesky paying customers. Why can't they just pay and leave us alo ne ? LOL

Those vasodilators can have side effects. But if you take one a day you cou ld take it after supper, or even cut the pills in half. If you have a BP me ter you might want to consider lowering the dosage is you are having side e ffects.

You know I am not an MD and I know you are not stupid enough to blindly tak e by advice, but I believe they pay too much attention to BP these days, at least some doctors. Think about out ancestors on the battlefield with swor ds and shields fighting for hours ad hours, what was their blood pressure 3

00/200 ? According to some doctors they should have just dropped dead. A la te friend of mine who got me into doing the research on nutrition a long ti me ago postulated it might be a copper deficiency. And believe it or not, h ypertension can also be caused by a sodium deficiency. The sodium in normal table salt is not absorbed and metabolised by the body because of the anti

-clumping agent, which might also inhibit other minerals' proper usage in t he body. If you use salt, get unrefied sea salt if possible. I got two kind s, one is from the dead sea and one is from up north, I usually mix them.

At least get the kind that clumps up. Evidence - back when salt would clump up in the summer and you had to put rice in it, doctors were not telling p eople to limit their salt intake. If you already use Kosher salt you might not have that problem but the unrefined stuff is still better because of th e mineral content. Most chronic conditions are caused by mineral deficienci es. Not all, but enough to make it a concern.

I think last time I went in my BP was like 140/70. The big spread indicates the heart is working well. When the spread goes down that is a bad sign. W hen it goes under 40 there might be a problem. I have talked to an MD about this. Also, some would think 140/70 is high but they did not say a word ab out it, or even suggest any medication.

i use very little salt these days. For one this unrefined stuff tastes diff erent. It is like it is not as strong, but it is easier to oversalt things. Take and put too much regular table salt on something and it's like oh wel l, but this stuff it is like UGH.

Take a spark plug wire off your car and...

Nevermind.

Good idea. However, I just took a 4th look at the schematic. There's a diode and 10K resistor from the gate to ground. Those are fed from a CMOS gate going through a capacitor (value unknown). I may have been wrong, but this thing now looks like an SCR instead of a Triac. If it is an SCR, the current through the parallel solenoids would have a rather high DC component. (Yeah, I know... indecision is the key to flexibility).

Pedro... could you post your schematic somewhere or could you give me permission to do so?

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Yes that is a possibilty, but in our over-regulated environment it'd require all sorts of recertification, almost like the aviation/aerospace industries. Having been exposed to THAT industry it's a path I don't want to tread out of MY wallet.

Ralph, should I presume that was with pre-ROHS vintage coils? As I mentioned in my O/P, the original pair (which were pre-ROHS) both lasted two decades. Subsequent ones were 15-18 months AND the oven gets less use nowadays as the kids migrate outwards. Not a statistically significant sampling for some, but sufficiently damning from where I stand/sit.

We haven't had the moving parts show any issues, just coils.

Nothing external has changed, except our supply voltage has been "harmonised" with rest-of-Oz i.e. reduced from the original 254V rms down to 240V rms.

Appreciate the question. It is seated down properly. The frame which sits over the coil/valve assembly as part of it is actually a snug fit (you need to slide the coil into it and align the holes, it's a press fit) and then the frame is held down over the valve plunger "cylinder" and screwed down. No unintended magnetic/air gaps.

There is also a "spacer" pressed into each end of the coil bobbin the ensure it is located correctly (radially).

Pics of the valve assemblies in situ:

and of the coil post-Dremel (*):

(*) I needed effective bobbin length/diameters to calculate turns and hence deduce a credible wire gauge to match the known DC resistance.

It's now at

alongside the pics.