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

Most of them were from days before the lead free junk.

When you pulse them about once every 60 seconds they do take a beating after 5 or 6 months. When they were first used in that application there were some complaints about how fast they were wearing out. However if you calculate the number of cycles , they lasted many more cycles than specified for by the manufactor.

You mentioned something about lowering the voltage from about 250 to 240 volts. Have you checked to see if the voltage is still around 240 ?

Duh. I thought C106D was a capacitor or typo error. It's a real part and most definitely an SCR: I could calculate the amount of heating from the DC present if you would kindly risk your life and obtain a scope waveform of what is present across the coil while energized.

I don't see any sign of heat damage around the solenoids in your photos. (I don't like using the gas line as an electrical ground, but that has nothing to do with the problem). The white wires connected to the spade lug connectors would have turned brown by now if there had been any overheating of the terminals. Therefore, my guess(tm) is that the solenoid terminals are NOT getting hot, which would certainly happen if they were melting the internal solder connections.

Measuring the terminal temperatures during operation would offer a clue. If they are anywhere near the melting point of RoHS solder of about 220C, it might be the culprit. However, since RoHS solder melts about 40C higher in temperature than leaded solder, I just don't see it. If you need some entertainment, try heating the terminals of one of the failed solenoid coils with a really hot soldering iron and see if you can reflow the solder inside the solenoid.

In other words, I don't think it's a solder problem.

Time for yet another guess(tm). I keep going back to the SCR. Besides the rectified AC problem, if there's any oscillation or funny waveforms feeding the SCR, the solenoids are going to get hit with some fast rise time pulses, which could produce some rather large voltage spikes in the solenoid. Tiny gauge wire means thin insulation which could be penetrated by the voltage. #34 single layer is good for 1500V. A high voltage spike could produce an arc between turns, followed by a shorted turn. Shorted turns really increase the current drain of an AC solenoid, which could be sufficient to fuse the #34 wire (5A).

If my guess(tm) is correct, then the problem will persist with almost any solenoid that uses thin #34 gauge wire. So, something must be done to prevent arcing between windings. Since the pulses are at a much higher frequency than 50Hz, methinks a small capacitor or snubber across the coil will take care of that. Maybe a spark gap across the capacitor in case the voltages are really high. Too many joules involved for an MOV to work.

Note: This was written before taking my morning vasodilator pill and is therefore presumed to be relatively coherent and sane.

--
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 didn't see any heat damage in the latest photos. Judge for thyself:

--
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

It is.

Western Australia's grid voltage was 254(+/-6%) as a result of the move from 40hz to 50Hz a long time back. Actually the single phase voltage was never *defined*, rather it was 440V phase-phase (3-phase system).

Although it was/is not interconnected with the eastern states' grid (which was 240V as far back as the landing or the Ark), pressure to fall in line was brought to bear, particularly as appliances etc made for the 240V system suffered reduced lifetime at 254V, although there were incandescent lamps made for 250V if one hunted them down.

So the supply authority relented and cranked the grid back to 240V where it has been ever since as a statutory requirement.

Fine on the history of Australia's grid. Too bad all the coutries of the world did not set one standard to go by. Most of the US seems to be around a split system for the homes of 120 and 240 volts at 60 Hz. There are some places that may have other odd values.

I worked in a large plant and most of the larger equipment was 480 volts

3 phase. The smaller equipment and offices were set for 120 or 240 volts.

Then we built a building of about 6 floors and while another and I were calibrating some 3 phase heaters we could only get about 380 volts out of the controler. Checked the incomming voltage and that was what we had. Seems that the engineer had bought some equipment from a country that required 380 volts. We were not told of this to start with and found out about it when we were checking out the equipment.

Years ago in the US it seems that the standard was 110 and 220 volts, then 115 and 230, now it is 120 and 240 volts. I hae a digital voltmeter on my ham radio desk that stays on all the time. It is usually around

122 volts, but I have seen it between 118 and 124 volts depending on the time of the year and day. I know it is accurate as I checked it against some meters at work that were calibrated by a lab to the government standrds.

Certainly is. Been around a while too.

Unlikely. Solenoids are cheap, I only have one life to give and it isn't going to be for this cause, however noble from the scientific research pov.

The original pair were showing darkened areas on the brown stuff, but that was two decades worth of higher utilisation.

There is quite a bit of earth bonding in there, excessive in my view, but I guess paranoia never killed anyone.

Typical oven operating temp is 200C. Some occasions are 220/230 for

30 minutes. Coils have limited prospects for ventilating airflow, and self-heating to boot, so they would reach ???

(snip)

When I have one of these failures, need to remember the characteristics. They go O/C when hot, with normal program resuming as they cool (at say 125C oven as a WAG). What I then do, after the mandatory outburst of expletives, is grab my diagnostic lead which I recall referring to earlier. This interposes a small naked 6V incandescent lamp in series with each coil. Then I fire the oven up again and wait for one or the other lamp to extinguish as the coil reaches the failure temp again. I do NOT recall the current spec of the lamps but suspect it is 50mA. They glow with decidedly equal brightness until one is interrupted, so any inter-layer etc shorts must also be transient.

If it *is* arcing, it reoccurs with regularity on these post-failure test runs - or not at all - as it would possibly take out the lamp or at least show substantially increased illumination prior to the return of O/C mode.

(snip)

I'm not supporting that hypothesis at this stage.

tiny wire is inherently unreliable

adding a transformer and using 24V valves is a good idea.

or use 120V valves and wire the 2 in series instead of parallel?

m