I mis-typed the alloy : it is actually Elmedur X.

Graphite pencils, Are basically just thick rods of high density graphite.  Like a pencil but without the wood, and no clay or binder in the graphite.  They are useful in diameters from about 5-10mm.

Okay, the devil is in the details. Skyworks does not appear to to be overly honest on which parts are certified for which purpose in their datasheet. Well, even Chinese friends put it more clearly! For QSOP part the isolation rating is very minuscule (down to 1kV!) so it will not qualify and will be plain dangerous even for basic insulation, which requires a mandatory ground connection anyway.

We of course are using wide-SOIC reinforced 5kV class chips for anything between live and low-voltage circuits, SO8  for functional insulation only.

Still, you are right that digital insulators are at somewhat elevated risk of getting shot-through on a several abuse. TI even has a special application note on this.

That isolator was Si8641, rated as basic insulation (it was in a SSOP package. The same component in a SOIC-W would be rated as reinforced insulation). It was burned internally without visible external damage. Showed some medium resistance on a megger between the sides

(And dead short between VDD and GND on the side that was burned out, but this can be expected)



For your resistor suggestion :

Connect the resistor from the minus side, and route the lamp wires such that they cannot short to each other (so the wires come right up to the lamp holder from 2 sides, and not in one cable). This will reduce significantly the chance of full 12V getting to the resistor

Protect the circuit by a motor overload breaker set to the precise current - like PKZM0-25 or similar

For the resistor construction :

 - As the resistor element use a packaging steel strip. Use the thinnest possible so it have high resistance/unit length. The element will be of large length (about 4m if folded by two in a 2m long duct)

 - The resistor is installed on busbar insulators.

 - Cable duct made of metal with ventilation slots as the outer housing. Install a unit of ~2m length (as much as can fit) under the car

It may be possible to fit 2x or 4x the resistor element running in parallel along the duct. The 4x configuration is the best here, as it allows to fit 2x 4m long resistors (so the resistors for both lamps) in a single 2m long duct

The heat dissipation <100W in a 2m long metal duct is not much, it is not going to get to any high temperatures

 I have seen some of the HPS span-wires in Michigan and YES , they DO suck !  There's even some of the LED versions on the freeway at the entrance/exit to rest areas !

Common practice for rural intersections here is to use one or two cobraheads at opposing corners and then add a spanwire flashing 4 way beacon if it requires extra attention. It works but I suppose it costs more than a single spanwire light.

Spanwire LED bucket lights, especially in 4000K or 5000K sound terrible. Bucket lights have no optical control and they probably should have used a spanwire gumball instead if they wanted to keep the spanwire.

@James
Thank you so much for your very detailed reply! I am very surprised to hear that oxy-propane was used in the industry, i would assume they would use natural gas or acetylene, but I guess not. Oxy-propane is very easily accessible, which is good.

I have never heard of Elmadur X, but naval bronze seems to be pretty cheap on Ebay if I ever find the need to make a pinch seal.

What do you mean by graphite pencils? Do you mean like literal writing/drawing pencil graphite? Or is this some industry terminology?

Nice to know about annealing, I guess I probably won't need to do that.

Thanks again for another excellent quality reply!

According to this PDF file , BLV orange MH lamps of 150W and over, have Na-Li, while 70W have Na only. Why they didn't added lithium also to the 70W orange MH lamp?

1) ChatGPT seems absolutely not trustworthy on such topics!  You can successfully work quartz with oxygen and most hydrogenated gases.  In the old days simple oxy-gas was used.  It has a higher flame temperature than oxy-hydrogen, 2776C vs about 2400C, but the latter has far greater heat capacity.  The flow rate of pumping methane into a gas burner is limited by its flame speed of about 2.2 feet per second : if you try to push in a greater flow of gas to get more heat, the flame will blow itself out if the gas speed exceeds the flame speed.  Hydrogen actually has a lower calorific value than methane (fewer bonds being broken and releasing their energy), but it does have a much higher flame speed of about 18 ft/sec.  So you can pump more of it through the burner and because of that the flame contains more heat despite the fact that its temperature is lower.    For that reason, hydrogen soon came to be preferred among lampmakers for working quartz.  It significantly reduces the heating time, and allows smaller burners than can be more precisely aimed.  Moreover due to the greater rate of gas glow it exerts more force on the quartz.  Glass is normally pulled approximately into the desired shape by gravity, but hot quartz doesn’t move unless you push it.  High velocity flames are very useful to help move it before the pinch hammers do the final job,

However, we always think of hydrogen as being a pretty rough and harsh fuel.  It packs in so much heat so fast that it can be a bit tough to control precisely.  The flames also make a hell of a noise, especially when you have large machines with dozens or hundreds of hydrogen fires.  In recent years many of the best lampmakers migrated to oxy-propane on the machines for their highest value and newest quartz lamps.  For instance all of the Sylvania BriteArc lamps developed by my factory since about 2010 ran through oxy-propane burners.  The flames are so much easier to control precisely, even though the heating time may take a little longer.  That gas may be even easier for you to obtain and will help you pump in the heat more rapidly than oxy-gas.

2) We tried dozens of different materials for pinch hammers over the years.  Carbon, graphite etc do not work because those materials are too soft and brittle.  The hammer is just that : a hammer.  You have to whack quartz really hard to make a pinch seal, and also incredibly fast, because it begins to cool and solidify the instant you stop the flames. For many years Naval Brass was the preferred material.  Iron and steel must be avoided because they stick too much.  Copper is too soft and wears out after making a few lamps.  For about the past 20 years the preference has been Elmadur X, a high copper alloy.  Osram discovered that this has about the best properties for reducing sticking and wear and it’s also quite easy to machine, and within a year or two of them sharing that knowledge most lampmakers were using that or some close equivalent.  Graphite was however maintained for some processes that use rollers to gradually push the quartz into shape, for instance on bulb forming or to roll down the diameter of big tubes before then making a narrower pinch-seal.  It does evaporate over time though.  Well actually it sublimes, it reacts with oxygen in the air and then disappears as carbon dioxide.  You can use graphite pencils if you try shrink-seals, that is what I usually use.  But it has to be extremely high density.  Almost everyone bought from Le Carbone in France, an ancient factory that used to be the old national works for making carbon filaments and arc lamp rods but now produces all kinds of high-tech carbon products.

3. Shrink seals are dead simple!  Your assumptions are all correct.  Much easier to make neat looking seals if you have a lathe.

4. Annealing is not so critical but we always did it anyway for high performance lamps.  After sealing in the electrodes but before dosing, they would go into a vacuum furnace at usually about 1800C for somewhere between 6 hours and 24 hours.  Most factories do not bother though because the vac furnaces for such high temperatures are hellish expensive.  I don’t know how much, but remember that every couple of years we had to replace their molybdenum heating elements and those cost something in the region of €100k each.  For your purposes, it would not be necessary.  Also in the factory our main reason for vac furnacing was not so much for annealing as to reduce the concentration of hydroxyl ions in the quartz.  That brought a dramatic improvement in the life of all metal halide types.  With pure mercury arc tubes we had a faster in-line process based on flaming the whole arc tube in soft bushy hydrogen flames while blowing argon or hydrogen into the lamp bodies.

Back to topic, browsing through the parts for work I found out that thermal fuses for some high temperatures (say 150+ degrees C) and high current of 20+ Amps are easily available. So making a ventilated metal box with a couple of 100W resistors (some power margin) and strategically placed thermal fuses will solve the problem cheaply/easily/safely and with zero worries about the interference to car electronics. Added benefit is that you are not limited to 100/50% power ratios.

https://www.setfuse.com/Products/Over-Temperature-Protection/Thermal-Link-OTCO-Organic-Type/RP-series.html



 

If I remember right, 82V is a rating for half-wave rectified 120V, for some really cheap designs consisting only a lamp and a diode.