exploding wirewound resistors

My problem with bulk resistors is that they are bulky. We're putting

32 SSRs on a VME module, with vibration requirements.

Looks like a Vishay 5-watt wirewound is reliable at 17 kilowatts, 1 msec, 17 joules/shot, 0.1 Hz. After a day or so more, I'll crank the voltage up and see what happens at higher energy. I think our FPGA can shut off the mosfets before 5 joules. We won't even promise to survive that kind of abuse.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I think JL wants a smaller R. But searching DK under ceramic R's I found these cement resistors.

Tempco stinks.

George H.

I'm pulsing the Vishay 5-watt axial wirewound now at 22 kilowatts for

1 millisecond, 0.1 Hz. There seem to be no indications on a data sheet of how resistors will survive extreme pulsing; just gotta try it.

I did blow the top off a Vishay 5-watt power-pad metal plate resistor.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It might make a joint, but 'normal' operating temperatures would crack the solder. One either hard-solders (silver/copper alloy) or welds to make a good electrical connection. Welding is cheaper.

What about titanium. I have some sheets. Absolutely amazing metal. Extremely difficult to work with. Doesn't want to weld or join with any other material.

Very good high temperature performance. Used in the SR-71. Extremely sensitive to chlorine embrittlement. Needs to be protected.

Could probably make good high surge resistors. The problem would be the welds.

It does burn rather well if it gets too hot.

John

Right. Good point.

George H.

Not too well in air, if it's thick (or until you get it /really too hot/..). Titanium is forged regularly, just like steel -- though not quite as easily!

The bright white grinding sparks are characteristic. :)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Well, you need to use inert gas or vacuum, of course; fluoride fluxes should do for soldering. Even under that atmosphere, you need to dispose of the existing oxide layer, whether by grinding, fluxing or some other (plasma?) process. Then you can try sticking things onto it.

There are plenty of metals that get along with titanium; I would think, if nothing else, an interface (diffusion barrier) layer can be made with Cr, Mo, Ni, maybe Al, or a PGM like Re. If Ni is good enough, Fe may be good enough (they are neighbors), in which case it could be bonded directly to that. Al might be fine, simply because its melting point is so low that it would be more like soft soldering.

I wonder what the tempco is like. Probably, much like with iron and nickel, there are alloys with stable resistance and acceptable ductility.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Surface-mount (plastic encapsulated) wirewound resistors seem to cook near their center and explode or crack near the middle. Axial wirewounds usually blow up at the end caps, probably at the weld. Welding the nichrome to the end cap seems to make a weak/hot spot or a stress point.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I'm pretty sure you can (in vacuum) e-beam weld titanium. To titanium, at least. I've often wondered if it would make a good soldering iron casing.

If it doesn't join to other metals, that just makes tip replacement easier. The pretty gold-color nitride... makes a passivating layer against oxidation, I hope?

That makes sense. My (old) truck battery wasn't charging properly, someone (maybe me) had put a steel lock washer in series with the charging connection. I chased it down with a volt meter. Moved the lock washer. Getting things in the right order can be important! (I've got more lock washer stories, but they cost a beer. :^)

In theory you want a big volume of resistive material.

(I've got this dream of a instrument to measure the thermal and electrical conductance of some stack of of materials. I'm guessing I'd mostly learn about contacts.)

George H.

Titanium (in my brief experience) was a magical interface metal. George H.

I think nickel is a better choice for this application. It is very easy to coat using electrolysis and it loves to coat with solder. The only problem is sourcing. I bought a bunch of pure nickel strips on ebay supposedly for spot welding AA batteries. They all turned out to be steel with a thin coating of nickel. Waste of money.

So I ordered some pure nickel ingots and sheets but I haven't had time to test them yet.

The idea is to make a Weller Gun soldering iron tip replacement with nickel coating to minimize the lead solder absorption and destruction of the copper tip. The homemade tips heat up in one or two seconds, and making a replacement tip from #14 gauge copper takes only minutes. But they don't last long, so nickel plating seems to be a good idea. It works well in other tips, so I expect the same in the Weller.

The game with the Weller is you can dump 140 or 250 Watts in a very small area very quickly. This is excellent for soldering heat sinks, grounding, and other hard to solder things wihout having to pull out the massive soldering irons that take forever to heat up.

I also have a soldering iron tip that attaches to a butane torch. This sort of works but it is difficult to control the temperature. Also it is difficult to keep the hot gas from the flame from destroying the insulation on the wires you are trying to solder or other flammable items in the vicinity.

el

rt

on

I can't think of anything I'd use a gun for rather than an iron. It's only quicker if you're sure you're only going to solder 1 or 2 joints, which is seldom the case. Their soldering power is the pits compared to a 60 or 120w iron. Mine sits idle decades after decade.

The gas torch is useful for desoldering, though the solder pot is less dest ruction prone.

NT

How can you tell the nickel from iron? They're both ferromagnetic (test on a Canadian dime). Density of iron 7.87 g/cc, of nickel 8.91, so I suppose you could measure and weigh...

Scrub the strip with a knife to remove any nickel coating. You really can't tell the difference between nickel and steel at this point.

Put the strip in a salt solution and leave it overnight. Iron rusts badly. Nickel is not affected.

Correspondingly: iron bubbles in acid (HCl or H2SO4), nickel dissolves extremely slowly (H2O2 helps -- it's almost as noble as copper). Nickel forms an emerald green solution (in either acid).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com

Good to know. Thanks. I don't like to deal with HCl, and I almost killed myself with H2SO4. I will never deal with it again, and I used HCl only rarely and under severely restricted conditions.

Another safer method is to immerse nickel in vinegar and make nickel acetate which you are going to need anyway for nickel plating. Set a current limited power supply to a suitable current. Attach the positive lead to the nickel and the negative lead to any conductive metal. The vinegar will turn green. Here is an example:

Now it is ready for plating. Change the cathode (negative lead) to the item you want to plate and adjust the current limit to a suitable level. Plating starts immediately:

The problem with the pure nickel strips from ebay is the nickel plating wore off quickly and the solution never turned green. I was left with the base steel which was useless for my application.