Agile Resistor?

BinaryMike

Pelagic EE
I used to buy lots of components from a local military surplus dealer because it was a good way to stock up on high quality parts that would have otherwise been far beyond my budget at the time. I happened to use four 100R 2W(?) wirewound resistors from that old stock recently, and learned a lesson the hard way: There are (rarely) ordinary-looking resistors that are deliberately manufactured with a large temperature coefficient! This example increases its resistance from 100R to about 120R when dissipating 0.6W. I checked six randomly chosen samples out of a bag of 100 or so and confirmed a similar resistance change in every one. I could induce the resistance change with a heat gun just as well as putting current through the samples. And they're definitely wirewound --- I cracked one open to make sure.

[100R] 001.JPG
 
Are they really mil surplus resistors from 1967? And I thought I had some old stock...

The leads are in remarkable condition if they are that old.
 
Are they really mil surplus resistors from 1967? And I thought I had some old stock...
The leads are in remarkable condition if they are that old.
I would have bought 'em in the late seventies or early eighties at the latest. They do still look almost like new --- probably from storage in a thick poly bag.
 
So this temperature co-efficient, PTC effect on wirewounds. Tell me more. Is it nichrome windings and only on low value/low wattage wirewounds that are designed that way?
 
Well, WW resistors are normally wound with a nichrome alloy that has near zero temperature coefficient. The subject resistors have clearly been wound with an alloy that has a large positive coefficient, but I haven't done any research to discover if that's likely nichrome or something else. It looks silvery. I've been involved with electronics professionally in one way or another since the early seventies and never encountered another example like this. The closest thing I'm aware of is copper wirewound resistors used for compensation in open-loop DC motor speed control circuits, where the copper's TC matches the motor's copper windings. The ones I've seen are indistinguishable from little RF inductors.
 
Back
Top Bottom