WR overlay designation for a fusible resistor?

Who can tell ? Why "Q" for a transistor ? Why "U" for an IC ? Why "VDD" ? Why "VSS" ? I bet if you think for a while, there's loads of these that don't 'appear' to make any sense, but must have to the o.e.d. I don't think that there is anything particularly special about fusible resistors in terms of the materials used to make the resistive element. As far as I know, they are still carbon or metal film, with a completely non-flammable coating. I think it is probably the internal connections that are 'necked' or something to make them deliberately vulnerable to excess current, or heat in the substrate, generated by excess dissipation.

Arfa

Wimpy resistor ?

Graham

By whom ?

In the UK we legend ICs as 'IC3' for example ! Too obvious ?

Now tell me a transistor is a 'Q' in the USA ! We use 'TR'.

Graham

Yes, modern 'fusible resistors' are typically made using metal film over a ceramic former and covered with a 'cement' coating. All of which is non-combustible and flameproof..

In comparison, carbon film covered with laquer is very combustible.

Graham

Fusible resistors are what it says on the tin - a resistor and a fuse. The MO or metal bit and then the fusible bit, so avoiding the possibility of seriously high temperatures building up in a fault condition, rather than just non-combustibility. I've only ever scraped one apart, long ago , out of curiosity, and there was a definite fusible part that I remember as a metalic? blob rather than a necking. Unfortunately I did not know of Wood's metal, then, and didn't try heating it up. I'll try scraping back a 2.2 ohm one, tomorrow.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

:W for Wood's metal for the fusible part ? :Googling "wood's metal" AND "fusible resistor" produces nothing

Can't say I have ever heard of Wood's Metal being used for fusible resistors in electrical or electronic equipment. I seem to remember that a major use for Wood's Metal was as the fusible element in thermal break-circuit fire detectors used back in the 30's - 50's. They were superseded by much better detectors.

There certainly used to be a pellet of Wood's in the 'daisy' fire sprinklers. I don't know of that is still the case. It's a simple, reliable, and more to the point 'on-the-spot' and unpowered system. The lid of my pressure cooker still has a Wood's metal pressure pressure relief valve, which coincidentally, I replaced just last week.

Arfa

Weak resistor ?

How about "CR" for a diode ? Copper (oxide) Rectifier, maybe ?

Arfa

reliable,

I scraped off one of a batch of 2.2 ohm 1/3W , called fusible resistors and heated with hot air gun to 180 deg C with no fusing anywhere. MO over ceramic construction with spiral cut into the oxide starting and ending 1/10 way in, so highest risistance in the axial central area between the 2 cuts in that area , so any excess current failure would be in that section. So little more than a standard MO resistor , but not having the continuous spiral. The scraped off coating seemed no more than slightly olive coloured varnish

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

:> :W for Wood's metal for the fusible part ? :> :Googling "wood's metal" AND "fusible resistor" produces nothing :>

:>

:> Can't say I have ever heard of Wood's Metal being used for fusible :> resistors in :> electrical or electronic equipment. I seem to remember that a major use :> for :> Wood's Metal was as the fusible element in thermal break-circuit fire :> detectors :> used back in the 30's - 50's. They were superseded by much better :> detectors. : :There certainly used to be a pellet of Wood's in the 'daisy' fire :sprinklers. I don't know of that is still the case. It's a simple, reliable, :and more to the point 'on-the-spot' and unpowered system. The lid of my :pressure cooker still has a Wood's metal pressure pressure relief valve, :which coincidentally, I replaced just last week. : :Arfa :

All of the sprinkler systems I have looked at in the past 50 years or so use the glass bulb which breaks due to expansion thus allowing the plug sealing the outlet orifice to pop out under water pressure.

You are correct regarding the pressure safety valve on a pressure cooker. It does have a slug of low melting point alloy but not sure if this is classified as Wood's Metal, although it would probably be very similar. Wood's Metal appears to have set proportions of bismuth (50%), lead (25%), tin (12.5%) and cadmium (12.5%) designed to melt at approx 150 deg F.

Crystal Rectifier (as in your original crystal set). Followed by point contact diodes.

Geo