Need help with Nernst lamp.

[Buk]

Another thought. If you wrapped wire around closely, you get a contact pattern as shown here in red

If you oversoldered it -- whilst it wouldn't wet the ceramic -- is would fill (some of) the voids and achieve a far greater area of contact.

Maybe overkill, but you might as well try and get as much of your current to flow through the ceramic, rather and just generating local hots spots.

Its a thought.

 

[shortbus=]

What voltage/temp do I need to make it glow?

Did you read the Wiki link? From the link -
" The Nernst lamp was an early form of incandescent lamp. Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence "

 

[RayRay1132]

Did you read the Wiki link? From the link -
" The Nernst lamp was an early form of incandescent lamp. Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence "

I know that. I just meant what temp does the ceramic need to be heated to to make it begin conducting? I talked to the seller and he said that nichrome wire doesn't get hot enough. Any ideas would be greatly appreciated.

 

[Buk]

I *think* this paper contains the information you need regarding conductivity versus temperature; but everything is measured in terms of eV and K, so it is heavy reading.

It does state:

This shows thatthe transition between the two activation energy regimes is very broadand that only temperatures far above 1000◦C reflect the true hightemperature activation energy without any influence of the bending ofthe curve. Such high temperature data are often not available and not surprisingly the reported supposed activation energies Ea are often too large.

Which I interpret to mean that it will not transition to incandecence until you reach 1000°C.

 

[shortbus=]

Do you not understand what incandescence is? You get the ceramic to what ever temperature it takes to THEN apply the electric to it. There is no standard temperature for all items that I'm aware of, it takes what it takes. This is the reason those lamps were never going to be used in any real number.

 

[Buk]

I just meant what temp does the ceramic need to be heated to to make it begin conducting? I talked to the seller and he said that nichrome wire doesn't get hot enough.

The NiChrome won't get the YSZ hot enough to incandesce on its own, but it should get it hot enough (~600°C) to allow the YSZ to start conducting enough that the (resistance to the) flow of current through the YSZ will start raising the temperature further.

If I were attempting this, I try a setup something like this:

Heater element consists of 60 turns (around a 2mm former) of 0.5mm NiChrome wire + 20mm & 50mm tails for a total length of 536mm.
(It's intended to slip inside the cruciple.)

The wire has a resistance of 5.551Ohm/M so the resistance of the coil is 2.9786 Ohms. Call it 3 Ohms.

If you put 30V through that you get a current of 10A; and a power disipation of 300W or 300J/s.

The cruciple has a volume of 420.97683694mm³ and the ebay page lists the density as 6.02g/cm³ giving it a mass of 2.5342805583788g, or 0.002534kg.

According to this pdf YSZ has a specific heat capacity of ~45000 J/Kg*°C. (At 600°C)

45000 * 0.002534 = 114.03J/°C

Which -- ignoring losses -- means the heater element ought to raise the temperature of the cruciple by a bit under 3°C per second.

So around 3 minutes to reach 600°C.

At that point, you'd probably need to switch the heater off and hope that the YSZ will conduct enough to continue raising the temp.

 

[RayRay1132]

The NiChrome won't get the YSZ hot enough to incandesce on its own, but it should get it hot enough (~600°C) to allow the YSZ to start conducting enough that the (resistance to the) flow of current through the YSZ will start raising the temperature further.

If I were attempting this, I try a setup something like this:
View attachment 132862

Heater element consists of 60 turns (around a 2mm former) of 0.5mm NiChrome wire + 20mm & 50mm tails for a total length of 536mm.
(It's intended to slip inside the cruciple.)

The wire has a resistance of 5.551Ohm/M so the resistance of the coil is 2.9786 Ohms. Call it 3 Ohms.

If you put 30V through that you get a current of 10A; and a power disipation of 300W or 300J/s.

The cruciple has a volume of 420.97683694mm³ and the ebay page lists the density as 6.02g/cm³ giving it a mass of 2.5342805583788g, or 0.002534kg.

According to this pdf YSZ has a specific heat capacity of ~45000 J/Kg*°C. (At 600°C)

45000 * 0.002534 = 114.03J/°C

Which -- ignoring losses -- means the heater element ought to raise the temperature of the cruciple by a bit under 3°C per second.

So around 3 minutes to reach 600°C.

At that point, you'd probably need to switch the heater off and hope that the YSZ will conduct enough to continue raising the temp.

Thanks this is very helpful, especially with the picture.

 

[Buk]

Thanks this is very helpful, especially with the picture.

Two things to note:

1. The turns of the heater coil should not be touching. They aren't in the picture, but it doesn't look that way at the angle posted:
   
2. The solder blobs are left over from the earlier post -- where they were intended to improve contact when measuring resistance at lower temps -- would drip off long before you reach 600°C; so best omit them if you see progress to higher temps.

 

[RayRay1132]

Two things to note:

1. The turns of the heater coil should not be touching. They aren't in the picture, but it doesn't look that way at the angle posted:View attachment 132863
2. The solder blobs are left over from the earlier post -- where they were intended to improve contact when measuring resistance at lower temps -- would drip off long before you reach 600°C; so best omit them if you see progress to higher temps.

View attachment 132864

What kind of wire would you recommend for the ends?

 

[Buk]

According to **broken link removed**20:80 NiChrome wire should be good to 1200°C continuous, So I'd use that there also.

 

[Buk]

Not ceramic and not NICHrome, but the shaft is 5mm diameter and the wire is 0.5mm:

RL is never as neat as simulation, but with a little care it is possible to get the wire tight enough that the resistance between the wire and shaft is <0.1 Ohms. (The lowest my MM will measure.)

Even without oversoldering, and despite that the soft iron gardening wire has an antirust coating that would have taken longer than I was prepared to spend scraping off.

If you wanted to really do the job right, you could over solder the NiChrome with 80% Silver Solder and a blowtorch. That doesn't melt until 1370°C, and doesn't flow till well above that, by which point the NiChrome would be done for.