two power resistors as element

[Bruce Raymond]

One thing missed in all of the discussions so far is that once
the water reaches 100 degC you have to dump in a *lot*
more heat to change its state from liquid to gas. I'm too
lazy to go look up the energy required for the state change,
but it's substantial.

- Bruce Raymond


"Roger Hamlett" <rogerspamignored@ttelmah.demon.co.uk> wrote in message
news:N4UPa.18390$4O4.2014460@newsfep2-win.server.ntli.net...

"nospam" <nospam@nospam.invalid> wrote in message
news:arvvgvk2na19ntkqa0di7leovv3rc4sr25@4ax.com...
Wolfgang Mahringer <wolfgang.mahringer@sbg.at> wrote:

Despite of what the others write: your resistors might even survive:
the water conducts heat much better than the air for which the
resistoir was designed.

That's not really true.

You have 2 thermal resistances:
i) Resistance wire (or body) to case
ii) Resistance case to ambient.

Resistance i) is _always_ the same, and mostly larger than resistance
ii)
Only resistance ii) is influenced (greatly reduced) by the water
"cooling".

And if the water heats up (presuming the resistors survive that long,
which i doubt, btw) the resistance ii) rises, because of the lesser
temp difference between resistor case and the fluid.

 

[nospam]

"Roger Hamlett" <rogerspamignored@ttelmah.demon.co.uk> wrote:

So the moral is even with oil cooling compact wire wound resistors don't
much like running with a 2500% overload.

I'd not expect
any design to get much beyond perhaps 10:1 (you were pushing 26:1 with your
experiment...).

Well yeah I know I was pushing it, some here seem to think the resistors
will vaporise as soon as you switch on.

I already ran the same resistor at 10 x rated in water and predicted it
would eventually fail from lead corrosion.

For 26 x rated I figured there would be too much localised boiling so went
for oil which likely offers significantly less cooling.

The resistor is wire wound on something then coated with a thin layer of
cement or ceramic loaded paint or something.

I would like to have tried one of the green glazed ceramic types, but
didn't have a suitable value and power supply kicking around.

 

[Roger Hamlett]

"nospam" <nospam@nospam.invalid> wrote in message
news:k3j0hvkjc1mtspvud6f9cl7a3kd5adlkhj@4ax.com...

"Roger Hamlett" <rogerspamignored@ttelmah.demon.co.uk> wrote:

So the moral is even with oil cooling compact wire wound resistors
don't
much like running with a 2500% overload.

I'd not expect
any design to get much beyond perhaps 10:1 (you were pushing 26:1 with
your
experiment...).

Well yeah I know I was pushing it, some here seem to think the resistors
will vaporise as soon as you switch on.
Yes. You will see, that with the right resistors, I am one of the people who

believe it is probably possible, though like you, also expect corrosion of
some form to be the most likely failure if the overload is 'reasonable'.

I already ran the same resistor at 10 x rated in water and predicted it
would eventually fail from lead corrosion.
Fair enough.

For 26 x rated I figured there would be too much localised boiling so went
for oil which likely offers significantly less cooling.

The resistor is wire wound on something then coated with a thin layer of
cement or ceramic loaded paint or something.

I would like to have tried one of the green glazed ceramic types, but
didn't have a suitable value and power supply kicking around.
You should try a much bigger one (evil grin - pictures of a person boiling a

300 gallon oil tank, with a couple of hundred watt 1 ohm resistors in
series, connected directly to the mains...).

Best Wishes