Wire Wound resistor failure?

[Bill Stock]

I've got an aquarium air pump on the pond to keep a hole open in the ice.
This prevents the potential build up of toxic gasses that might kill the
fish. However, aquarium pumps don't like the cold, it stiffens the
diaphragm. If it gets cold enough (likely), the diaphragm will crack.

So I was looking for a simple solution (cheap) to heat my pump. I read over
in the astronomy group, that they sometimes use wire wound resistors to
build dew boxes for their scopes/cameras. Fortunately I had a supply of 1.2K
ohm WR resistors that I scrounged many years ago. I think they're 5W (KKA5),
but I'm not sure. I wired four of them up to give me 1.25K,which should only
draw 12W. But they still generate significant heat. I'm getting about 30° C
over ambient after an hour in my test container.

Will this lead to an early demise of the resistors?

 

[Al]

I've got an aquarium air pump on the pond to keep a hole open in the ice.
This prevents the potential build up of toxic gasses that might kill the
fish. However, aquarium pumps don't like the cold, it stiffens the
diaphragm. If it gets cold enough (likely), the diaphragm will crack.

So I was looking for a simple solution (cheap) to heat my pump. I read over
in the astronomy group, that they sometimes use wire wound resistors to
build dew boxes for their scopes/cameras. Fortunately I had a supply of 1.2K
ohm WR resistors that I scrounged many years ago. I think they're 5W (KKA5),
but I'm not sure. I wired four of them up to give me 1.25K,which should only
draw 12W. But they still generate significant heat. I'm getting about 30° C
over ambient after an hour in my test container.

Will this lead to an early demise of the resistors?

From your description you have a series/parallel setup of four
resistors. If the resistors are reasonably close in value, the power
dissapated should distribute evenly between the four. Just make sure you
have a gap between the resistors so you don't create any hotspots. This
should work out just fine for you.

If you're still worried about it, immerse the resistors in a glass jar
filled with motor oil. The oil will transfer the heat to the glass jar
and keep the resistors happy. Of course you'll need a cap and a means to
feed the resistors inside of the closed jar.

Al

 

[Bill Stock]

From your description you have a series/parallel setup of four
resistors. If the resistors are reasonably close in value, the power
dissapated should distribute evenly between the four. Just make sure you
have a gap between the resistors so you don't create any hotspots. This
should work out just fine for you.

Thanks Al, yes a 2x2 setup. They're all spaced nicely. There's a slight
smell, but not too bad.

If you're still worried about it, immerse the resistors in a glass jar
filled with motor oil. The oil will transfer the heat to the glass jar
and keep the resistors happy. Of course you'll need a cap and a means to
feed the resistors inside of the closed jar.

Neat idea, but fish and oil don't mix.

I think once my air pump starts sucking cold air into the container, my 12W
will be overwhelmed. Perhaps a heated sock would be better.

 

[Al]

Neat idea, but fish and oil don't mix.

Uhhh, put the resistor assembly and the oil in a sealed glass jar and
immerse it in the pond next to your pump. If you are afraid the glass
will break, use a metal can, space the resistors away from the sides so
they don't short, and immerse the contraption.

If you really want to try something a little more complicated, heat the
pump's intake air. Attach your hose from the pump's air input to a
cannister which holds the resistors. The heated air impinges on your
pump diaphram and keeps it warm. The heated bubbles of air might be
useful in breaking up the ice. And make sure the heated air is not so
hot it damages your pump innards.

Al