I was cleaning up today and came across an old dummy load I had that looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint can
lid and looks to be a 50 watts worth of resistors. This looks like it may
have been a commercially built unit and I was wondering what the power
handling capability would be if the resistors were submerged in mineral oil.

Jimmie

Jimmie D wrote:
I was cleaning up today and came across an old dummy load I had that looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint can
lid and looks to be a 50 watts worth of resistors. This looks like it may
have been a commercially built unit and I was wondering what the power
handling capability would be if the resistors were submerged in mineral oil.

Jimmie

Hi Jimmie, It would probably take a kilowatt, the question is for how
long? I was looking at my old Heath "Cantenna" manual and it says it
will take 1000 watts for about 1 minute in mineral oil. The resistor
element appears to be ~50-100 watts. The Cantenna is in a gallon paint
can. If none of the 25 resistors "fuse" out due to current, your load
should take a kilowatt for at least 15 seconds, and take 100 watts
~forever.

Gary N4AST

wrote in message
ups.com...

Jimmie D wrote:
I was cleaning up today and came across an old dummy load I had that
looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint
can
lid and looks to be a 50 watts worth of resistors. This looks like it may
have been a commercially built unit and I was wondering what the power
handling capability would be if the resistors were submerged in mineral
oil.

Jimmie

Hi Jimmie, It would probably take a kilowatt, the question is for how
long? I was looking at my old Heath "Cantenna" manual and it says it
will take 1000 watts for about 1 minute in mineral oil. The resistor
element appears to be ~50-100 watts. The Cantenna is in a gallon paint
can. If none of the 25 resistors "fuse" out due to current, your load
should take a kilowatt for at least 15 seconds, and take 100 watts
~forever.

Gary N4AST

Thanks Gary
Would it be worth my while to remount it for a gallon container. I was
thinking of doing this anyway because the local paint store sells gallon
paint cans, Im not sure about he avaiability of the quart size cans. I have
access to lots of mineral oil. At work we use about 100 gal a year and I can
have all the used oil I want

On Sat, 20 Jan 2007 18:17:34 -0500, "Jimmie D"
wrote:


wrote in message
oups.com...

Jimmie D wrote:
I was cleaning up today and came across an old dummy load I had that
looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint
can
lid and looks to be a 50 watts worth of resistors. This looks like it may
have been a commercially built unit and I was wondering what the power
handling capability would be if the resistors were submerged in mineral
oil.

Jimmie

Hi Jimmie, It would probably take a kilowatt, the question is for how
long? I was looking at my old Heath "Cantenna" manual and it says it
will take 1000 watts for about 1 minute in mineral oil. The resistor
element appears to be ~50-100 watts. The Cantenna is in a gallon paint
can. If none of the 25 resistors "fuse" out due to current, your load
should take a kilowatt for at least 15 seconds, and take 100 watts
~forever.

Gary N4AST

Thanks Gary
Would it be worth my while to remount it for a gallon container. I was
thinking of doing this anyway because the local paint store sells gallon
paint cans, Im not sure about he avaiability of the quart size cans. I have
access to lots of mineral oil. At work we use about 100 gal a year and I can
have all the used oil I want


That certainly beats going into a drug store and buying the 8 or 10
bottles of mineral oil I needed to fill up my Cantenna back in olden
days :-) Strange looks at the check out counter...

bob
k5qwg

"Jimmie D" wrote in
:


wrote in message
ups.com...

Jimmie D wrote:
I was cleaning up today and came across an old dummy load I had that
looks
like it is made from 2 watt resistors. ITs mounted on a quart size
paint can
lid and looks to be a 50 watts worth of resistors. This looks like
it may have been a commercially built unit and I was wondering what
the power handling capability would be if the resistors were
submerged in mineral oil.

Jimmie

Hi Jimmie, It would probably take a kilowatt, the question is for
how long? I was looking at my old Heath "Cantenna" manual and it
says it will take 1000 watts for about 1 minute in mineral oil. The
resistor element appears to be ~50-100 watts. The Cantenna is in a
gallon paint can. If none of the 25 resistors "fuse" out due to
current, your load should take a kilowatt for at least 15 seconds,
and take 100 watts ~forever.

Gary N4AST

Thanks Gary
Would it be worth my while to remount it for a gallon container. I was
thinking of doing this anyway because the local paint store sells
gallon paint cans, Im not sure about he avaiability of the quart size
cans. I have access to lots of mineral oil. At work we use about 100
gal a year and I can have all the used oil I want


Hmmm, what is used oil? Used engine oil is regarded as carcinogenic in
these parts, hypoid gear oils similarly whether used or unused. Further,
it is full of conductive particles that may affect the load's its
performance, water and metals that may form corrosive products, and
possibly volatiles that will "boil off" when heated.

I don't know what automatic transmission fluid costs in your part of the
world, but here it is not an unreasonable cost to fill a load with new
ATF. Air tool oil is a source of light grade mineral oil that does not
contain detergent agents.

Long time ago, I favoured refrigeration (compressor) oil for filling
loads, but it is expensive, and the low moisture quality is soon lost if
the load is not as good as hermetically sealed. It is possible to do
that, I still have a load that I made using a hermetically sealed BNC
connecter (yes, a true glass seal) and the whole enclosure was brazed /
soldered up, closed like a 'fridge. I also experimented with refrigerant
in the loads, a lot more fuss and they didn't work any better than
compressor oil. Today, you would not be allowed to do that with R12!

Anyway, to your original question of how much increase in power rating
with oil immersion. In the long term, it is determined by the ability of
the load to dissipate heat from the outside for a tolerable rise in
temperature of the oil and the resistor element. In the short term, it
comes down to the resistor element, and different constructions will
behave differently (eg metal film vs composition).

Owen

"Owen Duffy" wrote in message
...
"Jimmie D" wrote in
:


wrote in message
ups.com...

Jimmie D wrote:
I was cleaning up today and came across an old dummy load I had that
looks
like it is made from 2 watt resistors. ITs mounted on a quart size
paint can
lid and looks to be a 50 watts worth of resistors. This looks like
it may have been a commercially built unit and I was wondering what
the power handling capability would be if the resistors were
submerged in mineral oil.

Jimmie

Hi Jimmie, It would probably take a kilowatt, the question is for
how long? I was looking at my old Heath "Cantenna" manual and it
says it will take 1000 watts for about 1 minute in mineral oil. The
resistor element appears to be ~50-100 watts. The Cantenna is in a
gallon paint can. If none of the 25 resistors "fuse" out due to
current, your load should take a kilowatt for at least 15 seconds,
and take 100 watts ~forever.

Gary N4AST

Thanks Gary
Would it be worth my while to remount it for a gallon container. I was
thinking of doing this anyway because the local paint store sells
gallon paint cans, Im not sure about he avaiability of the quart size
cans. I have access to lots of mineral oil. At work we use about 100
gal a year and I can have all the used oil I want


Hmmm, what is used oil? Used engine oil is regarded as carcinogenic in
these parts, hypoid gear oils similarly whether used or unused. Further,
it is full of conductive particles that may affect the load's its
performance, water and metals that may form corrosive products, and
possibly volatiles that will "boil off" when heated.

I don't know what automatic transmission fluid costs in your part of the
world, but here it is not an unreasonable cost to fill a load with new
ATF. Air tool oil is a source of light grade mineral oil that does not
contain detergent agents.

Long time ago, I favoured refrigeration (compressor) oil for filling
loads, but it is expensive, and the low moisture quality is soon lost if
the load is not as good as hermetically sealed. It is possible to do
that, I still have a load that I made using a hermetically sealed BNC
connecter (yes, a true glass seal) and the whole enclosure was brazed /
soldered up, closed like a 'fridge. I also experimented with refrigerant
in the loads, a lot more fuss and they didn't work any better than
compressor oil. Today, you would not be allowed to do that with R12!

I used to work on some fron cooled equipment. When the equipment got hot the
freon boiled and the vapor went up pipes into a radiator where it was cooled
by fans then driped back down the pipes. I guess you could do a dummy load
like this too.

I have a couple of the glass BNC connectors, I think they are like the ones
you are talking about. They are bulkhead connectors female on both sides. I
keep them around because I have never seen any others like them. They are
isolated. the shield doesnt connect to the mounting flange.


Anyway, to your original question of how much increase in power rating
with oil immersion. In the long term, it is determined by the ability of
the load to dissipate heat from the outside for a tolerable rise in
temperature of the oil and the resistor element. In the short term, it
comes down to the resistor element, and different constructions will
behave differently (eg metal film vs composition).

Owen

Owen Duffy wrote:
. . .
Anyway, to your original question of how much increase in power rating
with oil immersion. In the long term, it is determined by the ability of
the load to dissipate heat from the outside for a tolerable rise in
temperature of the oil and the resistor element. In the short term, it
comes down to the resistor element, and different constructions will
behave differently (eg metal film vs composition).

Yes, I'd worry a little about grossly overdriving carbon composition
resistors because of the thermal resistance between the heat-dissipating
composition material and the oil. There's going to be a temperature
gradient across the phenolic case that might be considerable -- in other
words, the resistance part could be much hotter than the oil. Resistors
which have the actual dissipating portion very close to the outside,
like film or (noninductive) wire wound resistors would be much better in
this regard. If you overstress carbon comp resistors you'll typically
get a permanent change in resistance. If the overstress is extreme,
they'll crack, explode, or catch fire. Make sure your oil and container
won't present a hazard if this happens.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Owen Duffy wrote:
. . .
Anyway, to your original question of how much increase in power rating
with oil immersion. In the long term, it is determined by the ability of
the load to dissipate heat from the outside for a tolerable rise in
temperature of the oil and the resistor element. In the short term, it
comes down to the resistor element, and different constructions will
behave differently (eg metal film vs composition).

Yes, I'd worry a little about grossly overdriving carbon composition
resistors because of the thermal resistance between the heat-dissipating
composition material and the oil. There's going to be a temperature
gradient across the phenolic case that might be considerable -- in other
words, the resistance part could be much hotter than the oil. Resistors
which have the actual dissipating portion very close to the outside, like
film or (noninductive) wire wound resistors would be much better in this
regard. If you overstress carbon comp resistors you'll typically get a
permanent change in resistance. If the overstress is extreme, they'll
crack, explode, or catch fire. Make sure your oil and container won't
present a hazard if this happens.

Roy Lewallen, W7EL

Thanks Roy, I too have given this some consideration and am thinking of
rebuiding the dummy load with metal film resistors heeping the brass buss
bars that came with the load. The actual measure DC resistance is near 60
ohms so I suspect some damage has already occured. The resistors have no
markings but I figure 220 ohms each. Its difficult to count the resistors
the way they are arranged. You dont pay 3 bucks for something like this at a
hamfest and expect it to 100%.

"Jimmie D" wrote in message
news:Ujwsh.298$ch1.65@bigfe9...
I was cleaning up today and came across an old dummy load I had that looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint
can lid and looks to be a 50 watts worth of resistors. This looks like it
may have been a commercially built unit and I was wondering what the power
handling capability would be if the resistors were submerged in mineral
oil.

Jimmie


Next time I ask about something I will have it in my hand when I discribe
it. Dummy load is 44 each 2.2K 2 watt resistors in paralell.
I think I will rebuild it using metal film.

"Jimmie D" wrote in message
news:Ujwsh.298$ch1.65@bigfe9...
I was cleaning up today and came across an old dummy load I had that
looks
like it is made from 2 watt resistors. ITs mounted on a quart size paint
can lid and looks to be a 50 watts worth of resistors. This looks like it
may have been a commercially built unit and I was wondering what the
power
handling capability would be if the resistors were submerged in mineral

Related, sort of:

When I was in Navy ET School in the 1960's, we had a shipboard radio
transmitter lab which used banks of incandescent lamps for the dummy loads.
By the time we got to that phase of the school we understood the concept of
matching to 50 ohms and I wondered then (and now) what the actual impedance
of a bank of light bulbs would be. (Of course it changed with the amount of
power applied, since it lit the lamps more or less brightly.)

Nobody ever explained it. It's a wonder we didn't make more smoke than
light. [AN/SRT-14, AN/SRT-15, for those who've been there & done that.]

Also, for excellent heat dissipation and no conduction, a liquid called
FC-75 is used as a heat transfer medium. They literally immerse the gear in
a tank of the stuff and run it though a liquid-to-liquid heat exchanger.
It's totally inert. We used it in our high power pulse transmitters in ECM
gear and it's apparently still around [
http://news.thomasnet.com/fullstory/473395 ] . No, I don't have any :-(