An ultra ultra conservative design. For RF, I'd prefer that the leads on
the individual resistors was nearly zero. I wrote up such a design for
Iowa Army MARS probably three decades ago. The wire lead inductance as
shown is much greater than that of the spiraled resistance elements and
besides those spirals are resistive which lowers the Q of that part of
the inductance.
I say ultra ultra conservative because I have tested that tin oxide
power resistor to 10 time rated power in air. Fortunately I had a 5
digit bridge to detect the change in resistance which showed only in the
least significant digit of the 5. That was a 10 ohm 3 or 5 watt made by
Mallory about 1964. With 10 times rated power applied the resistor body
glowed red and some of the coating burned off. The wire leads were so
hot they flame cut paper. His limit is the amount of oil that will begin
to boil with a few hundred watts. A gallon sized load will boil with a
KW applied for several minutes.
The dummy load for the Collins 821A-1 used lots of these metal film
power resistors. I didn't count them but they were the 10 or 13 watt
size mounted in disks of a bundle them in parallel filling a 4" tube.
Rough guess 19 to the bundle (since 19 rounds fill a circle very well),
and those disks stacked up about 6' long. In a metal tube for shielding
and to set the characteristic impedance and with an insulating tube to
control the oil flow. So with 4" long resistors, 6' would have 24 disks,
19 per disk would make 513 resistors at 10 watts each, 5 KW dissipation
without oil. We did burn up those dummy loads, sometimes from resistor
damage, but mostly from that insulating tube damage. The oil was
Dowtherm-A and the final longest lasting dummy loads mounted a 5 hp
centrifugal pump outlet directly at the end of the resistor column
because that load had to dissipate 125 KW carrier with 100% square wave
modulation that moved the average power up to about 240 KW per side of
the load. It was a 300 ohm balanced load made of two 150 ohm coaxial
loads. Likely each resistor was 120 ohms by my calculator. But with the
high velocity oil cooling we put over 460 watts to each resistor, 46
times rated power in air. Might have been there were 30 resistors to the
disk which would have reduce the overload factor. I didn't ever count them.
In air the hazard of those resistors is not to themselves, but to their
surroundings where the radiated heat may ignite less well temperature
qualified objects.
73, Jerry, K0CQ
On 4/2/2011 11:24 AM, Jeramy Ross wrote:
> Here's a simple and cheape solution that is easily homebrewed:
>
> http://k4eaa.com/dummy.html
>
> 73,
> Jeramy W5XTL
>
> On Sat, Apr 2, 2011 at 11:54 AM, Dr. Gerald N. Johnson
> <geraldj@weather.net> wrote:
>> While a 6AL5 and transformer could be cheap, the venerable HP 410B and
>> 410C used a smaller diode in the AC probe and they also made a 10:1
>> voltage divider and a coax tap adapter for this very purpose. So it can
>> be had ready made.
>>
>> One other technique is to use a water proof dummy load submerged in an
>> insulate water bath. Then monitor the temperature rise of the water. Its
>> best if the water is circulated and you know the amount of water and the
>> thermal mass of the dummy load. It can be calibrated by heating the
>> dummy load with AC or DC that's more easily measured than RF power and
>> setting the applied AC or DC power to achieve the same rate of
>> temperature rise of the water. Then there's no math, the test power
>> equals the applied Rf power. Called calorimetry and there are commercial
>> products for that too though probably neither readily available or
>> suited for 100 watts.
>>
>> Then there directional couplers with RF out that can go to an RF power
>> meter and head like the HP 432 and 436 families. Where the head can only
>> stand 10 milliwatts. And they can be used with a power attenuator. The
>> 432 and previous work with thermistors biased to get their resistance to
>> 50 ohms and the meter works by showing the reduced bias power required
>> to keep the thermistor resistance to 50 ohms. The later 436 family uses
>> thermocouples and load resistors sensing the temperature rise of the
>> load resistors and are much revered in the microwave industry, but a new
>> one with power head can cost the price an Eagle.
>>
>> 73, Jerry, K0CQ
>>
>> On 4/2/2011 9:46 AM, Roy wrote:
>>> Don't dismay...there's a way.
>>>
>>> A 52-ohm dummy load with a rectifier and filter cap yields an accurate dc
>>> voltage in proportion to the power being dissipated. Today's vom's and dvm's
>>> are pretty accurate (average the readings of all your meters). The rectifier
>>> can be a 6AL5 with its plates connected in parallel, and its heater
>>> connected to its cathode, so the output voltage rating is high enough for a
>>> kw. The plates are connected to the hot side of the dummy, and a 0.01 disc
>>> in parallel with say a 1-uF connect to the cathode for a positive dc voltage
>>> out with a hold time for peak readings. It's inherently self accurate and
>>> requires no calibration.
>>>
>>> 73, Roy K6XK
>>>
>>>
>>
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