[Test-Equipment] [Boatanchors] Bird Termaline Question - Engineering Update (long)

[Ron Youvan]

   On 02/17/2017 09:06 PM, Brian wrote:

> Hello Doug,

> For other readers coming to this thread late, I have left my previous response to Doug’s plea.

> Depending on how non-reactive you want the eventual load to be, you can accept a load resistor of slightly different dimensions from your measured 5” L x 9/16” dia. The diameter is going to be critical, in that you have no room to accept increased diameter, but you can accept lesser diameter, perhaps down to 0.58”. And if you intend using the load horizontally, you could use a solid carbon resistor – because horizontally, there will be no coolant convecting inside the resistor – solid or hollow. Also, because there will be no coolant flow inside the resistor, you could insert a 175 Ohm resistor inside the 70 Ohm resistor to get back to 50 Ohm overall. But, that 175 Ohm resistor needs to be able dissipate about 1/3 the load (43 W).

> When a dissipation rating is given for a resistor, it is usually specified in still air, usually at 20 C, horizontal and the maximum external body temperature will be specified – it could be 275 C. As radiated power is proportional to the 4th power of the temperature differential and the convected power approximately proportional to just the temperature differential, most of the dissipation will be by radiation and a little by convection.  But you will have this resistor immersed in oil – a good thermal conductor – and the oil will transfer the heat to the external walls and thence to the fins, thus increasing the radiative and convective area. There will be a time lag between heat leaving the resistor and its being radiated / convected from the outer fins. So, the load will be able to withstand temporary overloads.

> My suggestion, particularly as you have nothing to lose, is to find a tubular-bodied 175 or 180 Ohm resistor that will fit inside the current load resistor; it may be air-rated at 20 W, but in oil it will handle quite a bit more. Reassemble your Bird Termaline with the added resistor in parallel with the 70 Ohm tubular jobbie, fill the case with the correct oil, and measure the cold impedance. Then apply steady DC loads of 50 W, then 100 W, then 150 W (load Voltages of 50 V, 71 V and 87 V respectively) all the time clocking the ambient temperature and the final steady-state temperature on top on the Bird Termaline. If your mains power is relatively crap-free, you could drive the Bird Termaline with these as RMS AC Voltage values using a Variac; does your Voltmeter read true RMS? If not, stick with DC. Another possibility is to use an inline mains power meter in series with your Variac – don’t forget to subtract the no-load magnetising power the Variac requires – this may be different for each output Voltage. The beauty of a good inline mains power meter is that it couldn’t care less about waveform – all it’s concerned about is the in-phase product of V and I. Between each power run, disconnect the power, let the resistor cool and measure the IMPEDANCE. Hot, I expect the resistor’s impedance to creep a bit; but if the impedance back at room temperature has not departed from where it was BEFORE you started your power runs, you’re on a winner. My guess is that Bird’s design spec might say maximum temperature at the fins should not exceed 125 C, but that temperature will be related to the flash point of the oil.

   Five 35 Ohm 10 Watt metal film resistors, if they will fit in through
the center of the original resistor, would also do it.
   (2 Watt metal film resistors are the same size as a 1/2 Watt
composite carbon resistors, they are much smaller than others)
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-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.