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I'm looking for a resistor about 25 Ohm at 13.56 MHz (this is our operating frequency). The resistor must withstand at least 50 W (100 W is better!) at ~ 1 kV voltage amplitude. As I will use this for compensating the phase difference between the measured voltage and current from our voltage probe and a current transformer, It also must have a low capacitance and inductance. I originally thought a wirewound type resistor would be a good choice due to its strength for high power and high voltage, but I immediately realized that it might have bad inductance and, possbily, capacitance. What type of resistor is the best choice for me?

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    \$\begingroup\$ I am not quite sure what you are trying to do, but dropping 1kV over 25Ω is slightly more than 100W \$\endgroup\$ – PlasmaHH Mar 22 '17 at 10:02
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    \$\begingroup\$ 100W in an ISM-band system? I'm very curious to know what this system actually is. \$\endgroup\$ – pjc50 Mar 22 '17 at 10:04
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    \$\begingroup\$ It is from RF generator as a part of the plasma reactor (I don't know a type of the generator). I want to measure the impedance of the reactor by measuring its voltage and current. The problem is that there is some phase difference between these measurements, so I need to compensate this by using the resistor. A good resistor has an ignorable reactance, so I might use this to compensate such a phase (no phase shift between the voltage and current for resistor, in principle). Anyway, could you give me some suggestion about the resistor? \$\endgroup\$ – Donggyu Jang Mar 22 '17 at 10:13
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    \$\begingroup\$ You might want a water heater heating element. They aren't calibrated for RF resistance, but the inductance is low, and capacitance, they're shielded, and power dissipation is not a problem. \$\endgroup\$ – Whit3rd Mar 22 '17 at 10:22
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    \$\begingroup\$ Or check with the amateur radio guys. A dummy load for HF sounds like just the thing you need, and radio amateurs have lots of experience building that sort of thing. \$\endgroup\$ – JRE Mar 22 '17 at 10:26
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TGHLV25R0JE

Resistor, 25 ohm, 200W, non inductive, RF, SOT-227

http://www.digikey.com/product-detail/en/ohmite/TGHLV25R0JE/TGHLV25R0JE-ND/1124819

This resistor is made to be used with a heat-sink. In order to size the heat-sink you need to know how much wattage you need to eliminate and how much the temperature is allowed to rise across the heat sink.

Lets assume that you are working in a lab with ambient temperature Ta = 25C. The resistor datasheet also says the maximum allowed junction temperature of Tj_max = 155C. Therefore your maximum allowed temperature rise is...

dt = (155C - 25C) = 130C

Your post says that you have a maximum dissipation of W_max = 50W max. In that case the maximum allowed Junction to Ambient Thermal resistance R-ja-max is...

R_ja_max = dt / W_max = 130C / 50W = 2.6C/W

The resistor datasheet says that it has a thermal resistance of Rth_resistor = 0.35 Kelvin/Watt between the resistor baseplate and the heat-sink. R_ja_max includes all the thermal resistances along the whole thermal path. In a simple case this is Rth_resistor + Rth_heat_sink. Therefore the maximum thermal resistance of your heat-sink must be...

Rth_heat_sink = R-ja-max - Rth_resistor = 2.6C/W - 0.35C/W = 2.25C/W

You can pick any heat-sink that has less than 2.25C/W thermal resistance, so long as it fits mechanically. Alternatively if you have any large metal cabinets that are part of your equipment you can bolt the resistor to the frame (which will almost certainly have very low thermal resistance).

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  • \$\begingroup\$ Hello. I appreciate your suggestion which looks just fitted on my application. I'm just not familiar with a use of a heat sink. In mouser (mouser.com), there is a lot of heat sinks without a cooling fan. Can I simply mount the resistor on the heat sink without the fan or a rather large aluminum plate is just enough? \$\endgroup\$ – Donggyu Jang Mar 26 '17 at 11:25
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    \$\begingroup\$ @DonggyuJang Any resistor having less than 2.25C/W thermal resistance should work for your application. Or just bolt it to the chassis of your equipment. \$\endgroup\$ – user4574 Mar 26 '17 at 21:15
  • \$\begingroup\$ Oh..I see. The junction here means the part of the resistor which is supposed to be directly touched by the heat sink, right? I don't know what is the base plate of the resistor but I assumed that it is some bottom black plastic. And the thermal resistance of the heat-sink is the value evaluated without the cooling fan? I think this resistance can be lower if the cooling fan is used together. \$\endgroup\$ – Donggyu Jang Mar 27 '17 at 11:35
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    \$\begingroup\$ @DonggyuJang The junction is an imaginary hot-spot point within the resistor. The resistor is in a TO-227 package. The TO-227 package has a metal plate on the back side with places to mount it via two screws. In general the thermal resistance of a heat-sink lowers with increased airflow. The datasheet for the heat-sink will usually specify the value for no-fan, or for various air-flow amounts. \$\endgroup\$ – user4574 Mar 27 '17 at 12:22
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The old school way of doing this was to use many resistors in a series / parallel combination .Old ARRL handbooks describe this well .Wirewound resistors were not used due to inductive effects .Carbon resistors were used when I was a toddler .Metal film resistors would be suitable .When you bunch a lot of resistors together they will not do anywhere near thier power rating .I used a small 12V computer fan to cool my bunch of thru hole phillips PR03 metal film resistors .This worked fine for me at 3MHz .You could also try TO220 power resistors .They cost but you will not need so many of them and they easily bolt on to a heatsink just like a standard TO220 power transistor and better still the tab is isolated .

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I am sensing an XY problem here, 13.56MHz is not that high a frequency, and 100W is not that much power, so standard RF methods should be applicable for most measurements.

While plenty of folks make thick film TO220 resistors in reasonable power ratings (Watch the temperature derating on these, they tend to specify them at 25 degrees case temp), I cannot help but suspect it is the wrong answer.

What are you actually trying to achieve?

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