Hello.

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 possibly, capacitance. What type of resistor is the best choice for me?

 

You can two buy 100 Watt Hybrid 50 ohm Load Resistor like these and connect them in parallel for 25 Ohms.
http://www.ebay.com/itm/2-Pcs-New-1...f20fcca&pid=100623&rk=2&rkt=4&sd=390324755514

A good heat-sink and maybe a one-quart paint can filled with mineral oil to immerse that in would work for long term reliability.

 

Something is not right, you want a 25 Ohm resistor, but you have to choose whether you want it to be 50W, or 20kW as the 1kV amplitude would imply. Or is the 1kV just a peak transient value?

 

Something is not right, you want a 25 Ohm resistor, but you have to choose whether you want it to be 50W, or 20kW as the 1kV amplitude would imply. Or is the 1kV just a peak transient value?

Oh, you indicate what I was wrong. Yes, my RF generator can give out the power which can be set fixed. At 50 W to 25 ohm, a maximum voltage on the resistor is 50 V, if it is a sin wave. Yeah, I only need to concern about the power rating on the resistor. Could you recommend me a good resistor with reasonably low reactance values satisfying such a condition?

 

You can two buy 100 Watt Hybrid 50 ohm Load Resistor like these and connect them in parallel for 25 Ohms.
http://www.ebay.com/itm/2-Pcs-New-100-Watt-Hybrid-50-ohm-Load-Resistor-good-to-4-GHZ/371046407796?_trksid=p2047675.c100623.m-1&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=20160323102634&meid=107d8f8e62ad463e95e920748f20fcca&pid=100623&rk=2&rkt=4&sd=390324755514

A good heat-sink and maybe a one-quart paint can filled with mineral oil to immerse that in would work for long term reliability.

Hello. Thanks for giving me your suggestion.

I think what you told me is called an RF resistor that is not what I'm familiar yet. In this resistor, I'm wondering which parts are end-electrodes where connections from a circuit are made? There is a narrow metal strip coming out from the resistor and the relatively large metal part which has a hole. Are these metals what two end-electrodes of the resistor?

And you said it may be mounted on the heat sink. Any type of the sink is okay without a cooling fan? I appreciate it if you have some suggestion.

 

Hello,

There also exist some RF resistors in a kind of TO247 housing for easy mounting.
The german webshop BOX73 sells them:
http://www.box73.de/product_info.php?products_id=1583

I have attached a datasheet of them.

Bertus

 

OK, fundamental question here....How to you use a resistor to compensate phase?

The usual thing is a matching network where the capacitors are vacuum capacitors, http://www.jenningstech.com/technotes/vchrctr.shtml These are generally used for manual matching.

For auto-matching systems, a roller inductor is used. http://www.palstar.com/en/inductors

Matching networks: https://www.mksinst.com/product/Category.aspx?CategoryID=20
Didn't know that ENI is part of MKS now.

I've used this https://www.dxengineering.com/parts/mfj-250?seid=dxese1&cm_mmc=pla-google-_-shopping-_-dxese1-_-mfj to test an RF plasma generator

Directional couplers are use to sample the RF: https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

 

OK, fundamental question here....How to you use a resistor to compensate phase?

The usual thing is a matching network where the capacitors are vacuum capacitors, http://www.jenningstech.com/technotes/vchrctr.shtml These are generally used for manual matching.

For auto-matching systems, a roller inductor is used. http://www.palstar.com/en/inductors

Matching networks: https://www.mksinst.com/product/Category.aspx?CategoryID=20
Didn't know that ENI is part of MKS now.

I've used this https://www.dxengineering.com/parts/mfj-250?seid=dxese1&cm_mmc=pla-google-_-shopping-_-dxese1-_-mfj to test an RF plasma generator

Directional couplers are use to sample the RF: https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

Oh...you're right! I have a matching network box so the box + my plasma reactor must have an equivalent impedance of 50 ohm in purely resistance when the impedance matching is done. Thus, the RF input point of the box will be my measurement point where the phase between voltage and current measurements is compensated; at this point, the equivalent impedance toward the reactor is 50 ohm in pure resistance when the impedance matching is done, so the phase between the voltage and current must be zero. the measured phase difference in this condition is what I need to compensate for further measurement. I think my problem is now solved, with your idea

 

Yep, the generator needs to see as close to 50 ohms as possible, but exactly 50 is usually not obtainable. Usually, the measurements are forward and reflected power rather than SWR for plasma.

You generally won't get zero, but minimized reflective power. In the ENI auto network I used there were some capacitors that needed to be changed to give the matching network the range it needed.

One system was a sputtering system with a manual matching network. We operated between 2 and 5 milli-torr, but occaisionally, it was difficult to start the plasma. Increasing the pressure briefly (we had pressure control) allowed the plasma to strike. The auto-unit used it's initial tuning position of the tuned elements to create the strike.

On the automatic one, there was an output that measured the DC value of the plasma. This allowed the computer control to know when the plasma struck.