EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Crambone on May 10, 2020, 01:16:23 pm
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Is this a non conductive resistor, it needs to be used in a parasitic suppressor in VHF circuit?
https://www.mouser.com/ProductDetail/286-47-RC (https://www.mouser.com/ProductDetail/286-47-RC)
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Hard to say, it mostly depends on manufacturing process, is it a helix shape or no. 47R resisor will most likely be helix. Search for specialy made, low inductance resistors.
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I see that Mouser still stock Carbon composition resistors, the traditional non-inductive solution if TC and tolerance aren't an issue...
https://www.mouser.co.uk/Search/Refine?Keyword=composition+resistor+47ohm (https://www.mouser.co.uk/Search/Refine?Keyword=composition+resistor+47ohm)
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Non inductive resistor
https://en.wikipedia.org/wiki/M%C3%B6bius_resistor
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The data sheet, which covers the whole resistance range in that series, indicates a helical construction but there is no specification on inductance (typical for such parts). Solid resistors, such as carbon composition (carbon and talc mixed to obtain the resistance value) are not inductive, but high values (like 1 megohm, rather than 47 ohms) have interesting internal capacitance around the talc particles that lower the equivalent parallel resistance at RF. Helical and wirewound resistors often look capacitive at high frequencies, rather than inductive.
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Many engineering textbooks have a big section dedicated to ways of making noninductive resistors. Things like bifilar and Ayrton-Perry and others, explaining how each accomplishes its end. These of course are wire wound, which is the construction of choice for ruggedness and accuracy but only up to a few Megahertz. Skin effect and parasitic reactances are big obstacles.
The shape of the resistance element in bulk resistance/plastic units is a major determinant of frequency characteristic, as has been pointed out above. These days there are some very good resistors but they start to get expensive. Almost any fixture will have at least 1 or 2 pF of parasitic capacitance. They have to be tiny for high frequency but that limits the power handling. After a while it seems that one is chasing his own tail.
A carbon pile or block is one interesting solution. Again, its physical size is a major limitation. I would guess that a cube is a good shape for wide band operation.
My military surplus dummy load is made from a batch of noninductive resistors and is pretty good up to several megahertz. There are some interesting parts of the design that obviously have been thought out carefully. I don't fully understand it. But it's rated for a few hundred Watts, not too shabby.
I wonder about water loads, perhaps ionized to control resistivity. High power transmitter engineers must use something for testing.
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To clarify my statement above about capacitance in carbon composition resistors:
I had an application where I needed a high-value (1 M) resistor to bias a JFET gate, but I needed a high parallel resistance at 10 MHz. The parallel capacitance could be tuned out in the circuit. We tried different resistor constructions, and I was disappointed by old-fashioned Allen-Bradley molded carbon composition resistors. Researching this (pre-internet) effect, I found that the talc particles in the composition mix tended to short out nearby carbon particles at high frequencies, resulting in a lowered equivalent parallel resistance. This was confirmed by careful measurements with RF impedance measuring equipment. We replaced the CC resistors with a "carbon ink" formulation from IRC, in the same physical (leaded) size, which is probably now obsolete. Since then, I have found that the "flame-proof" cermet resistors are reasonable at this frequency range, along with thick-film surface-mount parts.
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The shape of the resistance element in bulk resistance/plastic units is a major determinant of frequency characteristic, as has been pointed out above. These days there are some very good resistors but they start to get expensive. Almost any fixture will have at least 1 or 2 pF of parasitic capacitance. They have to be tiny for high frequency but that limits the power handling. After a while it seems that one is chasing his own tail.
The parasitic elements can be eliminated by building the resistive element into a transmission line. Discs and cylinders are common shapes but various curved solids can be used to form a continuous transmission line of varying impedance.
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Don't know your specific application, but in amateur radio tube type
power amplifiers it was common to have a plain old 2 watt carbon
resistor usually of 47 ohms with about 4 turns of #18 wire wrapped
around the resistor and connected to each end in shorting fashion an
used to suppress the tendency of the amplifier to self-oscillate at VHF
frequencies.
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The construction is most likely spiral cut (via printing, etching, laser cutting, etc.). The (unrolled) length of the spiral is not much longer than the length of the component itself. The resistance is also in the sweet spot, close to Zo or so, where inductance and capacitance are least offensive; it's probably dominant resistive up to 100s of MHz. If the value were very small or very large, the reactance would be more noticeable at lower frequencies.
It will be fine.
Tim