Non-inductive wirewound resistors and the Ayrton-Perry winding - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: September 14, 2018, 12:28:27 pm
content: Hi all-I tried to find an answer but didn't come up with much about this. Across what frequency range can we safely assume one of these acts non-inductive? Do you think I'm safe up to a couple of MHz?Right now I'm specifically looking at these Vishays; even their datasheet doesn't promise anything in terms of "HOW non-inductive are we talking about?".Are there maybe other models from different vendors that *do* give you frequency specifics? Thanks...http://www.vishay.com/docs/31801/mra.pdf

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 14, 2018, 04:02:05 pm
content: kHz likely safe, several MHz unlikely; like they write, they do well for audio frequency range, but clearly not the choice for radio frequencies; Vishay has high power chip resistors with very good rf properties; I'm just going with those ones; they start in the range of 5W upto 50W dissipation

author: ConKbot
author_rank: Super Contributor
timestamp: September 14, 2018, 05:26:17 pm
content: Since it is two parallel counter-wound windings, you want each loop to still be electrically small. The 5w resistor is 4.24mm in diameter, 13.3mm in circumference. Multiply by 50 (you want to keep it under 1/50th of a wavelength) and again by 2 (ceramic core with high dielectric constant slowing down EM waves in it) and the loop should be electrically small up to 1.3m wavelength, or 230MHz.Take that with a grain of salt though, as they only mention audio in the datasheet, let alone HF for VHF radio. Id suspect a "few" MHz would be ok, and would want to test it over your frequencies of interest to be confident. Since they are in the <$10 price range, if you have the means to test one, give it a shot.The test could just be a scope and sig-gen, and some coax, seing how one performs as a 50 ohm termination resistor, or make a divider with a non-inductive resistor (0603 thick film will be fine) and seeing how it performs at frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2018, 07:42:35 pm
content: Essentially the mechanical length of the part. Hmm, it'll be more than that, because of the zig-zag current path, rather than a straight wire. In any case, you can't get less inductance than the straight-line length equivalent, no matter how closely matched the windings are.It's been my experience that regular wirewounds poop out in the low MHz, i.e., X_L = R. That doesn't mean they're necessarily useful up to the low MHz. If you need a pure resistance at, say, 200kHz (where a regular wirewound may be ~10% inductive), you should still get a noninductive part.If you need a very low inductance (~nH say), or equivalently a very high resistive bandwidth, consider a thick film heatsinked part. Also try to design the circuit for modest value resistances: the higher you go (relative to the transmission line impedance of the part's geometry), the more capacitive (and lower bandwidth) you get. The lower you go, same but inductive.Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: September 14, 2018, 09:41:10 pm
content: Hi its never a good sign if a product property is claimed ("non-inductive") but vital data is msiing completeley.My guess is:- inductance depends on resistance value- smaller than straight coil, but several times the inductance of a straight conductor of the same lengthGotchas:- Watch for resonances.Measurement Method:- get one and solder it across an SMA socket with minimum lead length- Attach it to a VNA and check.I tried this with a lot of component with either no specs or no trustworthy specs, like here:https://electronicprojectsforfun.wordpress.com/componentology/

author: gbaddeley
author_rank: Regular Contributor
timestamp: September 15, 2018, 01:36:24 am
content: Quote from: Wolfgang on September 14, 2018, 09:41:10 pmAgree. At some high frequency, the wirewound resistor will increase in impedance, to a point where it resonates with its own self capacitance (usually at a low Q). Beyond that the impedance becomes capacitive. The leads and layout contribute to the resonance.

author: cvanc
author_rank: Frequent Contributor
timestamp: September 15, 2018, 03:07:09 pm
content: Thanks everyone. I've emailed Vishay asking for more detail about this specific product. If I get a response I'll post it here.