[ARC5] Resistor Excursions

[Tom Lee]

Let’s do some math.

As I mentioned, a 0.5W classic through-hole resistor will have a shunt capacitance of around 0.5 pF with pigtails clipped short. The uncertainty of that value is perhaps 30%.

At 10MHz the reactance will be about 30 kilohms or so. That means you would not expect a significant effect from the capacitance unless your resistor was at least some kilohms. And even a 10 k resistor will appear predominantly resistive at that frequency.

A blanket assertion that a resistor will stay resistive up to some frequency is neither trustworthy nor useful without reference to resistance value or frequency. These obviously matter. I guarantee that a megohm resistor will not be primarily resistive at VHF, yet that’s what your claim implies.

I’d like to see the papers you refer to, as I’ve had this exact type of exchange many times over the years. Somehow the actual papers never get cited, so I have no idea over what range of R and f the datasets cover. That’s precisely why I made measurements of my own. The data shows that a shunt capacitance model is the simplest one that covers behaviors reasonably well up to around a GHz. Like all models, this one is wrong. But it is also useful.

As an old boss of mine liked to say, an ounce of data trumps a pound of opinion.

Tom



Sent from an iThing; please forgive the typos and brevity

> On Apr 20, 2024, at 13:26, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> If you measure them correctly, they do exhibit the “stays resistive” characteristic up at least into the VHF region. There are papers on this going back into at least the 1930’s. Yes, your test setup has to be pretty carefully done to take out any incidental stray C …. Last time I did that sort of testing was in the 1970’s.
> 
> Bob
> 
>> On Apr 19, 2024, at 7:35 PM, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
>> 
>>  What is the source of your information?  I have measured many CC resistors using a Boonton RX-Meter and found they become more reactive at RF. While many think film resistors become inductive I have not found that true. Curiously enough the examples in the Boonton handbook show the same thing.
>>  I was taught the same thing about CC resistors but it seems not to be true.
>> 
>>> On 4/19/2024 4:23 PM, Bob kb8tq wrote:
>>> Hi
>>> If you *do* decide to replace CC’s with a different type of resistors, do a deep dive into just how weird CC’s are at RF. Unlike pretty much all other resistors, they drop value, but stay resistive as frequency goes up. While that sounds a lot like a resistor model that includes a cap, it’s not. The phase shift with “pure resistive” is not the same ….
>>> How much does this matter? It very much depends on how tricky the circuit happens to be. In many cases … not a big deal.
>>> Bob
>> 
>> --
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> SKCC 19998
> 
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: https://www.qsl.net/donate.html