Chokes can be wound over the bodies of high-value
>wire-ended resistors, the parallel resistance value
>perhaps having some desirable effect on the ultimate
>response.
I am having trouble seeing any advantage of doing this instead of simply putting a resistor and an inductor (on a form if you insist on winding it yourself) next to each other.
I see several disadvantages:
You are soldering to the leads of the resistor, not holes in a PWB. This requires hand work and is likely to put too much heat into the resistor.
You can't read the color codes on the resistor.
A resistor is unlikely to be the optimal diameter and length - parameters you can easily control with a winding form.
I am having trouble seeing why you're having trouble seeing any advantage in doing this. A high value resistor is, in many cases, readily available from a junk box and makes a good coil form.
So, what will the heat do? Cause the value to change? If so, it doesn't matter because you can use almost any value of resistor anyway. Just as long as it's high enough to get you the Q you need. Also, sometimes you don't want to put a coil on a PCB. Like when it's used on the plate cap of an RF amplifier. And, they've been used in solid state RF amplifiers as well.
Why would you need to as long as it's just used as a coil former and any value over a 10 to 1 range would suffice?
As I recall, optimal length/diameter ration is supposed to be about 1 for highest Q. So, just wind enough wire on the resistor to get the length/diameter ratio you want.
Maybe you are not aware that this technique has been used for many years to make coils and chokes? In fact, I once rebuilt an RF choke which was used in the plate circuit in an old Gates commercial 1 kW AM transmitter. I think it was a 2W, 47R wound full of 18 gauge magnet wire.
This was common practice in hobbyist applications in the 20's thru the
90's. Hams did this all the time and it's quite effective where all you need is a few uH or mH choke. This probably arose when a resistor was a few pennies and coil forms were much more expensive or hard to come by. This is called cheap, fast and dirty. I believe there were even a few formulae published in the Ham Radio Handbook for determining L based on various resistor sizes and wire gages and number of turns.
PWB's were unheard of when this practice started. Marconi and Edison probably did this as much as anyone at the time.
Not if you have any skill with a soldering iron. The resistance is irrelevant in any case, see below.
Irrelevant. It's a choke now, not a resistor. The resistance is likely to be very much larger than the resistance of the wire windings of the coil and won't even have any effect on the Q of the coil unless it's sub-milliohm.
It depends on the application, doesn't it? Often, coil forms were arrayed on hooks on the wall in the lab. When you needed a coil, solid wire was wound on the forms and the forms removed, leaving the coil to hold it's own shape. Air core coils could be tuned by spreading the turns or squeezing them closer as needed. In those days you used whatever forms came to hand. The techs also knew how to solder without burning things like resistors. I remember winding coils on round number 2 lead pencils for forms. You must never have worked in a radio hobby shop or learned electronics from an old ham radio operator otherwise you would have more sense than to raise these issues.
In a production shop where something were to be reproduced 10,000,000 times by automated assembly it's necessary to choose a part to meet those requirements. Where one is only looking to experiment or fix an RFI or other design issue on a prototype basis, the resistor-choke is a valid choice. It can be evaluated later and a proper part can be chosen based on empirical measurements of the choke.
Considering common 1/8-watt to 2W resistors, a lot of ground is covered in the convenience department. Those of us who work with a range of frequencies (dc-3GHz) appreciate coaxial forms.
Thanks! I was unaware of that effect. I have, of course, seen inductors wrapped around resistors, but that was in the days of point-to-point wiring, and I always assumed that it was a mere convenience.
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