Author Topic: inductor wound over a resistor?  (Read 5090 times)

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Online coppercone2Topic starter

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inductor wound over a resistor?
« on: August 24, 2022, 01:33:43 am »
I am curious about this. The 461A amplifier I spared for switch addition has this method, a wire of maybe 3 turns wound around a carbon resistor.

What is the point of this and how is this designed? Would a modern design still use it, or does it have something to do with lack of proper inductor materials? Does this have anything to do with the resistor material acting differently in the magnetic field, or would it work equally well if it was in parallel with the resistor?

Not much information about this, other then its a form of de-tuning.
 

Offline joeqsmith

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Re: inductor wound over a resistor?
« Reply #1 on: August 24, 2022, 02:00:20 am »
On the plate supply?
***
Like this for example?
« Last Edit: August 24, 2022, 02:04:59 am by joeqsmith »
 
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Offline mag_therm

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Re: inductor wound over a resistor?
« Reply #2 on: August 24, 2022, 02:05:23 am »
It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics  of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.
 

Offline joeqsmith

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Re: inductor wound over a resistor?
« Reply #3 on: August 24, 2022, 02:10:51 am »
Not sure if this helps as I have no idea what you are looking at but this is from ARRL handbook.
 
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Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #4 on: August 24, 2022, 02:23:52 am »
well this is a VHF transistor LNA but it looks very similar. I don't see which one is in the manual schematic so I would have to get the box out of storage to look to see which circuit it is on, but IIRC it might not have been listed in the schematic because when I was checking it I thought I found a shorted resistor (since at the time I knew the schematic value) until I noticed the coil around it. I wonder why they would put it in a solid state amp. I guess I have to get the box open again and take a look so I can identify which resistor they put it over (that box is a 5 stage stagger tuned VHF amp that uses differently tuned LC coils from 1KHz - 160MHz with a max output of ~7dbm)

But anyway, there is no interaction between the magnetic material and the carbon composition material, its just there for ideal placement reasons?
« Last Edit: August 24, 2022, 02:31:13 am by coppercone2 »
 

Offline Stray Electron

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Re: inductor wound over a resistor?
« Reply #5 on: August 24, 2022, 03:10:55 am »
  This has been discussed in this forum before.  Resisters were often cheaper than forms for coils so some manufacturers simply used a high value resistor as the body for winding a coil onto.  The give away is that it was usually used with in DC, or low frequency AC, signal paths and the impedance of the resistor is MUCH greater than the coil impedance so the resistance had no significant effect on the operation of the circuit.

  A true story: When I was in college one of my profs brought in a wing tip light out of his Cessna aircraft that wasn't working.  The light had a coil and resistor combo like that and no one could understand why the light wasn't working since there was continuity through the resistor/coil.  But I measured it and the resistance through the combo was about 10,000 Ohms IIRC so needless to say the lamp simply wasn't getting enough current to operate.  I checked the coil closely and found a burnt spot in the wire. I carefully unwound the coil until I reached the burnt spot and I then soldered the ends back together again and wound the wire back onto the coil. I checked the DC resistance and it was only about 1 Ohm. We reconnected everything and checked it and it worked like a champ. He reinstalled it in his aircraft and never had any further trouble with it.  I should add that the light assembly also had a small DC motor in it that rotated a reflector around the lamp so that it made the light appear to blink even though the lamp was actually on all of the time. The coil was in series with the lamp and motor to help block any AC noise from feeding back into the aircraft electrical system.
 
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Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #6 on: August 24, 2022, 03:20:21 am »
thats a little weird that they would allow so much confusion to occur to save very very small amounts of money on low production equipment (in general it seems that it is confusing to anyone trouble shooting the circuit) and that for some reason I think a composition resistor makes for a worse bobbin for the coil, since I think it can short out, or are these resistors short circuit proof?

https://groups.google.com/g/sci.electronics.basics/c/PPL_AXC6ydU?pli=1

it says they can bake themselves into a low resistance
 

Offline vk6zgo

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Re: inductor wound over a resistor?
« Reply #7 on: August 24, 2022, 04:56:17 am »
thats a little weird that they would allow so much confusion to occur to save very very small amounts of money on low production equipment (in general it seems that it is confusing to anyone trouble shooting the circuit) and that for some reason I think a composition resistor makes for a worse bobbin for the coil, since I think it can short out, or are these resistors short circuit proof?

https://groups.google.com/g/sci.electronics.basics/c/PPL_AXC6ydU?pli=1

it says they can bake themselves into a low resistance

Over a long time, carbon composition resistors tend to increase in value, due to heating & other causes.
If grossly overloaded, any resistor can go short circuit, but going low isn't the default just from having a "hard life".
 

Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #8 on: August 24, 2022, 05:52:34 am »
still compared to no parts being there its a bit... improper.. surprised it got popular

its like one of those... hey you know we have some many of these lying around and the shape is kinda similar...

Ok I feel like the problem is that the thing you replace it with should be non-electrical whatever it is. Something really not in the circuit, not dependant on reasonable parameters (i.e. not what happens frequencies greatly outside of bandwidth. Anything else seems bootleg when the part is not supposed to be part of the circuit. It almost seems that ANY resistor (or component) used there, even for creative cheapness, should not be applicable to circuit diagrams without crazy stuff like electrometers and material parameter analyzers. Like a lego brick would make more sense to me then a resistor (but assuming it was a high temperature lego brick), with the point being that a lego brick intuitively has nothing to do with electronic circuits.

I was really hoping that they would have a partial electrical reason. Not that what you said is what is happening in that schematic (it may be a intentionally damped inductor like Joe Q said), but I just really don't like it for some reason. If it was in mega ohms it would start to look like some generically applicable widget I guess, at least in my mind, but when its like Kohms that seems dodgy catagorically (it starts to remind me of a damp piece of wood being used in a circuit for isolation reasons (shitty breadboard))

Like as a building material its like saying ima put up a interior wall using old stereos ..... or like someone just stuffing old couch cushions and mattresses in the wall and covering it with sheet rock as insulation (like actually doing it and selling it to someone as home contracting not just some temp survive in wood cabin for a week before rescue). I mean I guess those things seem like they might work but damnnn .. like if you tried to ratify that in some kind of code it would sound like some survival manual psycho shit like the page about destroying equipment on ww2 navy gear (pickaxes are ok and dynamite is good too!)
« Last Edit: August 24, 2022, 06:16:23 am by coppercone2 »
 

Offline fourfathom

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Re: inductor wound over a resistor?
« Reply #9 on: August 24, 2022, 06:48:29 am »
It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics  of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.

This is the correct answer. 

The resistor is a medium value and dissipates or dampens any potential VHF parasitic oscillations.  At the much lower operating frequency the inductor has a low reactance and effectively shunts the resistor.
We'll search out every place a sick, twisted, solitary misfit might run to! -- I'll start with Radio Shack.
 
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Offline A.Z.

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Re: inductor wound over a resistor?
« Reply #10 on: August 24, 2022, 05:44:25 pm »
It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics  of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.

This is the correct answer. 

The resistor is a medium value and dissipates or dampens any potential VHF parasitic oscillations.  At the much lower operating frequency the inductor has a low reactance and effectively shunts the resistor.

and, as shown in a previous post, it was common in a number of homemade (but commercial too) HF tube amplifiers, I've used it several times with 6kd6, 813, pl519...  based amps
 

Offline RoV

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Re: inductor wound over a resistor?
« Reply #11 on: August 24, 2022, 07:44:09 pm »
A few oldies...

1573258-0

The two on the left and the one on the bottom were recovered by tube b&w TV sets. The two on the top right (sorry one is damaged  ;) ) were commercial units for hobbists/repair, bought in the late seventies. One of them is covered with vax, that was quite common.
 
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Offline G0HZU

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Re: inductor wound over a resistor?
« Reply #12 on: August 24, 2022, 07:50:29 pm »
I am curious about this. The 461A amplifier I spared for switch addition has this method, a wire of maybe 3 turns wound around a carbon resistor.

What is the point of this and how is this designed? Would a modern design still use it, or does it have something to do with lack of proper inductor materials? Does this have anything to do with the resistor material acting differently in the magnetic field, or would it work equally well if it was in parallel with the resistor?

Not much information about this, other then its a form of de-tuning.

It looks to be a simple parallel RL network that is used to define the amount of negative feedback vs frequency in each amplifier stage .

At low frequency the inductor has very low reactance so the resistor is effectively shorted. This means that the other 180R resistor in series with the RL network provides quite heavy negative feedback. This will tend to reduce the excess gain of the stage at lower frequencies.

At higher frequencies the parallel RL network will look like a resistance in series with some inductive reactance and this will tend to reduce the negative feedback. So this maintains gain at higher frequencies and the aim would be to get equal gain across LF through 150MHz.

My guess is that you have to tweak each inductor to get a flat frequency response in the 5 stage amplifier. Each one of these RL networks will be optimised for a particular part of the LF through 150MHz bandwidth of the amplifier. So you would have to know which one to tweak to flatten a particular part of the frequency range. It should be possible to get a really flat response.

Without this tailored negative feedback the amplifier would have lots more gain at low frequencies compared to the gain up at VHF.

I suspect this 'old school' coil over resistor construction method was used out of convenience and it also saves space. It would be unsuitable for today's fully automated assembly systems using SMD though.
 

Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #13 on: August 24, 2022, 08:22:20 pm »
if you mill a slot under the part it might be possible to add a coil to a SMD part
 

Offline JohnG

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Re: inductor wound over a resistor?
« Reply #14 on: August 25, 2022, 01:30:23 pm »
+1 for damped inductor.

I have also seen this in series with audio amplifier outputs, where it allow you to control the impedance seen by the amplifier in the ultrasonic range. This is done because loudspeaker/cable combinations have an impedance in this range that only your favorite deity knows and you don't want your amp oscillating at 100 kHz or 1 MHz or some such frequency.

John
"Reality is that which, when you quit believing in it, doesn't go away." Philip K. Dick (RIP).
 

Offline G0HZU

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Re: inductor wound over a resistor?
« Reply #15 on: August 25, 2022, 02:28:07 pm »
Maybe I'm the only one who is looking at the actual HP 461A circuit so I've attached it below if it helps focus on the real reason the parallel RL network is there.

Each amplifier stage uses resistive negative feedback with an emitter resistor and there is also resistive feedback between collector and base using the RL network in series with a 180R resistor.

Both forms of feedback help with stabilising the gain and input impedance over a wide bandwidth. The reason the inductor is there is to help flatten the frequency response across 1kHz to 150MHz as I described in my previous post. The inductor will help flatten the gain at lower frequencies and it may also help to slightly boost gain up at the higher frequencies. Each coil in each parallel RL network can be adjusted to help to achieve a flat frequency response across 1kHz to 150MHz.

This application of the parallel RL network in the HP 461A isn't quite the same as the other suggestions about fitting the parallel RL network as a parasitic suppressor between the output of the amplifier and any external load.

In the case of the HP 461A amplifier this RL network is part of the negative feedback and the primary reason it is there is to help flatten the frequency response of the overall 5 stage amplifier.
 
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Offline fourfathom

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Re: inductor wound over a resistor?
« Reply #16 on: August 25, 2022, 03:02:05 pm »
Maybe I'm the only one who is looking at the actual HP 461A circuit so I've attached it below if it helps focus on the real reason the parallel RL network is there.

Each amplifier stage uses resistive negative feedback with an emitter resistor and there is also resistive feedback between collector and base using the RL network in series with a 180R resistor.
Apparently you are correct!  I'm curious though, those inductors are shown as variable. How can that be if they are wound on resistor bodies?  Do they just squeeze the windings together as needed during calibration?
We'll search out every place a sick, twisted, solitary misfit might run to! -- I'll start with Radio Shack.
 

Offline G0HZU

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Re: inductor wound over a resistor?
« Reply #17 on: August 25, 2022, 04:19:38 pm »
I'm not sure how they are adjusted. I would expect HP to configure the coil separately so it could be that this isn't the RL network in question but I couldn't see another one in the service manual. Maybe an extra RL network was added to this amplifier?

It does seem strange that the final amplifier stage output feeds direct from an emitter follower with no series resistance. Maybe an extra RL network got added here and it isn't on the circuit diagram?
 

Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #18 on: August 25, 2022, 04:36:33 pm »
No it’s not the stage inductors it’s only labeled as a resistor on the schematic. I can take a look today since this seems popular to find the schematic value. There is only one of these iirc and it’s not the adjustable ones. I think the final stage is what I’m talking about. I only saw one of those combo resistors. The adjustable ones are covered in wax like tuned rod inductors
« Last Edit: August 25, 2022, 04:40:01 pm by coppercone2 »
 

Offline G0HZU

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Re: inductor wound over a resistor?
« Reply #19 on: August 25, 2022, 04:54:07 pm »
OK thanks. I've seen an RL network like this used as a parasitic suppressor as others have already suggested, or as RL networks in power supply feeds (to minimise voltage drop at DC) or as part of an RF feedback network as I described earlier.

I'd expect to see some deliberately added ESR at the amplifier output because it uses a direct connection to an emitter follower as the output. If a fairly fast BJT was used here I'd expect it to produce some negative resistance up towards UHF. Slower parts are going to be OK (in terms of stability) without any added ESR. However, you did say the part was already on the schematic marked as a resistor so I guess it can't be fitted here unless there is a revised schematic available that shows a series resistor at the amplifier output connector?

« Last Edit: August 25, 2022, 05:00:14 pm by G0HZU »
 

Offline Warpspeed

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Re: inductor wound over a resistor?
« Reply #20 on: August 26, 2022, 02:05:34 am »
Agree with fourfathom,
Its the original high impedance grandfather of the modern low impedance ferrite bead.

Its a lossy series impedance added to damp out nasty ringing and resonances, even oscillation !
 

Offline Warpspeed

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Re: inductor wound over a resistor?
« Reply #21 on: August 26, 2022, 02:22:05 am »
Another ancient technique using these curios components you don't see today, were shunt peaking coils in wideband amplifier applications such as video amplifiers and oscilloscope vertical amplifiers..

Back in the tube era, to lift up a sagging high frequency response, it was common to place an inductor in series with the anode load resistor.
The rising load impedance lifted the high frequency gain.  But sometimes the gain might rise so high up near a resonance, instability might result.
The small inductor then may have a damping resistor placed in parallel to tame the beast.  So you might see a few turns wound on a resistor body.  Cheap and effective.

Sometimes this component was selected on test during manufacture, and the schematic might show an adjustable inductor, although in practice the part was selected by trial and error,  not tweaked.
 

Online coppercone2Topic starter

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Re: inductor wound over a resistor?
« Reply #22 on: August 26, 2022, 05:15:03 am »
I think its the L1 near the 50 ohm input resistor. It has a HP part number i can't read. I originally must have read the schematic backwards (the board is weird because the cable goes to the back rather then have the input be right at the front, IMO the input should be in the front and the output should snake back to the front since it already has the amplification noise). So HP does not provide a resistor value, I thought it was a high value resistor near the other side of the PCB. But it does look like a coil over a resistor, but its only about 3 turns. It's varnished very heavily.

god this scan must have been fed through the machine 1000 times
« Last Edit: August 26, 2022, 05:26:09 am by coppercone2 »
 

Offline radiolistener

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Re: inductor wound over a resistor?
« Reply #23 on: August 26, 2022, 08:41:49 am »
It is used as R+L in parallel circuit. But sometimes high resistance resistors can be used just as a coil frame
 

Offline G0HZU

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Re: inductor wound over a resistor?
« Reply #24 on: August 26, 2022, 12:09:23 pm »
If the inductor is L1 then this is shown in the fuzzy schematic below. The input BJT is an emitter follower (highish input impedance) so some shunt resistance is used to set the input impedance to be approximately 50 ohms. If you work out the equivalent resistance of all the input resistors it will be close to 50 ohms. i.e. there's 1800R, 59R and (499+55R) at the input.

However, up at VHF the emitter follower will have an input capacitance of a few pF and this will spoil the input match up at VHF. I think L1 is there in series with the 59 ohm resistor to compensate for the input capacitance of the BJT and therefore L1 will probably be adjusted to minimise the input VSWR up at VHF.

 


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