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Registered Member #3108
Joined: Thu Aug 12 2010, 05:37PM
Location: Worthing, England
Posts: 72
One day, I was pulling apart an ATX computer power supply to salvage some parts to complete my project I was working on. Moments later I found an inductor with a SCREW used as an core !!
As far as I can see, the PSU has not been opened before and tampered with, it must be the manufacture who decide to go cheap and use screws instead.. I have to admit, I am quite shocked!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Russell Haley wrote ...
Proud Mary wrote ...
without adding ohmic resistance.
Please clarify.
Stout enamelled copper wire is used in this inductor, which is clearly intended to pass significant current, and will have a resistance in the milliohm range. At the same time, an inductor wound with such wire will have a high Q, which may cause unwanted 'ringing,' gain peaking, or other destabilising resonance effects. Since R cannot be placed in series with L to lower Q and so widen bandwidth in this circuit, the designer has chosen to use a steel or aluminium screw as a 'shorted turn,' which has the effect of reducing Q without adding ohmic resistance to the inductor.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Looking at the photos it seems that this inductor may be a trap in series with one of the high current outputs. The main inductor is to the right in one picture. Depending on the frequency targeted an aluminum screw would indeed lower the Q whilst a steel one would increase it.
In radio work we used a 'tuning wand' One end was brass, the other powdered iron. When a coil was thought to be needing adjustment, by watching a curve on a scope, if bringing the brass end caused the desired response, the coil inductance was decreased by spreading out turns. if the iron end, then the opposite. If both ends caused undesired effect, it was left alone.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
Proud Mary wrote ...
Russell Haley wrote ...
Proud Mary wrote ...
without adding ohmic resistance.
Please clarify.
Stout enamelled copper wire is used in this inductor, which is clearly intended to pass significant current, and will have a resistance in the milliohm range. At the same time, an inductor wound with such wire will have a high Q, which may cause unwanted 'ringing,' gain peaking, or other destabilising resonance effects. Since R cannot be placed in series with L to lower Q and so widen bandwidth in this circuit, the designer has chosen to use a steel or aluminium screw as a 'shorted turn,' which has the effect of reducing Q without adding ohmic resistance to the inductor.
I suppose this arrangement dissipates the heat more easily than a smaller, cheaper, lower Q inductor?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
The cynic in me wants to raise a more mundane possibility such as they ran out of ferrite/iron powder cores on the assembly line one day and so they decided to slap in a screw to see if they could keep producing power supplies that day.
Also, does it look like the screw was used as a coilform - i.e. do the turns fit neatly into the screws threads? Could also be that it was a cost-effective way to make a high power low-Q inductor like Mary guessed. I would guess that screws are cheaper than magnetic cores simply by economy of scale.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Mattski wrote ...
The cynic in me wants to raise a more mundane possibility such as they ran out of ferrite/iron powder cores on the assembly line one day and so they decided to slap in a screw to see if they could keep producing power supplies that day.
Also, does it look like the screw was used as a coilform - i.e. do the turns fit neatly into the screws threads? Could also be that it was a cost-effective way to make a high power low-Q inductor like Mary guessed. I would guess that screws are cheaper than magnetic cores simply by economy of scale.
I think you are right! Once I'd got carried away with my shorted turn d-Qer notion, I stopped seeing that the screw is not part of an obvious magnetic circuit, in the suicidal way that using a U-bolt to hold down a torroidal transformer would be.
Perhaps it was an individual initiative to save a productivity bonus, or a collective effort to please management accountants. But if it works...
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Inductor with a screw as an core
