I am trying to house some circuits in a box and have just found an ideal box without having to go to the trouble of making one. Just one slight problem is that not everything will quite fit but if I alter the position of a capacitor then everything will be perfect. It would involve re-siting an electrolytic capacitor to be in the middle of a torroidal inductor. I have never seen anything like that before so I'm wondering if I should attempt this unconventional approach or is there some good reason why I should not? The inductor is 150μH rated for 5A and the capacitor is 1000μF 35V and is used for smoothing in a power supply.

 

I do not see an issues unless it would cause undue excess lead
L in the C routing.

Regards, Dana.

 

Since both leads of the capacitor are on the same side of the hole (as opposed to threading a wire through the hole as in a current sense transformer), induced currents into the capacitor conductors should mostly cancel out. Check the output noise waveform in before and after circuits to see any effects.

If the inductor already is running warm, it will get noticeably warmer. Probably not to the point where the magnetic properties of the core change enough to matter, but something to keep an eye on.

ak

 

Shouldn't be a problem.

 

Thanks for your quick replies folks. I'll get my soldering iron out.

 

If the inductor already is running warm, it will get noticeably warmer. Probably not to the point where the magnetic properties of the core change enough to matter, but something to keep an eye on.
ak

Good point about inductor temperature.

Also, if the inductor is running hot it will reduce the life of the capacitor. The cap is (potentially) positioned right in the middle of an oven.

 

The inductor is a bit overkill for the job. It is rated at 5A and the most I draw is 2A. It stays very cool even when run at 2A for two hours in an ambient approaching 30deg.C. It is fixed to the circuit board with hot melt glue which has shown no signs of softening.

 

The inductor is a bit overkill for the job. It is rated at 5A and the most I draw is 2A. It stays very cool even when run at 2A for two hours in an ambient approaching 30deg.C. It is fixed to the circuit board with hot melt glue which has shown no signs of softening.

Then I agree that that it would be safe to put the cap inside the inductor.

Let us know how it works.

 

I do not see an issues unless it would cause undue excess lead
L in the C routing.

Regards, Dana.

Putting a decent sized MLCC where the electro used to be might fix that.

 

Let us know how it works.

I will do that Richard. It might take a little time to do so because I have things dismantled at the moment. The power supply in my picture is for a NiCd battery charger and will be coupled to a control circuit board and fitted (very tight squeeze) into a redundant casing for another old charger unit. I now have to provide an extra two contacts to the battery pack so I am thinking of ordering some nickel strip for this purpose. I can't think of any other readily available material that will be suitable to use.

 

nickel strip

Where do you get that?

Pure nickel or nickel-plated something? Battery contacts tend to be nickel-plated steel. The plating is done after the parts are stamped and formed, so the edges are covered.

ak

 

There's pure Ni and Ni plated steel on Ebay. I think I would be better with the plated steel as it will be more springy than pure nickel

 

I now have to provide an extra two contacts to the battery pack so I am thinking of ordering some nickel strip for this purpose. I can't think of any other readily available material that will be suitable to use.

Where do you get that?

Pure nickel or nickel-plated something? Battery contacts tend to be nickel-plated steel. The plating is done after the parts are stamped and formed, so the edges are covered.

ak

My choice would be a strip cut out of a "tin" can. But, I am really cheap.

 

I had thought of that as tin can sides are useful for all sorts of things so I have kept a good store of them before the World gets completely overtaken by plastic. The problems I saw with tin cans were not a great deal of springiness and their ability to rust after little more than a year. I suppose I could try tinning the surfaces with solder followed by a quick wipe with a moleskin.
I am really cheap too and I have just discovered a good alternative to hook up wire. It's telephone cable. I have some six core stuff, the cores are not bonded to the outer sheath so they pull out readily. The inner conductors are tough enough to resist damage from a craft knife blade if that's what you use for stripping the insulation and if you run out of colours for identification then you can put a tiny piece of coloured shrink tube on either end. I'll bet there's a lot of you out there who already do this!

 

If the inductor already is running warm, it will get noticeably warmer. Probably not to the point where the magnetic properties of the core change enough to matter, but something to keep an eye on.

Why do you think that the inductor will get warmer? All of the magnetic field is kept inside the core itself and there is none "in the hole", so there will be no interaction between the two devices magnetically. And air circulation in the center is virtually zero now, so placing the cap in the middle wouldn't degrade circulation any more than it is now.

 

Why do you think that the inductor will get warmer?

Experience. this is not an original idea.

air circulation in the center is virtually zero now, so placing the cap in the middle wouldn't degrade circulation any more than it is now.

Yes, it will. Most of the heat leaves the inductor body through radiation and convection.

Inside-the-hole heat will radiate into the capacitor until they reach some kind of thermal equilibrium, at which point the only radiation into the ambient air is through the top of the cap. this is a serious increase in thermal impedance.

Since there is no hole in the PC board to allow true air "flow" through the hole, convection cooling is through air flow around the body, some of which swirls down into the hole through turbulence. That part of the convection cooling is lost when the hole is plugged.

ak

 

Experience. this is not an original idea.

Yes, it will. Most of the heat leaves the inductor body through radiation and convection.

Inside-the-hole heat will radiate into the capacitor until they reach some kind of thermal equilibrium, at which point the only radiation into the ambient air is through the top of the cap. this is a serious increase in thermal impedance.

Since there is no hole in the PC board to allow true air "flow" through the hole, convection cooling is through air flow around the body, some of which swirls down into the hole through turbulence. That part of the convection cooling is lost when the hole is plugged.

ak

Okay, sounds good - I was just curious as to your reasoning.

 

Mount the capacitor on the underside.