DC power supply question

[Douglas JohnstonParticipant @douglasjohnston98463]

I am building a dc power supply for a project and am using a toroidal transformer along with an electrolytic smoothing capacitor and bridge rectifier. Now I want to make the whole thing as compact as possible and noticed that the capacitor fits very neatly inside the hole in the centre of the toroid but I worry that this might not be a good idea due to the magnetic field in that space. I know that there are people with electronic brains the size of planets on this forum so is this a good idea or a definite no-no ?

Doug

[Douglas JohnstonParticipant @douglasjohnston98463]

[Andrew TinsleyParticipant @andrewtinsley63637]

The normal way of securing a toroidal transformer is through the hole in the middle of the toroid, so may not be a good place. Also the transformer generates heat which isn't the best thing in the world for an electrolytic capacitor.

Otherwise go ahead and put the capacitor there, it isn't going to matter a great deal and there is no "electronic" reason why not.

Just be careful when securing the toroidal transformer that you don't create a "shorted turn" . Easy to do if you are not aware of this potential problem. It won't do the transformer any good at all!

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

Any conductive material in the "donut hole" of a toroidial transformer is a one "turn" secondary and will have an AC voltage induced in it. The voltage is low, typically about 0.1V but is capable of high current. If the ends of the conductor are connected togethe externally it will pass hgh current. A classic example of this is a bolt through the middle holding the transformer to the bottom of a case. All works until you put the lid on and it touches the end of the bolt and Buzzzt fuse blows.

While a capacitor does not make an obvious short circuit, I would NOT put anything in the middle of the toroid.

This raises the question of how you are mounting the toroid….

Robert G8RPI.

[John HaineParticipant @johnhaine32865]

There is no field in the centre of a toroidal transformer – that's the point of them. BUT I think you could get some very odd effects if the cap is the usual metal case type with the negative case grounded to a metal bottom plate and fed by a bridge rectifier from the secondary. Essentially ANY conductor laced through the centre of a toroid couples very strongly to the windings and the cap case could in effect be a winding. So I would avoid it.

The usual mounting kit for a toroid is a couple of big insulating washers and a sort of pressed tin "hat" arrangement with a single screw through its centre to clamp the toroid down – ensure the top plate is only connected through the screw to ground to avoid a shorted turn.

[Anonymous]

Posted by Douglas Johnston on 15/06/2021 14:40:48:

…………………due to the magnetic field in that space.

There isn't a magnetic field in the centre hole, at least in theory. The whole point of a toroid is that the magnetic field is contained purely in the core material.

However, it wouldn't be good idea to put the capacitor in the centre for the reasons mentioned above; heat, danger of creating a shorted turn and problems in clamping the toroid.

Andrew

[Douglas JohnstonParticipant @douglasjohnston98463]

The transformer can be glued in place to avoid using the central hole but probably not a good idea to put the capacitor in there. I was forgetting that the outer case of the capacitor is aluminium and would probably get eddy currents in it. It was just a thought to save space but not one of my better thoughts.

Doug

Just noticed the above response about there being no magnetic field in the middle – you live and learn.

 

Edited By Douglas Johnston on 15/06/2021 16:40:14

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