'Ferrite Pot Core Pairs'? - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Wrangler · 17th Apr 2020, 8:11 am

I think those cores are not gapped, not at those Al figures, and the core in the transformer sketch I saw earlier in the thread showed a gapless core gapped by the insertion of a non-metallic shim washer which will space the whole core apart.

Washer gapping leaves a gap in the centre post and an equal gap in the outer path, so you have two gaps in series.

Gapped pot cores have a gap only in the centre post, and leave the outer ungapped. This leaves the outer as a better magnetic screen so less field is radiated. Gapping washers are cheap, but grinding down the centre post only without doing the same to the outer is fairly expensive compared to the flat grind across the whole face of an ungapped core. The price disadvantage is accepted in normal production stuff because of the better screening.

As a half-way house, some gapped cores are made by having the bottom part made as an ungapped part, and the centre post of the upper half ground to the full gap. The upper half only has the Al value printed on it, and the lower half often has the threaded bush for an adjuster glued in. You can play games withthese parts! Use two bases together to get an ungapped core if you didn't have one in stock. Use two tops together to get an extra-low Al. All with unspoilt screening.

For this job, back when I was doing a lot of ferrite inductors and transformers, I'd have probably tried designing around a Siemens N28 material RM8 pot core probably Al 630. We kept T38 material mostly as ungapped cores.

David

Uncle Bulgaria · 22nd Apr 2020, 9:34 pm

You slightly misconstrued me there, David - I meant I wondered whether the cores shown were gappable or if they were some kind of one-piece. Perhaps this isn't even a possibility!

Regardless, thank you for the information which will come in handy very soon - I have some cores from kind members on the way. They will be thanked publicly when I write up the project!

The PCB has run out at EPE, so they are awaiting some more. There will be a hiatus of some weeks, but as I've got other parts that arrived today I wondered whether putting together the HT generator would be possible on some perfboard to check the function of that part of the circuit. Can anyone see a problem with that, apart from the safety considerations?

I considered making the whole circuit on perfboard, but knowing how I make everything expand to fill the time available I think we would have been here until the cows come home.

Many thanks for all the help. I think I'll start a new thread soon on the reformer itself and my (no doubt) struggles with it.

Radio Wrangler · 23rd Apr 2020, 12:26 am

Ah, sorry.

confusion, will be my epitaph, as I climb the cracked and broken path...

(so now you know my taste in music!)

Well I guess that supply chains of all sorts are going to take quite some time after the virus thing has ended before they get back to normal... if they ever do. I don't suppose it's just fashion houses on the high street which are going under.

David

ian_rodger · 23rd Apr 2020, 8:19 pm

I built the inverter part on perf board; only issues I had were centered on the ferrite cores, messing around with gap etc. I still have it somewhere ....

ian_rodger · 23rd Apr 2020, 8:39 pm

Found it!

Radio Wrangler · 23rd Apr 2020, 10:47 pm

That's a big RM series core, no centre hole for adjustment.

David

Diabolical Artificer · 25th Apr 2020, 7:06 pm

Found some more pot cores J if you need any, see pics.

Andy.

Radio Wrangler · 25th Apr 2020, 7:47 pm

The 'square' pot cores are RM types, not sure of RM6 or RM8 size maybe. The T38 designation is a good low frequency material by Siemens (later EPCOS)
OL marking means gapless. There is no centre hole as these cores are intended for transformers rather than adjustable inductors

The material is good for what's wanted.

The core size is probably adequate but winding might be uncomfortable.

The application does need it to be gapped. But the clips won't fit if the centre post is shimmed to hold tha halves apart. And you don't have a centre hole to pass a screw through.

It could be made to work. Wound with fine wire, gapped with a shim and then taped together.

David

Uncle Bulgaria · 28th Apr 2020, 2:42 pm

Thanks all, and Diabolical Artificer for the latest kind offer. I think I have enough cores to be going on with and have just stripped an FX3440 of its enormous amount of wire in preparation for investigating impedances...

This one has clips on, but I imagine the shim is so very thin that they're not going to be affected by the additional material at all.

17th Apr 2020, 8:11 am	#41
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 'Ferrite Pot Core Pairs'? I think those cores are not gapped, not at those Al figures, and the core in the transformer sketch I saw earlier in the thread showed a gapless core gapped by the insertion of a non-metallic shim washer which will space the whole core apart. Washer gapping leaves a gap in the centre post and an equal gap in the outer path, so you have two gaps in series. Gapped pot cores have a gap only in the centre post, and leave the outer ungapped. This leaves the outer as a better magnetic screen so less field is radiated. Gapping washers are cheap, but grinding down the centre post only without doing the same to the outer is fairly expensive compared to the flat grind across the whole face of an ungapped core. The price disadvantage is accepted in normal production stuff because of the better screening. As a half-way house, some gapped cores are made by having the bottom part made as an ungapped part, and the centre post of the upper half ground to the full gap. The upper half only has the Al value printed on it, and the lower half often has the threaded bush for an adjuster glued in. You can play games withthese parts! Use two bases together to get an ungapped core if you didn't have one in stock. Use two tops together to get an extra-low Al. All with unspoilt screening. For this job, back when I was doing a lot of ferrite inductors and transformers, I'd have probably tried designing around a Siemens N28 material RM8 pot core probably Al 630. We kept T38 material mostly as ungapped cores. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

22nd Apr 2020, 9:34 pm	#42
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: 'Ferrite Pot Core Pairs'? You slightly misconstrued me there, David - I meant I wondered whether the cores shown were gappable or if they were some kind of one-piece. Perhaps this isn't even a possibility! Regardless, thank you for the information which will come in handy very soon - I have some cores from kind members on the way. They will be thanked publicly when I write up the project! The PCB has run out at EPE, so they are awaiting some more. There will be a hiatus of some weeks, but as I've got other parts that arrived today I wondered whether putting together the HT generator would be possible on some perfboard to check the function of that part of the circuit. Can anyone see a problem with that, apart from the safety considerations? I considered making the whole circuit on perfboard, but knowing how I make everything expand to fill the time available I think we would have been here until the cows come home. Many thanks for all the help. I think I'll start a new thread soon on the reformer itself and my (no doubt) struggles with it.

23rd Apr 2020, 12:26 am	#43
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 'Ferrite Pot Core Pairs'? Ah, sorry. confusion, will be my epitaph, as I climb the cracked and broken path... (so now you know my taste in music!) Well I guess that supply chains of all sorts are going to take quite some time after the virus thing has ended before they get back to normal... if they ever do. I don't suppose it's just fashion houses on the high street which are going under. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

23rd Apr 2020, 8:19 pm	#44
ian_rodger Pentode Join Date: Jan 2015 Location: Greenock, Inverclyde, UK. Posts: 161	Re: 'Ferrite Pot Core Pairs'? I built the inverter part on perf board; only issues I had were centered on the ferrite cores, messing around with gap etc. I still have it somewhere ....

23rd Apr 2020, 8:39 pm	#45
ian_rodger Pentode Join Date: Jan 2015 Location: Greenock, Inverclyde, UK. Posts: 161	Re: 'Ferrite Pot Core Pairs'? Found it! Attached Thumbnails

23rd Apr 2020, 10:47 pm	#46
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 'Ferrite Pot Core Pairs'? That's a big RM series core, no centre hole for adjustment. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Apr 2020, 7:06 pm	#47
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: 'Ferrite Pot Core Pairs'? Found some more pot cores J if you need any, see pics. Andy. Attached Thumbnails __________________ Curiosity hasn't killed this cat...so far.

25th Apr 2020, 7:47 pm	#48
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 'Ferrite Pot Core Pairs'? The 'square' pot cores are RM types, not sure of RM6 or RM8 size maybe. The T38 designation is a good low frequency material by Siemens (later EPCOS) OL marking means gapless. There is no centre hole as these cores are intended for transformers rather than adjustable inductors The material is good for what's wanted. The core size is probably adequate but winding might be uncomfortable. The application does need it to be gapped. But the clips won't fit if the centre post is shimmed to hold tha halves apart. And you don't have a centre hole to pass a screw through. It could be made to work. Wound with fine wire, gapped with a shim and then taped together. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Apr 2020, 2:42 pm	#49
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: 'Ferrite Pot Core Pairs'? Thanks all, and Diabolical Artificer for the latest kind offer. I think I have enough cores to be going on with and have just stripped an FX3440 of its enormous amount of wire in preparation for investigating impedances... This one has clips on, but I imagine the shim is so very thin that they're not going to be affected by the additional material at all.