Mystery 'Capacity Divider' - UK Vintage Radio Repair and Restoration Discussion Forum

John M0GLN · 12th Mar 2020, 9:33 am

These three photos were taken by a friend a few years ago, they are the only photos he has and whatever happened to the instrument is unknown.
It's called a 'capacity divider' and although I can't see it having anything to do with radios as such, the BNC socket labelled 10M, 12.5PF, 12.5PF suggests something electronic.

I wonder if anyone can identify this piece of equipment please?

Thanks

John

Radio Wrangler · 12th Mar 2020, 9:52 am

It's a high voltage range extender for a voltmeter.

Charging a larger capacitor near ground via a smaller capacitor from the unknown voltage puts a calculable fraction of the unknown voltage on the larger capacitor.

Used because it's a lot more difficult to make reliable high voltage resistors than it is to make high voltage capacitors. Used for measuring high voltage AC, can be used on dc as a one-shot measurement but that's difficult.

The capacitance of whatever you feed it to and the cable are in parallel with the output capacitor and affect the accuracy, hence the design value is carefully labelled.

David

rontech · 12th Mar 2020, 9:53 am

The rounded top cap and the heavily insulated lead with the croc clip suggest something for EHT testing. Also the socket labelled 10 ( megohm? )

rontech · 12th Mar 2020, 9:54 am

Quote:

Originally Posted by Radio Wrangler

It's a high voltage range extender for a voltmeter.

Charging a larger capacitor near ground via a smaller capacitor from the unknown voltage puts a calculable fraction of the unknown voltage on the larger capacitor.

Used because it's a lot more difficult to make reliable high voltage resistors than it is to make high voltage capacitors. Used for measuring high voltage AC, can be used on dc as a one-shot measurement but that's difficult.

The capacitance of whatever you feed it to and the cable are in parallel with the output capacitor and affect the accuracy, hence the design value is carefully labelled.

David

Our posts just crossed lol

John M0GLN · 12th Mar 2020, 10:07 am

It never ceases to amaze me how quickly these questions regarding seeminingly obscure items get answered, thank you for your replies and satisfying our curiousity.

Thanks

John

Radio Wrangler · 12th Mar 2020, 2:00 pm

S'easy! We're some very obscure people!

David

Radio Wrangler · 12th Mar 2020, 2:03 pm

Don't tell me you're not going to put it is as a puzzle in some non-technical magazine as a puzzle "What do you think this is for?" sort of thing. With the label on the far side in the photo, of course.

The replies could be somewhat amusing.

David

12th Mar 2020, 9:33 am	#1
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Mystery 'Capacity Divider' These three photos were taken by a friend a few years ago, they are the only photos he has and whatever happened to the instrument is unknown. It's called a 'capacity divider' and although I can't see it having anything to do with radios as such, the BNC socket labelled 10M, 12.5PF, 12.5PF suggests something electronic. I wonder if anyone can identify this piece of equipment please? Thanks John Attached Thumbnails

12th Mar 2020, 9:52 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Mystery 'Capacity Divider' It's a high voltage range extender for a voltmeter. Charging a larger capacitor near ground via a smaller capacitor from the unknown voltage puts a calculable fraction of the unknown voltage on the larger capacitor. Used because it's a lot more difficult to make reliable high voltage resistors than it is to make high voltage capacitors. Used for measuring high voltage AC, can be used on dc as a one-shot measurement but that's difficult. The capacitance of whatever you feed it to and the cable are in parallel with the output capacitor and affect the accuracy, hence the design value is carefully labelled. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

12th Mar 2020, 9:53 am	#3
rontech Heptode Join Date: Jun 2017 Location: Southport, Merseyside, UK. Posts: 646	Re: Mystery 'Capacity Divider' The rounded top cap and the heavily insulated lead with the croc clip suggest something for EHT testing. Also the socket labelled 10 ( megohm? ) __________________ Time flies like an arrow, fruit flies like a banana

12th Mar 2020, 10:07 am	#5
John M0GLN Octode Join Date: Jan 2004 Location: Southport, Merseyside, UK. Posts: 1,156	Re: Mystery 'Capacity Divider' It never ceases to amaze me how quickly these questions regarding seeminingly obscure items get answered, thank you for your replies and satisfying our curiousity. Thanks John

12th Mar 2020, 2:00 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Mystery 'Capacity Divider' S'easy! We're some very obscure people! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

12th Mar 2020, 2:03 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Mystery 'Capacity Divider' Don't tell me you're not going to put it is as a puzzle in some non-technical magazine as a puzzle "What do you think this is for?" sort of thing. With the label on the far side in the photo, of course. The replies could be somewhat amusing. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done