Coaxial cable piece for SMPS sense resistor scoping?

[treez]

Hello,
We need a piece of coaxial cable so that we can scope the source current sense resistor in our 160W Boost converter. Do you know of a relevant part number and manufacturer? We can find nothing in our extensive search.

As such, we need to buy an approx one meter length of flexible coaoxial cable with a BNC Plug on one end so it can be plugged into an oscilloscope.
We are not bothered what is on the other end as we would cut it off, and solder the centre conductor to one end of our source sense resistor, and the rolled up outer conductor to the ground end of this resistor, so that we can see the waveform min this sense resistor.
(I think I asked similar before, but not the same as this, I did search for similar but nothing came up)

 

[volker@muehlhaus]

Thin coax cables like RG174/U should be fine. This is pretty mainstream stuff if you buy cable and connector separately. But assemblies are also available, here's an example:

https://www.l-com.com/coaxial-rg174...ansmission-lines-with-straight-bnc-connectors

 

[chuckey]

Try Maplins:- **broken link removed**
Frank

 

[betwixt]

..or even a plain BNC plug and co-ax you make up yourself. It's easy enough to do.

Brian.

 

[treez]

Betwixt, you are right, though we would have to pay for the coaxial stripper, then pay for the coaxial crimp tool which crimps the outer sheath to the cylinder bit of the BNC connector , then pay for the the other crimp tool which crimps the "point" to the inner conductor, then pay for a 50 metre reel of suitable coaxial cable, since nobody sells it in 1 metre lengths any more. Our boss wont like that, so we may pay this company to do one metre length for us for £20...

An electrical assembly company has offered to make us a 1 metre piece of 50 ohm RG316/U coax with a BNC plug on one end for our purpose, and for £20.
It does need to be this kind of coax because it has a thick braid that can be split, then rolled up into a substantial”wire” which can be soldered to the test point.
Some of the black off-the-shelf coaxial cables which are sold with a BNC plug at each end could be used, -we could just cut one end off and use it, but they have very thin outer sheaths which are not subsubstantial enough to correctly solder to the resistor.
The coax does need to be 50 Ohm because it has to go into a oscilloscope channel. (x1 setting)


RG316/U Coax
https://uk.rs-online.com/web/p/coax...22677633D4E4F4E45267573743D3636352D3538373726

BNC connector plug
https://uk.rs-online.com/web/p/bnc-...5522677633D4E4F4E45267573743D3730303933343426

Does anyone know any better?

 

[betwixt]

Those are crimp types so you do need the special tool but you can get compression types with solderable center pins and a nut you tighten to hold the cable in place. If you use RG58 cable and standard plugs it works out much cheaper.

Brian.

 

[treez]

Thanks, I had forgotten about screw-on ones.
Now its just to find somewhere that will sell just a metre or two of RG58. I believe we still need the stripper tool, but I think they are around £10, not too bad.

 

[FvM]

If you decide for RG58 cable, you can buy a 2m BNC cable and cut it in the middle. Personally I would prefer RG174 or possibly RG178 (High temperature - "solder resistant"). But the cable type is rarely used for ready-made BNC patch cables.

Some catalog distributors are still selling coaxial cable per metre. Also most electronics DIY shops do. I presume many engineers are using just a paper cutter rather than a cable stripper.

 

[chuckey]

You do not need a stripper tool, just a sharp knife and a little bit of dexterity. Just what sort of bandwidth are you aiming for with this cable?. Normal RF coax suffers from a change of impedance in the low RF frequencies, that is why video cable was invented, this is guaranteed flat from DC to 10 MHZ over very large distances (> 100m).
Frank