Homemade Coaxial Shunt - Page 1
Forum section: electronics / projects

author: 13
author_rank: Contributor
timestamp: December 19, 2018, 05:31:32 pm
content: Hello,I recently made a coaxial shunt to measure short, high current pulses. The shunt consists of two concentric tubes of copper attached together on one end via a machined aluminum end plate as shown in the pictures. A BNC connector is bolted to the aluminum end plate with the center connector of the BNC connected across the inner copper tube. I am still working on a way to connect the outer tube to my bus bars so if anyone has a good idea let me know. I know that I should use manganin for the shunt but I cannot find a tube for a decent price. I guess I'll just deal with the horrible temp coef of copper. Also is it worth coating the outside copper cylinder with silver to prevent corrosion (I have alot of Silver Nitrate)? Overall, I plan on using this shunt to verify my homemade Rogoswki coils as I posted about earlier. If you see something I should fix/ improve let me know!13

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2018, 07:35:25 pm
content: Neato, but copper is the worst choice not just for tempco but also because its bulk time constant is very long indeed -- that's an even better reason to use proper materials (if not manganin, then nichrome sheet rolled up?) especially when high speed is demanded.To put that another way: the frequency where the reactance dominates over resistance is relatively low for copper, and relatively high for resistance metals.Simply making the shunt smaller, also helps greatly with inductance. Inductance goes as the physical length.If you can afford the inductance in your current path, then there isn't much bad about a single-turn strip (many current shunts are sold in this format) -- what's nice is you can loop the sense line through two turns, positioned inside the current loop, to cancel out the EMF. You still have the added (10nH or whatever) in the current path, but the signal at least comes out square.Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 19, 2018, 10:17:31 pm
content: Why dont you cut the middle tube and solder there 20 or 30 of this resistors WSHP2818R0100FEA in parallel? Otherwise I dont think your going to measure much

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 19, 2018, 11:41:43 pm
content: I would add that direct contact of copper and aluminium is asking for trouble due to galvanic corrosion.Could You please specify amplitude, rise time and duration of pulses You want to measure?

author: JohnG
author_rank: Frequent Contributor
timestamp: December 20, 2018, 02:59:11 pm
content: Quote from: T3sl4co1l on December 19, 2018, 07:35:25 pmThis is true. The shunt element should be much thinner (4-5x) than the skin depth of the maximum frequency of interest, otherwise the high frequencies will be exaggerated. You can use manganin foil and roll it into a tube. The seam will have negligible effect, even if you don't connect it. In fact, you are probably better off not overlapping it.Quote from: doktor pyta on December 19, 2018, 11:41:43 pmThis is so true. The design of these types of shunts is not trivial at the frequencies where these types of shunts are justified.Also, sometimes the measurements depend a great deal on the rest of the system, and how the shunt connects to it physically, especially at higher frequencies.John

author: 13
author_rank: Contributor
timestamp: December 20, 2018, 05:42:37 pm
content: Thanks for all the responses!The shunt will be used to measure currents in the magnitude of tens of thousands of amps from shorting large capacitors hence its large size. I plan on making a larger shunt based off the results from this shunt.Quote from: MasterTech on December 19, 2018, 10:17:31 pmI would do this but the cost of 20 to 30 resistors of the rated power would cost the same as buying a similar rated shunt.Quote from: doktor pyta on December 19, 2018, 11:41:43 pmThat's a good point about galvanic corrosion! I will switch the end plates with copper instead. As for the amplitude my ultimate goal is to measure pulses upwards of 100,000 amps with rise times of 100 micro seconds.Quote from: T3sl4co1l on December 19, 2018, 07:35:25 pmI found some suppliers of manganin sheets on Alibaba for "cheap" ~$30 per Kg. However, I am unsure of how the sheet will handle currents in the tens of thousands of amps. I don't care if I add inductance to the loop. I originally was going to use an old 50,000 amp shunt but was told it would give erroneous voltages due to the inductance of the shunt its self. Would brass or aluminum be better shunt material?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2018, 06:07:52 pm
content: Aluminum would not be much better than copper. Brass is already little better. One might consider non magnetic stainless steel as a material with a relatively high resistivity and low TC. Not as good as Manganin of cause but still reasonably low TC.The coaxial shunt would still have inductance, unless one uses it in a matched impedance setup (e.g. 50 Ohms shunt). The advantage might be that one can calculate the inductance relatively accurate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2018, 06:18:19 pm
content: Yeah, stainless is like "nichrome-lite". If you can't get the latter, it's still better than others.If you are having trouble getting low enough resistance, just keep increasing the thickness and width, and reducing the length, of the shunt.BTW, nichrome and such can be soldered, but it's somewhat difficult. Acid flux is needed. Or braze, that's even better.Tim