What’s this symbol? - Page 1
Forum section: electronics / beginners

author: @rt
author_rank: Super Contributor
timestamp: March 13, 2017, 08:37:27 am
content: And what’s it for in series with a superconducting resistor?This is close to the supply input before the DP power switch where the device has a +5/+12V external supply.It might make sense to me if perhaps the 0 Ohm resistors were supposed to be fuses and the manufacturer cheaped out,and the unknown component is something variable with current.Cheers

author: @rt
author_rank: Super Contributor
timestamp: March 13, 2017, 08:40:40 am
content: Ok, it’s a thermistor. Still the question remains. What’s it good for in parallel with a superconducting resistor.

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2017, 03:53:01 pm
content: "NTC" means negative temperature co-efficient. The resistance decreases as the part warms up.If there is a pair of pads for a zero-ohm resistor inparallelwith the part, that probably means that the designer gives the option to stuff one or the other part, but they have different pads. The usual joke is that the second part is 0 ohms, +/- 5%, rather than it superconduct.

author: james_s
author_rank: Super Contributor
timestamp: March 13, 2017, 05:10:16 pm
content: I'd love to find some 0 Ohm resistors that were near the -5% end of that tolerance.

author: SeanB
author_rank: Super Contributor
timestamp: March 13, 2017, 05:15:41 pm
content: Could be they have an option to use one of a few different size NTC devices, or a polyfuse in some size, and in some applications they simply just rely on supply impedance to limit current into the circuit during start up transients.

author: @rt
author_rank: Super Contributor
timestamp: March 14, 2017, 12:22:17 am
content: Thanks guysWell in the product they omitted the thermistor,and used the link resistor.The thermistor footprint looks like a good place to mount fuses.

author: sokoloff
author_rank: Super Contributor
timestamp: March 14, 2017, 12:46:04 am
content: Quote from: james_s on March 13, 2017, 05:10:16 pmThat would still be 0 ohms, though...

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 14, 2017, 12:52:02 am
content: Quote from: sokoloff on March 14, 2017, 12:46:04 am

author: technogeeky
author_rank: Frequent Contributor
timestamp: March 14, 2017, 03:21:23 am
content: I still don't understand if "0 ohm" and superconducting are being used here as a joke or not. I also seem to think that the word series and parallel were mixed up in the original post. But in any case:Thermistors are often used as quench protection triggers in superconducting power circuits. In this application (it's used all over the place at the LHC at CERN and presumably other places with intentionally- and accidentally-quenching circuits), it's used to provide a more sensitive readout for quench protection hardware. This arrangement allows for just enough of a faster readout for voltage monitoring equipment to react to an oncoming superconducting magnet quench. If you are monitoring the superconductor's resistance directly (or the voltage drop across it), it's basically impossible to react fast enough to stop an impending quench. But if you use an NTC device and monitor its resistance/voltage drop, you will get a warning signal readout just fast enough to intentionally force the quench on the superconducting link in question (and usually some nearby links, to cut off the quench from cascading).If this is all a joke, then I'm with Richard.

author: Brumby
author_rank: Unknown
timestamp: March 14, 2017, 05:01:33 am
content: Quote from: sokoloff on March 14, 2017, 12:46:04 amThey wouldhaveto be superconducting to bewithin tolerance.

author: @rt
author_rank: Super Contributor
timestamp: March 14, 2017, 05:44:30 am
content: I mixed up series with parallel in the second post.Quote from: technogeeky on March 14, 2017, 03:21:23 am

author: james_s
author_rank: Super Contributor
timestamp: March 14, 2017, 06:42:33 am
content: If they were less than zero Ohms they would be producing energy rather than dissipating it.

author: tooki
author_rank: Super Contributor
timestamp: March 14, 2017, 10:35:41 am
content: Quote from: james_s on March 14, 2017, 06:42:33 amReminds me that I need to calibrate my Keithley 2015, which shows -2.5 mOhm in 4W ohms mode when shorted...