Disk Thyristor "teardown" - Page 1
Forum section: electronics / projects

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 06, 2016, 05:11:20 am
content: Hi,I'm usually around high voltage disk thyristors, but never had the time to open one. So I came across a broken one, rated at 8500V and 1600A max and decided to put it to good use, cracking it open with a lathePics are self explanatory, 4 silicon unions, PNPN, the gate signal is taken to the middle so as to minimize and distrubite diffusion current at firing, thus improving di/dt characteristics of the thyristor.Forgot to mention... as a curiosity this things wont work properly ( ie, low on resistance) unless subjected to a huge pressure. This particular model needs around 4200kg of equivalent weight on top of ithave a good day all!

author: zlymex
author_rank: Frequent Contributor
timestamp: May 06, 2016, 02:56:11 pm
content: Cool.Looks very similar as photo below

author: lincoln
author_rank: Regular Contributor
timestamp: May 06, 2016, 09:03:04 pm
content: You left the chuck key in on the lathe?Cool part though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2016, 10:13:45 pm
content: Neat!FYI, those discs should be molybdenum, I think..? Might be worth finding a place to recycle them at.Moly is noticeably heavy, even compared to steel. Some chemical tests could also be performed; I forget what all it's resistant to.Tim

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 07, 2016, 01:35:20 am
content: Those are awesome. We called them "hockey pucks" up here in Canada...

author: EPTech
author_rank: Regular Contributor
timestamp: May 11, 2016, 01:03:33 am
content: Hi There,Since you are around these things on a regular basis, maybe you can answer this question.I had to replace some of these in a soft starter a while ago. I also replaced it's counterpart just to be on the safe side. I decided to measure the supposedly good one but the typical gate to kathode resistance was not there. Intuitively I put some more pressure on the probes and all of the sudden the resistance was there. The ones of the box has the resistance even though they were not compressed. So my question is: Does the thyristor need to be compressed for the gate to operate correctly or is this considered a defect. I know they need to be compressed to make good contact with the rail assembly but this gate behavior looks weird to me.Thanks in advance. Nice teardown.

author: retrolefty
author_rank: Super Contributor
timestamp: May 11, 2016, 01:11:32 am
content: Quote from: EPTech on May 11, 2016, 01:03:33 amYes, most (or all?) need to be compressed to a minimum pressure before they will conduct. A datasheet should cover the requirements for any specific 'hockey puck' thyristor device. It's not a notable burden as these kinds of devices need good pressure against a heat sink anyway to operate at their specs.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 11, 2016, 04:43:52 am
content: I worked with SCRs in the late 1980s and early 1990s. I built 50kW inverters (half-bridge) and 80kW (full-bridge) running at 10 kHz.I started with Brown Boveri (BBC) reverse conducting thyristors. This is essentially a monolithic SCR and diode on the same piece of silicon.I very quickly changed to using separate SCRs and diodes manufactured by Westcode.These high current SCR are similar to a Darlington transistor:The centre connection is the gate of the smaller SCR, the pilot SCR. The fingers that can be seen on the silicon:are the cathode of the pilot SCR and the Gate of the main SCR.A low frequency SCR will have a simple pattern, may be even concentric circles. Higher frequency SCRs will have more complex shapes. This is done to increase the periphery of the Main Gate. This increases the DI/DT rating of the SCR. Conduction starts on the edge of the gate fingers and spreads across the cathode. Using more of the area for the pilot SCR reduces the current handling of the part.In the design I stacked the capsules:SCRDiodeDiodeSCRInsulatorSCRDiodeDiodeSCRThere were busbars between the semiconductors.A single clamp was used to apply pressure to the assembly. The clamping force was around 9kN (2000 lbs)The silicon was mounted on molybdenum (anode side). Moly matches the thermal coefficient of expansion of the silicon.On the cathode side, there was a silver shim. The pole pieces in the ceramic capsule are plated copper.There is no need to use a lathe to open the capsules. You can cut around the flange with tin-snips.Regards,Jay_Diddy_B