On 18.8.16 21:20, Jim Thompson wrote:> On 18 Aug 2016 11:04:56 -0700, Winfield Hill > <hill@rowland.harvard.edu> wrote: > >> A major meltdown last Friday for my 8ns 1kV SiC MOSFET >> pulser. After set of tests at 400V and 4MHz repetition >> rate, with everything working fine, I had to increase >> the frequency. A loose hand slip and I was at 10MHz. >> That's when the smoke appeared. Blew out a 1-ohm gate >> resistor, both TO-247 MOSFETs, both 24V gate drivers, >> the flying regulator and the 150Mbps 50kV/us isolator, >> and a 0.1-ohm current-sense resistor! Plus a scorch >> damage region on the PCB. In the painful process of >> getting it going again took out another gate driver. >> (No more stock now for those.) >> >> So yesterday after Tuesday's rebuild, I took it slow >> and easy, checking temps with the Flir IR camera as I >> went, looking for the operating limits. Can do 10MHz >> in long bursts, check, 8MHz continuous with HV off, >> check, 5MHz at 500V, nice. >> >> 500V, 5MHz, 5A square waves anyone? >> >> OK, saved scope screen shots, turned everything off, >> went home. Now I find the burned out 100-watt 50-ohm >> non-inductive resistor, mounted on 100W heatsink + fan. >> MP9100, "Caddock's Micronox� resistance film fired >> onto flat ceramic substrate, 100 Watts at +25�C Case". >> (Need 50-ohm back termination to drive output coax.) >> Sheesh, but thankfully my 150-watt attenuator is OK. >> Those are expensive, even on eBay! > > Back in my amateur radio days (K7ZAE, 2M, ~1964), I made a dummy load > with two sheets of FR-4 and 40 2K/2W/Composition resistors in parallel > between the two sheets (and a fan of course). Gave an outstanding > good VSWR at 144MHz: > > ================= > _|_ _|_ _|_ > | || || | > | || || | > |_ _||_ _||_ _| > | | | > ================= > > ...Jim ThompsonI had a similar thing with two parallel groups of 1 kohm 2 Watt carbon composition resistors. There was 40 resistors in a group, and two groups in series. I used brass sheet instead of FR4. -- -Tauno Voipio (OH2UG)
Damn, burned out my 100-watt 50-ohm resistor!
Started by ●August 18, 2016
Reply by ●August 19, 20162016-08-19
Reply by ●August 19, 20162016-08-19
On Fri, 19 Aug 2016 16:58:48 -0000 (UTC), Chris <cbx@noreply.com> wrote:>On Thu, 18 Aug 2016 11:20:34 -0700, Jim Thompson wrote: > >> Back in my amateur radio days (K7ZAE, 2M, ~1964), I made a dummy load >> with two sheets of FR-4 and 40 2K/2W/Composition resistors in parallel >> between the two sheets (and a fan of course). Gave an outstanding good >> VSWR at 144MHz: >> >> ================= >> _|_ _|_ _|_ >> | || || | >> | || || | >> |_ _||_ _||_ _| >> | | | >> ================= > >Thanks for providing the little diagram, there, Jim. I was finding it >kinda hard to picture parallel resistors. ;->Yep. It was an 8 x 5 array spaced far enough apart to allow good air flow. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●August 19, 20162016-08-19
On Fri, 19 Aug 2016 22:48:02 +0300, Tauno Voipio wrote:> I had a similar thing with two parallel groups of 1 kohm 2 Watt carbon > composition resistors. There was 40 resistors in a group, > and two groups in series. I used brass sheet instead of FR4.I housed mine in a bean can (Heinz). Carefully removed the label, topped up with oil, soldered the lid back on and replaced the label. Dead flat VSWR up to about 7Mhz. This was immediately before I learned the importance of minimal lead length. :-/
Reply by ●August 19, 20162016-08-19
On Fri, 19 Aug 2016 22:14:26 -0000 (UTC), Cursitor Doom <curd@notformail.com> wrote:>On Fri, 19 Aug 2016 22:48:02 +0300, Tauno Voipio wrote: > >> I had a similar thing with two parallel groups of 1 kohm 2 Watt carbon >> composition resistors. There was 40 resistors in a group, >> and two groups in series. I used brass sheet instead of FR4. > >I housed mine in a bean can (Heinz). Carefully removed the label, topped >up with oil, soldered the lid back on and replaced the label. Dead flat >VSWR up to about 7Mhz. This was immediately before I learned the >importance of minimal lead length. :-/You used the wrong bean can, should have been Bush's ;-) ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●August 19, 20162016-08-19
On 2016-08-18 11:04, Winfield Hill wrote:> A major meltdown last Friday for my 8ns 1kV SiC MOSFET > pulser. After set of tests at 400V and 4MHz repetition > rate, with everything working fine, I had to increase > the frequency. A loose hand slip and I was at 10MHz. > That's when the smoke appeared. Blew out a 1-ohm gate > resistor, both TO-247 MOSFETs, both 24V gate drivers, > the flying regulator and the 150Mbps 50kV/us isolator, > and a 0.1-ohm current-sense resistor! Plus a scorch > damage region on the PCB. In the painful process of > getting it going again took out another gate driver. > (No more stock now for those.) > > So yesterday after Tuesday's rebuild, I took it slow > and easy, checking temps with the Flir IR camera as I > went, looking for the operating limits. Can do 10MHz > in long bursts, check, 8MHz continuous with HV off, > check, 5MHz at 500V, nice. > > 500V, 5MHz, 5A square waves anyone? > > OK, saved scope screen shots, turned everything off, > went home. Now I find the burned out 100-watt 50-ohm > non-inductive resistor, mounted on 100W heatsink + fan. > MP9100, "Caddock's Micronox� resistance film fired > onto flat ceramic substrate, 100 Watts at +25�C Case". > (Need 50-ohm back termination to drive output coax.) > Sheesh, but thankfully my 150-watt attenuator is OK. > Those are expensive, even on eBay! >The only 50ohm resistor I had that could really withstand power was the one I built myself. Specially shaped copper structures to keep it a real 50ohms even at hundreds of MHz, lots of 2W resistors, then the whole thing submerged in tractor oil. It lasted 30+ years and had to take the occasional kilowatt. A few years ago I had to scrap it because the bucket it was house in (a huge metal honey bucket from a bee keeper) began rusting out and leaking. -- Regards, Joerg http://www.analogconsultants.com/
