Damn, burned out my 100-watt 50-ohm resistor!

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 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.


 (Need 50-ohm back termination to drive output coax.)
 Sheesh, but thankfully my 150-watt attenuator is OK.
 Those are expensive, even on eBay!


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
On 18 Aug 2016 11:04:56 -0700, Winfield Hill

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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:

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                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Damn, burned out my 100-watt 50-ohm resistor!
Jim Thompson wrote...
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 Yep.  Mmodern RF power resistors are nice, my
 100-watt TO-247 part is only $11 on DigiKey,  
 http://www.digikey.com/product-search/en?keywords=MP9100-50.0F-ND

 But as I said to Dan, I worried for my precision
 attenuator, those are invaluable in RF power work.
 E.g., $500 at Fairview Microwave for 150W units.


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
On 18 Aug 2016 12:11:54 -0700, Winfield Hill

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That will give you indigestion ;-)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thu, 18 Aug 2016 11:20:34 -0700, Jim Thompson wrote:

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Thanks for providing the little diagram, there, Jim. I was finding it  
kinda hard to picture parallel resistors. ;->


Re: Damn, burned out my 100-watt 50-ohm resistor!
wrote:

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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      |
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Re: Damn, burned out my 100-watt 50-ohm resistor!
On 18.8.16 21:20, Jim Thompson wrote:
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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.

--  

-Tauno Voipio (OH2UG)


Re: Damn, burned out my 100-watt 50-ohm resistor!
On Fri, 19 Aug 2016 22:48:02 +0300, Tauno Voipio wrote:

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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. :-/


Re: Damn, burned out my 100-watt 50-ohm resistor!
On Fri, 19 Aug 2016 22:14:26 -0000 (UTC), Cursitor Doom

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You used the wrong bean can, should have been Bush's ;-)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thursday, August 18, 2016 at 2:05:10 PM UTC-4, Winfield Hill wrote:
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Look at " RF Termination Microwave Resistor Dummy Load RFP 150W 50ohm 150wa
tt G150N50W4B "   on Ebay.

  
eBay item number:

311601466625

Dan

Re: Damn, burned out my 100-watt 50-ohm resistor!
snipped-for-privacy@krl.org wrote...
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 Hey Dan, that's a simple dummy load, not a GHz bandwidth
 precision 20dB attenuator, with configurable connectors.


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thursday, August 18, 2016 at 2:42:25 PM UTC-4, Winfield Hill wrote:
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I am just confused.  I thought you said your 100 watt 50 ohm load was burned out.  And that your 150 watt attenuator was okay.

                                     Dan


Re: Damn, burned out my 100-watt 50-ohm resistor!
snipped-for-privacy@krl.org wrote...
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 The two are in series and dissipate the same power
 level.  Right, I'm thankful the $11 100W TO-247 guy
 burned out, and saved the $250 150W attenuator guy.  

 Next I'm going to tear up the PCB layout, double
 the power capability of the heatsink + fan, and
 use two of the 100-watt resistors in parallel.


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thursday, August 18, 2016 at 5:08:42 PM UTC-4, Winfield Hill wrote:

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Still confused.  Assuming the resistors are 50 ohm, would you not need to put two in series and parallel those with two more 50 ohm resistors ?

Do not worry about replying.  I am sure you found a better way like using 100 ohm resistors.

                                      Dan


Re: Damn, burned out my 100-watt 50-ohm resistor!
snipped-for-privacy@krl.org wrote...
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 Right, although 100 ohms is the highest value available
 and there's low inventory at Digi-Key, two 25-ohm in
 series is better.  I'm beginning to like these guys.


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thursday, August 18, 2016 at 5:49:27 PM UTC-4, Winfield Hill wrote:
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Do those have a ceramic back?  
We had issues with production over torquing the mounting  
screw and cracking the ceramic... split lock washers
were also removed and replaced with expensive belleville.
(besides the torque wrench/driver.)

George H.  


  

Re: Damn, burned out my 100-watt 50-ohm resistor!
George Herold wrote...
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 Oh, boy, yes they do.  I've just been using
 a screw w/o anything, and making it tight.
 Sounds like trouble brewing.  What if I'm
 tightening down on a flexible Sil-Pad?


--  
 Thanks,
    - Win

Re: Damn, burned out my 100-watt 50-ohm resistor!
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You may want to use something other than just a screw through the
resistor, so that the pressure on the top of the device is spread
out.  Possibly a large washer?  Some people prefer to avoid using the
screw entirely, and like to clamp the device to the heatsink... run a
flat, rigid metal bar across the top of it and screw down the ends of
the bar, or use a clamping spring.

If you do use a screw, respect the device manufacturer's torque
specification for the part.

If I recall correctly, Sil-Pads have rather poor thermal conductivity,
compared to some other solutions.  They might reduce pressure stress,
but make it harder to keep to the "25C case temperature" part of the
rating spec.




Re: Damn, burned out my 100-watt 50-ohm resistor!
On Thursday, August 18, 2016 at 8:50:17 PM UTC-4, Winfield Hill wrote:
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I wouldn't use a sil pad, unless there was some voltage  
isolation involved.  Belleville washer and proper torque
was caddock's recommendation.  If you are doing them all,
you could do the torque by hand.  Production people like  
to make things "tight", and some of them are strong as an ox.

George H.  

Re: Damn, burned out my 100-watt 50-ohm resistor!
George Herold wrote...
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 Thanks, George!


--  
 Thanks,
    - Win

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