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In Reply to: RE: TO220 Schottky posted by Triode_Kingdom on July 22, 2014 at 05:44:19
Jack
That's what I always use and I grind or cut the top part of the diode off of the TO-220 case and cover it with heat shrink.
Honest amplification is better than excessive 2nd order distortion anytime.
Follow Ups:
"cut the top part of the diode off of the TO-220 case "
Thanks, good tip. Do you also know if there's a less-expensive alternative to these in the 1KV range? Like I said, prices have come down a lot, but these are still a little expensive for a HV supply. I'm thinking of a bridge that might need four or five per leg. For everything else, the TO220 Crees are very affordable.
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Buy Chinese. Bury freedom.
These Cree diodes are available without the back, in the TO263 case.
A bit more delicate to work with than the TO220 and harder to see.
I used six in a string on the tail of a 6N6P; I ordered a couple extra and they went to good use!
1200V:
http://www.digikey.com/product-detail/en/C4D02120E/C4D02120E-ND/2679414
600V:
http://www.digikey.com/product-detail/en/CSD01060E/CSD01060E-ND/1244254
Most other values are available.
....I was wondering if coating the back metal tab (either 220 or 263) with Permatex Blue Sensor Safe silicone would alleviate the mechanical noise?
No idea as to the dielectric strength, or if it's cause for concern?
Cheers,
W
I don't but Eli might.
Honest amplification is better than excessive 2nd order distortion anytime.
MUR860G FAST DIODE, 8A, 600V, TO-220AC are a buck a pop at newark.com.
Cleet
A usual (of late) Digikey has 0 stock.
Mouser is well stocked.
...Wondering if Newark still ships each item separately?
I'm wondering if it's worth the effort to snub a TO-220 bridge?
I put a snub over my secondary winding (a 220uF and 330ohm resistor in parallel across the secondary) and given TO-220 are pretty clean I am wondering if I should bother.
I need to do it all by ear, no breadboards here, so a lot of effort to A-B.
Thanks
Cleet
You don't need to "snub" Schottky diodes as they do not have a p-n junction to deal with.
However, they do turn on/off which the RRSF can minimize the effects of.
I leave the filter across the secondary winding. This can be seen on the oscope.
220uf is rather large - consider 0.01uf - 0.033uf HV snubber caps.
Verify your voltage with the link.
Cheers,
W
Ok cool.
Yeah, meant to type .022uF. I think my caffeine levels were depleted when I typed "220" instead of ".022". :-(
thanks
Cleet
Sure thing.
PM me if you want the RRSF schemo for both the FWR. and FWBR.
Cheers,
W
"I'm thinking of a bridge that might need four or five per leg."
You MUST use voltage equalizing resistors, when stacking high PIV Schottky diodes. Cree explicitly warns of cascading failure.
Whenever possible, use Cree part # C3D02060F, which is 600 PIV/2 A. rated. Notice the insulated construction, as opposed to a "flying" metal tab. Mouser's Q1 price for the part is 96 cents.
If you choke I/P filter, make certain to protect the SS diodes against inductive kick back spikes. A very high WVDC 0.01 μF. cap. or appropriately rated MOV does the job.
Eli D.
Thanks Eli, those are the same precautions I take when stacking any SS rectifiers. Max, I don't think those .01 caps will serve to equalize the voltages across the series diodes. The fact that you haven't experienced a failure might simply be a testament to the consistency of the devices, plus the fact each diode in the string is overrated. If it were me, I would give some thought to adding equalizing resistors before something goes awry.
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Buy Chinese. Bury freedom.
Valve Wizard Merlin Blencowe seems to think caps can do the job...
Edits: 07/24/14
up to 1040V using a high voltage probe. Results are shown in the attached graph. at 1040V, the difference between the highest and lowest diodes with caps across them was a MAX of 1.6V.
Pretty darn close!
I don't believe you can accurately measure the PRV seen by individual diodes using that method. As for Merlin Blencowe, he's simply incorrect on this point. Don't get me wrong; if you're happy with it and don't want to make changes, that's fine with me. Nevertheless, the need to use resistors to balance voltages across SS diodes is founded in well-established engineering going back more than four decades. I would personally never build a series string without them.
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Buy Chinese. Bury freedom.
The newer ARRL handbook recommends against using voltage balancing resistors as they seem to cause more failures than they prevent.
According to the 2005 ARRL Handbook (page 17.9) "shunting resistors are generally not needed across diodes in series rectifier strings. In fact, shunt resistors can actually create problems because they can produce a low-impedance source of damaging current to any diode that may have reached avalanche potential."
I know how this might sound, but I no longer consider the ARRL an authoritative source on electrical theory. Walter Maxwell years ago had significant differences of opinion with ARRL staff and was practically banned from publishing under their logo. Walter was a highly regarded satellite/microwave scientist best known in the amateur ranks for his work and publications on choke baluns. I bought his last hardback book on the subject back in the mid-'90s, sold at a discount after ARRL banished him from the ranks.As someone who works frequently with RF magnetics in my day job, I can also point out several areas of ARRL-approved construction having to do with baluns and core materials that violate basic magnetic theory, and over which I have also voiced concerns. The errors have existed for more than 30 years, and like Maxwell, I had no luck ever convincing ARRL staff that any of those designs - still being published AFAIK - should be revisited.
With this as (partial) background, I hope you'll understand why I say it will take more than a quote from the ARRL handbook to convince me. I continue to use the Handbook for easy reference to formulas and the like, but I consider all else to be questionable at best. If there is another source for this new concept, I'd be interested to know more. Otherwise, well, you get the point... :)
Jack
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Buy Chinese. Bury freedom.
Edits: 07/24/14
I don't want to let go of this without at least a cursory examination of the quotation you posted:
"shunt resistors can actually create problems because they can produce a low-impedance source of damaging current to any diode that may have reached avalanche potential."
This claim raises at least one troubling question. If a diode in the string has reached avalanche potential, in other words, its PRV has been exceeded, are the resistors really the issue of concern? Isn't this a theoretical problem that doesn't exist in properly designed circuitry? If a diode avalanches, and the current path through the resistor causes junction failure, it only means the designer failed to install a sufficient number of diodes relative to the total PRV of the string.
I am also skeptical of the author's claim that the resistors provide a low-impedance source for current flow. Values for these resistors are typically in the vicinity of 470k. This seems almost insignificant as a parallel value when considering the impedance of a diode that has avalanched.
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Buy Chinese. Bury freedom.
Here's a discussion on the topic among some hams, with a couple references cited as well. Apparently among the ham community balancing resistors went out with drum brakes.All I can say for sure is that in 7 months time I haven't had any issues using 10nF caps for the balancing duties, in a 2500VAC bridge.
Edits: 07/24/14
I don't think that discussion contradicts what I've been saying on this point. According to one of the contributors, "if you have the equipment, check a few of your diodes and see if they will support say 10 microamps reverse ok. If not, then they should have equalizing resistors." The same person states earlier that "the diodes made today mostly don't require them. Back resistances and switching times are pretty well controlled." He goes on to say that rather than using resistors, he would add diodes to the string.This agrees completely with my previous post, where I said "The fact that you haven't experienced a failure might simply be a testament to the consistency of the devices, plus the fact each diode in the string is overrated."
The bottom line here - in my opinion - is that the balancing resistors provide a safety mechanism that isn't present otherwise. If the particular diodes are well-matched (and impervious to small reverse currents) such that the resistors aren't required, so be it. At that point, it simply becomes a matter of design philosophy. One has to balance A) the possibility of over-voltage or line spikes that might cause the PRV to be exceeded due to whatever mismatching remains, and B) a generalized concept of adding some number of additional diodes for protection. I see nothing to indicate that equalizing resistors cause any harm, so I use the number of diodes indicated by the expected PRV, regardless of the diodes in use, and I add the resistors to be sure they have the best chance for a long and useful life. :)
Jack
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Buy Chinese. Bury freedom.
Edits: 07/24/14
I constructed a bridge using 20 of the 1200V Cree Schottky diodes and used .01uF 1250V caps to equalize the voltage instead of resistors (see picture). It's been running for 7 months now rectifying the 2500VAC supply in my Midlife Crisis 833C amps with zero issues, and the caps also serve to snub kickbacks from the choke-input supply. B+ is ~2300VDC.
I was trying to keep the bridge compact and minimize heat generation, and it seems to have worked well. Since the picture was taken (and before use) I removed the unused solder pads on the perf boards near the high voltage areas and gave the connections a coat of corona dope to help prevent arc-overs.
I am guessing the Jolida 6BQ5 is around 400 VDC...
IF the Jolida amp uses a 4 diode bridge rectifier, 600 PIV Schottky diodes will be fine, from an electrical perspective. Remember my previous warning about mechanical issues! The size of xx4007 diodes is small, while TO220 cases are big.
Eli D.
In a full-wave bridge with capacitive input filter, lightly loaded (as it will be at turn-on), peak reverse voltage across a rectifier unit is 1.4 times the secondary RMS voltage. If the supply is delivering 400V under load, the secondary RMS is probably about 350-375V. The unloaded RMS will be higher. That implies a required rectifier PRV upwards of 550V, but that provides no safety margin for line variations and transients. I would not use less than 1KV PRV in each leg (or 2 x 600V series) for this application.
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Buy Chinese. Bury freedom.
wanted to know what the voltage value for the snubber caps should be , .01mfd @ ?wvdc, thanks!
"wanted to know what the voltage value for the snubber caps should be , .01mfd @ ?wvdc, thanks!"
I've only ever used ceramics for this, always rated at least 1KV. Being as Eli is recommending a different type cap, he's probably better qualified to answer your question.
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Buy Chinese. Bury freedom.
Ceramics aren't bad and many a part labeled for snubber service contains polypropylene, but Mr. Joppa (a man to heed) provided info. as to why polyester is better.FWIW, I'd like to see something greater than 1000 WVDC parts be used, even in an EL84/6BQ5 amp. However, a 1 KV rated polyester part probably will survive around "12" W. tubes. Remember, metalized film caps. are "self healing".
A quick search did not show any polyester dielectric parts rated above 1 KV. :> ( The "best" solution may be ceramic dielectric caps. in series with carbon comp. resistors.
edit: fixed typo
Eli D.
Edits: 07/25/14
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