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Author Topic: Choosing Diodes for HV power supplys.  (Read 25217 times)
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WQ9E
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« Reply #25 on: June 25, 2009, 09:04:50 PM »

Clark,

A 1 amp rated would be more than sufficient.  To put things into perspective, the replacement transformer sold by P. Dahl for the Collins KW-1 was rated for .7 A CCS and the replacement for the Johnson Desk was rated at .6 A CCS.  Both of these were full 1 KW carrier input rigs and the modulators and finals were supplied by the single HV supply.

But conservative is good and I wouldn't hesitate to use the best transformer you can find.  When I built my homebrew amp several years ago I went with a P. Dahl 1800 volt 2 A CCS transformer and I have no regrets about this choice.  At the time I planned to use the amp as a linear amp for AM as well as contesting use so I built it on the stout side.

This is the hamfest season so hopefully something good will show up.  A parted out broadcast transmitter could also be a good source for you.  I am currently looking for a bit smaller transformer to build a power supply for the Johnson Invader 2000 I picked up a few months ago.  I have a lot of projects in progress so I expect I will stumble across a suitable transformer in plenty of time before this restoration project bubbles to the top of the stack.   Of course I came across a couple of perfect transformers last year when I wasn't looking for one-a little voice told me to buy them so note to self:  listen to little voice next time and it will save you big money!

Rodger WQ9E
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ke7trp
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« Reply #26 on: June 25, 2009, 09:21:22 PM »

YEah.. Thats kinda what I thought..   really.. 1800 to 2500 volt 1 amp would be PLENTY.   This big power supply was major overkill for my needs. I have an engineering note from 1968 where they tested it at 2570 watts input from measuring voltage and current to the power supply. AM mode, 100% mod.  So as you can see this was built tough..  Later it was run at 2100watts input through the 70s.

Its now rumored that it was used as a low power AM transmitter for a college campus at some point. Who really knows.

Thanks for the lessons everyone.. I am going to see what pops up.  If not, I can at least get my money out of it.

Clark
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« Reply #27 on: June 25, 2009, 10:22:19 PM »

But conservative is good and I wouldn't hesitate to use the best transformer you can find.  When I built my homebrew amp several years ago I went with a P. Dahl 1800 volt 2 A CCS transformer and I have no regrets about this choice.  At the time I planned to use the amp as a linear amp for AM as well as contesting use so I built it on the stout side.
Rodger WQ9E

Rodger,

The p.s. I'm building for my amp for AM has a Dahl 1.5 A CCS 2.25 KV -- I went with an increase in A CCS rather than voltage & that sucker weighs 70 lbs.  abt. 3 times the weight of the stock B+ transformer so I hope I'm on the right track.

73

Rob K5UJ
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WQ9E
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« Reply #28 on: June 26, 2009, 08:28:09 AM »

Rob,

You should be well covered on the transformer end of things!

Make sure you have good filter caps (oil type preferred if you can find them).  I got mine from Fair when I built mine (eight 7.1 uf units in parallel).

If your design leaves the HV on all the time the amp is on then a step start circuit would be a good idea to reduce inrush.  Otherwise, using inrush current limiters can be used if it is a PTT switched HV to provide inrush current protection.

Post some pictures when you get a chance!

Rodger WQ9E
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WD5JKO
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« Reply #29 on: June 26, 2009, 09:43:38 AM »


The group consensus has been for some time that oil caps are best if they can be found, are within budget, and if they fit in the space available. Last year I took devils advocate on why electrolytic caps might be a worthy choice, and I discussed ways to make the series strings less fragile. Sometimes taking a position like this is similar to taking a "Fight Global Warming" poster to a GOP rally! Grin

Here is the link to the discussion, and my post is near the end:

http://amfone.net/Amforum/index.php?topic=17729.0

73,
Jim
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WQ9E
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« Reply #30 on: June 26, 2009, 10:06:05 AM »

Jim,

I remember that post and it is good information.  Most of my gear does have electrolytic caps since that was original design and there isn't room for oil filled replacement even if it was economically feasible.  Your points on increasing reliability and life are spot on.

For homebrew HV, I try to use oil caps when practical but Jim is right that electrolytics are fine and when used properly they can save a lot of money and have a long life.  I would much rather use known quality electrolytics than take a chance on some of the "energy storage" caps run near their stated voltage ratings.  I know that some of them are fine but others definitely do not like the high ripple in a capacitor input filter. 

Certainly oil filled caps can fail also.  In my early days of ham radio I was at a contest station that was using 3 of the big Alpha amps and one of them suffered a capacitor explosion about 2 hours into the contest.  The amp was new so the failure was due to some form of manufacturing defect.  It caused quite a mess with oil throughout the amp and smoke from the oil getting pulled into the cooling system and then baking on the tube anode as the breakers tripped and killed the airflow.

Rodger WQ9E
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« Reply #31 on: June 26, 2009, 12:09:16 PM »

Engineering is always about tradeoffs. I wonder about the lower cost for electrolytics though. When you add in the cost for the equalizing resistors and the extra caps for a safety factor, does the lower cost still hold?

Guess I need to get the Mouser catalog out .....




The group consensus has been for some time that oil caps are best if they can be found, are within budget, and if they fit in the space available. Last year I took devils advocate on why electrolytic caps might be a worthy choice, and I discussed ways to make the series strings less fragile. Sometimes taking a position like this is similar to taking a "Fight Global Warming" poster to a GOP rally! Grin

Here is the link to the discussion, and my post is near the end:

http://amfone.net/Amforum/index.php?topic=17729.0

73,
Jim
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« Reply #32 on: June 26, 2009, 12:58:48 PM »

Rob,

You should be well covered on the transformer end of things!

Make sure you have good filter caps (oil type preferred if you can find them).  I got mine from Fair when I built mine (eight 7.1 uf units in parallel).

If your design leaves the HV on all the time the amp is on then a step start circuit would be a good idea to reduce inrush.  Otherwise, using inrush current limiters can be used if it is a PTT switched HV to provide inrush current protection.

Post some pictures when you get a chance!

Rodger WQ9E

Hi Rodger,

Well, I got three 1.2 KV 88 uF oil cans from RF parts; I plan to put them in series with a 200K 50 w. resistor across each one.
h.v. will be on all the time.  I'm gg to change the stock step start resistor.  The stock setup is typical design I think with a relay that puts one leg of the primary through a resistor before it closes to short the resistor.  I'm gg to swap out the stock 10 ohm 25 w. R for a 10 ohm 200 w. one.  I hope this will hold the startup current to 12 A for a half second.  gg to use 10 Fairchild RURG8060 diodes on each leg of a full wave bridge.  Across each one will have 500 v. .01 uF disc and one 300 K 1 watt metal film resistor.  Oh I am looking forward to posting pix but I'm taking my time enjoying the project.  Need to build a cabinet.  I plan to build a wood cabinet with the bottom on wheels, and the back panel fixed to the bottom.  On it i'll anchor the caps and rectifier breadboard. and put a fan and transformer on the bottom panel.  bottom and sides should be around 3/4 inch thick.  I want the other three sides and top to be on hinges with latches so they can all fold down to give easy access to the components.  Probably won't be finished until fall some time. I have to start building the rectifier next.` I'm going to try cutting strips of copper strap about 12" long and maybe 1/4 to 1/2 inch wide and put one along the length of the holes in breadboard, poke the component leads through from the other side and solder them to the strap.  when all thesoldering is done, clip the copper strap to make 1/2 inch gaps under each diode.  I hope this strap will make a low resistance h.v. connection and act as fins to radiate heat.   Like I said, I'm no engineer; this p.s. will be either way overkill or burn the house down  Smiley

73

Rob K5UJ
 
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ke7trp
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« Reply #33 on: June 30, 2009, 03:30:27 AM »

I very nice fellow AMer came by the house today with BC610E transformer.   The problem is that its huge.. 

NICE PEOPLE DO EXIST!!!!    Grin

To fit it on the PS deck of this rig, I am going to have to remove the Two 4hy@500ma chokes and the two 2MF at 4kv oil caps.   There is just no room to fit this Trans in!

I can keep the chokes if I move them on the deck.  The Oil caps are to large.. Very old style.. 5 in X 5 X 6 tall each!. 

So If I can come up with a smaller 4kv cap, this might all fit.. Anyone have any suggestions on how much MF I actualy need here?  I have 4MF now total at 4kv.  I found to very slim line caps that are stacked would be about the same size as one of the old caps.. These are 4MF each at 4KV.. I would have 8MF total. 

I am half tempted to build an enclosure for the Trans with Seperate input and HV lines running out and into the transmitter. That would make this project alot easier. External Variac, External HV Smiley

By the way, I got the 1N5408s for the Rectifier board.  1k at 3 amps each.  So that issue is settled.

What would you do?Huh  The old Transformer was 220 volt run off two ganged variacs.  This one is 120 volt...

Thanks For all the help!   

Clark


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KE6DF
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« Reply #34 on: June 30, 2009, 10:57:29 AM »

I had two of those transformers when I was a teenager -- one on a BC610 power supply chassis and one separate.

I schlepped them around from move to move but somewhere when I was about 30 I got tired of moving the things (100lbs each) and threw them into a dumpster.

That was a shame. I should arrest myself for crimes against boatanchors -- wish I still had them.

One memory of them as the first time I hooked up the powersupply.

The transformers have a 120 volt secondary with two taps designed for like 105V and 115V if I remember right.

By mistake I hooked the 120VAC BETWEEN the 105 and 110 taps. I must have had nearly 50KV coming out of the transformer.

I was working on it on my back porch out-door workbench.

I turned it on, the MW tubes arched over, probably a few other places arced over, there was a loud band, every dog in the neighborhood started barking, the neighbors came running out of their houses to see what had happened.

I blew the filter caps -- they started leaking soon after and I threw them away. I blew the rectifier tubes. But surprisingly the transformer and chokes survived.

Sometimes I wonder how I made it past age 18.

Dave



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WD5JKO
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« Reply #35 on: June 30, 2009, 11:38:30 AM »



Clark,

   I had one of those, and for years it followed me around QTH to QTH. I even used it once too with 3b28's.

If you want to run the rig off 220v, then you have a problem. Maybe look for a 220/110 autotransformer rated for the application.

That thing with FW CT rectification and a choke input filter will want to be a diode killer....unless you take precautions.


Look at the following online book, section 1.4.3:

http://books.google.com/books?id=pv9nUimtuvsC&pg=PA11&lpg=PA11&dq=PIV+diode+for+power+supply&source=bl&ots=d4qlrvH2jH&sig=0DVCnii4JldoiX8k1voS3F5OlfQ&hl=en&ei=USNKSriUFI2KMujn_KMB&sa=X&oi=book_result&ct=result&resnum=10

Lets say this transformer is 5000 volts CT, then we have 2500v RMS each side of center, or about 3500v peak. The diode PIV needed will be double that or 7000v PIV. As a safety factor double that again, and we need 14,000v PIV. That means you need AT LEAST seven 1N5408's on each leg in series, and with a safety factor, you need no less than 10, and preferably 14. Still at 20 cents each this is cheap.

Now with a choke input filter you need to guard against the back EMF from the choke when the load is suddenly removed, such as with CW KEY UP. This will require a series R-C snubber across the choke (preferably), or from choke input to ground. Some of the Bill Orr handbooks cover this with formulas for the R and C. A rough guess might be to make the R equal to the load resistance of the supply (RL = E/I), and make it about 5 watts WW. The C could be somewhere around 0.05 UFD at a voltage rating that is about 1.5 times the peak AC voltage going into the filter. Making the C too large will get the resistor hot from normal conditions. Might be best to find the real calculations.

Adding a 120v varistor across the AC primary is a good thing, such as a V130L20A

This is a case where a pair of 866's or 3B28's are a worthy candidate to consider instead of diodes....

I prefer diodes where the transformer has NO CT, is lower voltage, where either a FW bridge or a FW voltage doubler is used. With big High voltage FW CT rectification we need sky high diode PIV ratings, or Xenon, or MV tube rectifiers.

Comments anyone?

Regards,
Jim
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« Reply #36 on: June 30, 2009, 12:50:10 PM »

Some of the BC610 chokes had spark gaps -- similar to modulation transformers -- to deal with the power off/on surges.

That probably was to protect the chokes not the rectifiers, however, and a spark gap probably would still let too high a reverse voltage spike get to the diode rectifier.
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« Reply #37 on: July 02, 2009, 10:37:20 AM »

The proper procedure is to calculate the minimum L for that choke and then add a bit. This will minimize the min-max swing and make any rectifier happy. The 866 is especially prone to surge current arcing where a 1N5408 is impervious in an engineered PS.  This is a good place for a 5-25H or 6-30H swinging choke. Many follow the 5X swing ratio.

All my HV supplies use SS and swingers followed by lots of C

When I was 17 I joined Army MARS out of Mitchell Field and dragged home a pair of those BC-610 transformers plus 250TH's and lots of other parts. Never could even warm up that iron with PP 250TH's and 810 modulators runing thru a '610 mod xfmr. Drove it with a Viking I. Made about 5 contacts on 15M before the neighbors came over with the tar and feathers. After that I learned about shielding, coax, and low pass filters. This was in the day of TV's with 21mc IF's. Band changing was a PITA and that was my last PP plug in coil amp. Onward to the pi network and bandswitching.

Carl
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« Reply #38 on: July 03, 2009, 04:11:12 AM »

I have a 5600 volt, 1.5 amp, center taped. Weighs in around 100 lbs. Shipping would kill ya.
Regards,
Gary
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« Reply #39 on: July 03, 2009, 10:29:14 AM »



  I remember using a open frame BC610 transformer setting on the floor for my P-P 805 rig of the mid 1980's. One day I was bare foot, and on the air. Picture me with D-104 firmly clamped with my right hand, and I was stretching somewhat by extending my left leg. Then I realized what I was doing and looked under the table. My left big toe must have been a fraction of an inch from one of the HV terminals on that transformer. If I had touched, I think I would have seen my maker..

True story! Shocked

Jim
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« Reply #40 on: July 03, 2009, 02:06:04 PM »



  I remember using a open frame BC610 transformer setting on the floor for my P-P 805 rig of the mid 1980's. One day I was bare foot, and on the air. Picture me with D-104 firmly clamped with my right hand, and I was stretching somewhat by extending my left leg. Then I realized what I was doing and looked under the table. My left big toe must have been a fraction of an inch from one of the HV terminals on that transformer. If I had touched, I think I would have seen my maker..

True story! Shocked

Jim
WD5JKO

Reminds me of a friend I had back in the 80s (wierd, lots of HV probs in the 80s Smiley ).

He had his D104 in lt hand, keyed up, adjusting something in his Heathkit Warrior (811A amp).

Well, the dummy had a small gold chain on his neck, and it came in contact with the plate assy, connecting the parasitics, Cblock and the RFC choke.  I didn't attend the funeral, but was told the chain was left in his neck, it had imbeded itself so deep.  He fell forward ON to the amplifier when he was electrocuted.

RIP, 5-4 Beamer, we all miss ya .....  San Diego, Ca.  1984, IIRC.  Coulda been 85 or 86, though.


--Shane
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« Reply #41 on: July 04, 2009, 08:53:28 AM »


Shane,

  Sorry to hear about your friend. Yes the 1980's were shocking for many of us.

My worst shock was at work. I worked at a place that made ion beam accelerators. This particular model went to 120,000 volts, and was the first production unit built following the prototype. Back then (1983), lock out tag out (LOTO), and grounding hooks were non existent.

One day I had to do some calibration on a variable gas flow valve that was within a module that was normally at elevated voltage. To perform the task I needed "control power' on. I had been working 80 hour weeks, and all I was worried about was getting this calibration done. The high voltage supplies were powered up, but not commanded to do anything.

While I had both arms in the 'gas box' and my feet on the concrete floor, a co worker went up to the operator console and proceeded to turn on the 'extraction' voltage which was a 40 KV @ 50 ma power supply. The supply was custom designed that if powered up into a low impedance, and the current went to 50 ma, it would sustain that current for 1 second, shut off 1 second, and then try again two more times.

My body took three one second bursts of 50ma from my arms down through my legs to the floor. It was really quite incredible the sensations, and also my inability to pull away during the 1 second period between the bursts. After the episode I walked away and sat down. Later I went home, and received a call from the boss saying there is work to do, and why am I at home. So I went into work, and worked till midnight. Of course he left at 5 pm.

That night I woke up from sleep, all sweaty, and kind of jumping around in bed. I never went to the doctor. Several days later I noticed that the skin on the bottom of my feet started flaking off in large pieces as if I had severe sunburn. That 40KV @50ma had to arc through the soles of my shoes, and in doing so damaged my skin.

I am still in the industry, and deal with much higher voltages these days. The power supplies are now up to 100ma, and these never shut off into a short; they just go constant current forever...

The safety aspects of this industry today are much more stringent, to the point of being a PITA, but the good part is that accidents now are very rare.

Sorry for my 2nd off topic post...

Jim
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« Reply #42 on: July 07, 2009, 07:56:27 PM »

K2AW Silicon Alley rectifiers will solve all of your HV needs.
Big manly diode stacks, like in a BC transmitter.

Fred
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