Choosing Diodes for HV power supplys.

[ke7trp]

I am rebuilding my HV power supply.  This supply is 2 amps at 4000 volts.  It will run two 4-400s up at max voltage and power.

The power supply had 872s but sometime in its life someone removed those and installed strings of diodes. 16 of them. SR-222 is the part number. I cant find any information on them. 

Anyone have any idea what the volt and current rating of an SR-222 is? 

I am going to remove them and install newer style diodes.  I have a 100 pack of 1000PIV 1 amp diodes (FR-107).  It would be convenient for me to use them.  I have now been told by several people to use 1000 PIV at 3amps. (1n5408)

For the life of me, I cant figure out why the 1 amp diodes would not be enough.  Can anyone take a moment to explain why the 3 amp is recomended? 

Thanks for your help!

Clark

[WQ9E]

Clark,

I will take a stab at this:  Do you need your power supply to run at its full rated current or just enough to run two 4-400s at their max voltage (4000) and rated power (well under 1 Amp plate current at this voltage)?  If you are not going to exceed the 4-400 power capability you should be able to use the diodes you have on hand since a unit using 872 rectifiers would have been designed with a choke input filter (lower peak charging current) and the full wave CT circuit is splitting the total current between the two strings.

However, if at any time you plan to run the power supply at its full 2 A rating then you really should go to a higher rated rectifier.  If the power supply had used (or is modified to) capacitor input filtering then the peak charging current is high enough that the additional capability of the 1N5408 is certainly desirable. 

The 1N5408 has about 7 times the safe peak current capability of the FR-107 and provides a much better safety margin and that is what I would use.  I get mine from Mouser where they are around a dime each in quantity 100.  Personally, I would buy some 1N5408 rectifiers for this supply and save your FR-107 for replacing tube rectifiers in receivers and other lower power applications.  Although I would assume most modern parts have a fairly conservative rating, some of the earlier Japanese sourced rectifier diodes depended upon the heat sink ability of their leads to meet their current ratings and were a source of failure when the shortened leads and circuit board lands did not provide enough thermal transfer.  Given the total cost of a 4KV 2A supply, the cost of good rectifiers is insignificant and a good safety factor is highly desirable.

Rodger WQ9E

[K5UJ]

Hi Clark,

Firstly, I am not an engineer, I need to get that out of the way.  As a result I tend to over-engineer things, and also not provide enough capacity so when I fire up my p.s. for the first time I'm going to have a fire extinguisher on hand  

Anyway, I'll take a general stab at your question because I've been going through this exact same thing with diode selection and know enough to be dangerous.  From what I have read, your 1 A 1 kv piv diodes would be okay at face value if you strung 8 of them together (I'm assuming you are building a full wave bridge) on each leg.  You should have at least twice the PIV of your p.s. voltage.  You'd also have 8A.

Here's where the problems may come in and I think there are two:  When you first power up the supply there's a big surge current.  I think this depends on the winding resistance of your transformer.  I read somewhere that if you have a tranny rated at 1/2 amp CCS you can have an inrush of 400 A; for 1 A or 1.5 it can be 800 A.  You have an initial current surge on the diodes that can be pretty high for the first 1/2 cycle of AC, not 400 or 800 A but perhaps higher than your diodes can handle.  
I can't say much more because I don't know what your transformer is and what those diodes you have are.  If you can find them listed in mouser.com then you may find a data sheet on them with the current specs for the one-time initial current max.

The other thing is that the diodes aren't conducting all the time.  Your filter caps are supplying the current except at the peak when they discharge, then the diodes conduct.  I don't fully understand this but at that point the diodes conduct a lot of current.  This is known as the repetitive peak current limit.    ARRL says the diodes should be rated at 10 to 20 times the average current your p.s. puts out.

My guess is the guys who recommended the 3A diodes think they can handle these two factors, the repetitive peak current and the initial surge.  

BTW, this is all stuff I've been trying to sort out and got this information from the ARRL handbook so I recommend reading the chapter in it about power supplies, ch. 11 in mine.  In the later editions (mine is 2001) they say there is now no need for cap and resistor parallel with each diode but I'm using them anyway on mine because that's how I see the rectifiers done in the commercial ham amps and I figure the engineers at companies that ship out thousands of amps and have to deal with repairs know more about reliability than the ARRL.

73

Rob K5UJ

[WD5JKO]

It is hard to follow the two prior posts that pretty much covered it all. So I will instead relate a story.

Some years ago (1980's) a company I worked for used many 1N400X (X = 1-7) diodes in manufactured goods. Then some 'brilliant' marketing guy decided that they would buy all 1N4001's, and then get a low paid employee to screen the 1N4001's for PIV. As it turned out many would pass for the higher PIV types. These were then sorted based upon PIV.

After a while we experienced unusually high infant failures where these diodes often failed. I was in the R&D group and called upon to investigate. Watching the lady screen diodes on the Tektronix curve tracer I noticed that she would run up the voltage until the diode avalanched just a few micro-amps of current. Sometimes I'd see something like an avalanche at 1100 volts, and then it changed to 500 volts. The diode once avalanched changed forever.

This started a search for 'controlled avalanche' rectifiers. I recall an old GE diode that we called 'rat-turds' where these would avalanche cleanly like a zener diode. Boosting the current did not damage them. I once cranked things up until the diode body was red like a 4-65 plate, and that GE diode never failed. It was amazing. Back to the 1N4001-7 series, the avalanche once done destroyed these devices, and if they still seemed to work, they often failed quickly afterward.

Diodes that cannot avalanche gracefully really needed the classic resistor-cap shunt combination commonly seen on older construction articles.

Most of the diodes today are 'controlled avalanche' devices and therefore we don't need to provide the shunt R-C across each diode. This includes the 1N4007 and 1N5408. This might sound like a contradiction. I will say that a 1N4007 built in 1980 is not the same as one built today. I could be wrong, but I believe that to be correct.

I still prefer the 3 amp rated devices because they are so cheap, so there is no $ cost savings choosing the 1 amp 1N4007.

73,
Jim
WD5JKO

[w4bfs]

one additional thing that is often overlooked ... when you have a choke input filter and you turn the hv supply off .... if current is flowing in the choke and the load is dropped OR the hv turned off then its Tesla time and a large magnitude voltage spike may be created by back emf .... to protect the supply diodes a small value capacitor may be used either from the input side of the choke to B- or across the choke (with attention paid to possible resonance effects) .... I have done this for a long time with no failures ...hope this helps ... John

[KD6VXI]

Purchase the 6A10 diode.  Be done with it.  Yes, they are a bit more expensive, but they are pretty near indestructable.

6A CCS is more than enough

I've used them on some of my larger stuff, and I know of others. 

--Shane

[KD6VXI]

Anyway, I'll take a general stab at your question because I've been going through this exact same thing with diode selection and know enough to be dangerous.  From what I have read, your 1 A 1 kv piv diodes would be okay at face value if you strung 8 of them together (I'm assuming you are building a full wave bridge) on each leg.  You should have at least twice the PIV of your p.s. voltage.  You'd also have 8A.

I might be missing something, but 8 1A diodes in series is still 1A, not 8A.

MIGHT be the source of them fires when you first fire the supplies up

6A10.  400A surge capability.  6A forward.

'Nuff said.

--Shane

[ke7trp]

Thanks for taking the time to post guys.  I think I will just order the 5408s. I can get them cheap and they are overkill for the supply.  I understand now that I could use the 1amp diodes for the 500 mills I might pull with the 4-400s.  But one time something goes wrong or I mistune it one time, I could pop the string.  I would feel safer with a diode setup that matched the 2 am rating of the transformer. 

I had the FR 107s and I figured why not.. But I dont like doing things twice and this PS is heavy..

By the way.. THe power supply has two 4HY chokes and two oil cans around 3MF each.  It also uses 50K at 400! watt resisistors.  The largest resistors I have seen.  The Transfomer is rated at 8900 Volts at 2 amps. and is run on a built in to the Rig, Dual gang Variac.  This explains why I only need to have the Variac 1/3 of the way to get 3000 volts.

THanks again!


Clark

[WQ9E]

Clark,

Sounds like a nice supply!  You might consider adding a mechanical stop to the dial controlling your Variac to avoid an accident from over-voltage.  It is possible a shack visitor or cleaner may set the Variac to "full throttle" and when you next apply mains power you may get quite a nasty surprise.  There is probably room to add a collar (with an extra long set screw sticking out) to the shaft; you would then mount a screw with nuts to hold it to the panel sticking out far enough that it would contact the set screw if someone attempted to turn the Variac too far.

Rodger WQ9E

[ke7trp]

Roger..I am going to do this today.. The deck is out and there is easy access.  That is a great idea..  I am also going to put a check list next to it on a clip board with knob readings.

Thanks!


Clark

[KD6VXI]

Roger..I am going to do this today.. The deck is out and there is easy access.  That is a great idea..  I am also going to put a check list next to it on a clip board with knob readings.

Thanks!


Clark

You can also mount a microswitch to control a circuit you want to come on (ptt, etc) when the voltage gets up to snuff...

IE, as the variac is turned, the mech stop you guys are talking about comes about, and as it hits the stop, it mechanically STOPS, as well as hits a small microswitch, enabling PTT or something (some use it to bypass the variac once it's fully up to voltage, as a pseudo step-start).

Somethin to think about, another safety feature, etc.

--Shane

[ke7trp]

The power transformer for the plate is shorted.   From CT its 450 ohms on one leg and 20 on the other.  This is an 8900 volt 2 amp transformer. To expensive to replace. I am going to part out this entire unit. I will be selling the RF deck, LV supply, 810 modulator deck and the chokes and caps from the power supply. 

Clark

[KD6VXI]

The power transformer for the plate is shorted.   From CT its 450 ohms on one leg and 20 on the other.  This is an 8900 volt 2 amp transformer. To expensive to replace. I am going to part out this entire unit. I will be selling the RF deck, LV supply, 810 modulator deck and the chokes and caps from the power supply. 

Clark

WTF.  You had the thing working, and had Pout on SSB.

Wha Happen?

As to the xformer, you could convert to cap input and FWB with a pole big....  You have a variac to get teh voltage anywhere you want.

--Shane

[ke7trp]

Dont know..  I installed the new Transformer for the mod deck.. Plugged it in and keyed it.. The Volts went up to 3000 and then fell. That was it.. It held one key up. Its from 1962 and its old.  The more and more I look at this thing the more we find that needs to be changed and or upgraded.  Not sure where I would get that transformer. So it makes sense to us to part it it out. There are LOTS of good parts that some guys will need. I can get my money back (break even) on it. 


I might keep the rack.. Then get some shelves.  I could rack the R390, SP600, 1051, Speakers,  The Globe Champion 300 and what ever else I can fit in there.. Then roll the unit into the room. Sit the Globe 500 Next to it and have all my Am gear in a compact station.


Clark

[W9GT]

Clark,

I understand your frustration, but it would be a shame not to keep and revive that nice rig.  There are lots of transformers around.  You might even be able to find an old pole pig that would work for you.  If you can't find one locally, you might find one on-line.  Shipping might be a little expensive, but ya never know what might be out there.

73,  Jack, W9GT

[WQ9E]

Clark,

Sorry about your loss!

However,  I wouldn't part it out yet because you should have a lot of options for the plate transformer.  The 4-400s will be happy over a wide range of plate voltages and they run pretty well even at lower voltages.  For example, I normally use and load my Johnson Desk (pair of 4-400s modulated by 810s) to 300 watts carrier out in the low voltage position which has a bit under 1,500 volts on the final plates. 

Assuming you are planning to run this at the current legal limits you have a wide range of voltages to choose from and a bit of modulator mis-match can be tolerated since it was designed for a lot more audio than you need at today's lower limit.  Of course you could also go to a full wave bridge if that is what your replacement transformer needs to have to provide the correct voltage.  So, why not give it some time and thought and see what you can come up with....

73, Rodger WQ9E

[K5UJ]

Clark,

I know how extremely disappointing and frustrating this type of thing is, but don't do anything rash.  let it go for a day or two before you start pulling it apart.  Often, other options, solutions or possibilities reveal themselves then you take a step back.

I might be missing something, but 8 1A diodes in series is still 1A, not 8A.

MIGHT be the source of them fires when you first fire the supplies up

6A10.  400A surge capability.  6A forward.

'Nuff said.

--Shane

Shane, you are correct.  Of course, series....well I said I wasn't an engineer    thanks for the correction.

73

Rob K5UJ

[K5UJ]

Re the 6A10s those are what Ten Tec uses in their Centurion rectifier, five of them per leg on a 2150 v. secondary.   Interestingly, Ameritron, in their AL82 with the 2650 v. secondary uses five 3A diodes, each 1000 PIV.   When Ameritron is bragging about their p.s. voltage in their catalog, they don't mention anything about the rectifier.  Not sure if one is over engineered and the other is cheap.

[ke7trp]

Its going to be hard to find a transformer like this one with that much volts.    I dont want to have to redesign the power supply and put a bunch of money into it.  It was a nice old transmitter that I picked up.. Looking back I know the guy was dishonest. It had a shorted Mod transformer AND now a shorted Plate Trans.

I think its best if I just part the old thing out and let it die like the previous 3 owners. This thing is bad luck!

The good news is that I got the Globe 500C modulator deck totaly finished.. I could run my outboard audio gear but never a D104 from the front input.  After pouring over the schematic a friend pointed out my Rookie mistake. The Loading resistor that runs from the Grid of the First mic tube was doubled up.. There was a factory one installed on the side of the tube socket.. I came along and added another to ground thinking it was not there.. With the correct input and grid resistor to ground it sounds great and has full audio!



Clark

[WQ9E]

Clark,

Sounds like it might have spent a fair amount of time in a high humidity environment at some point in its life leading to the transformer failures.

You don't need the kind of current and voltage the original transformer provided and if you find any reasonable transformer that is either 2000 to 4000 volts each side of center tap or find one that is 2000 to 4000 volts with no center tap and just add two more rectifier strings.  To run at the current power levels you only need a transformer that can reliably run at 500 mils or less depending upon the voltage.  Why not put a want out on this site and see what pops up?  Unless you want to keep it as a variable voltage supply (no need to really) then you may be able to use the current variac as trading fodder towards a needed plate transformer.  Unfortunately your current transformer is very heavy or otherwise it might have some value to another member to use as a core for rewinding.  If the primary is still good, you could remove the HV winding and with not that much wire or work make the "mother of all filament transformers" for building a tube with handles rig in the future.

I can understand not throwing a bunch of money at it but you might find something cheap here on the site.

Good luck whichever direction you take!

Rodger WQ9E

[ke7trp]

Ebay.    350100613889





C

[WQ9E]

Clark,

That should work.  The 540VA rating is CCS so you could easily draw what you need in ICAS.

[ke7trp]

So like 5600 volts rectified?    Feed with the dual gang variacs in the unit on the 125 v tap?   I need to get this down to 3000 volts and not realy any higher.. What about the current rating at the lower voltage?   I would like a constant al night long 600 to 750 watts am

Clark

[WQ9E]

Clark,

With a choke input filter the output voltage is going to be about 90% of the RMS input voltage or 3,600 volts in this case.  If you went to a capacitor input filter it would be around 5,600 no load probably dropping to around 4,800 under load but you definitely don't want to do that.  If your line voltage is actually 120 then the 125 volt tap will give a slight further reduction.

This transformer should be safely sufficient to run 550 watts input plus the modulator in normal service but much higher than that and you may be stressing it a bit.  Hopefully someone here has had more experience with this particular series of Hammond transformer and will chime in.  I cannot remember you design but is the modulator also getting its power from the same HV supply?

Rodger WQ9E

[ke7trp]

The modulator has its own supply. A lare 2650-0-2650 transformer pictured.

Further thought.... It would seem to me that I need about an amp of current.. 3000 volts.  I dont have to stay Center tap.  I could even ditch the chokes and run 32 diodes and some bleeders.  Maybe a 2500 to 2800 volt trans at 1 amp would do it. That would give me PLENTY of plate voltage and current. What kills me is that just a few weeks ago, I had a transformer out of a Henry 2k console. It would have worked just fine I bet.  I sold it off.


I am going to give it a week to see what pops up.  After that, I will just part it out and move on.

Thanks for the help and advice.   This blown transformer has caused me to hit the books and I am learning all about HV supplys now with this thread.

Clark