The AM Forum

I am building my diode stack for my 813X813 rig.  How many 1n5408s (3A 1000V) in each leg of a full wave rectifier?

I was planning on using ten in each leg with a .01uF cap (1000V) across each..........

Thanks

Rich

I like the 5408's; the 3A is a nice safety foctor.
5 diodes per 1KV should werk nicely.

what voltage from the 2ndary of the iron??


klc

The secondary is 3600/4600, I was going to start with the 3600. 

Rich

Do I need balancing caps and resistors across the diodes?  I am seeing information that that is an outdated practice.........


Rich

The last stack I made did not use the resistors and caps. It is in a 1500 volt supply for my 812 pair rig and works fine.

Bill KA8WTK

Hi Rich,

You're correct - the old equalizing resistor/caps aren't needed with modern diodes. I haven't used them for decades. I have a stack in bridge config for about 15 years with no problems.   Be sure to use a fine wire, like from an RG-213 shield in series with the diode stack for protection. Place the wire on a pair of ceramic stand-offs spaced 2" apart.  It has saved my stack from dead shorts many times.


Yes, five 1KV diodes per leg, per 1KV, is a nice, conservative safety factor. I do about that myself. You never know what transients will occur to stress them when you deal with transformers and chokes. (inductance)   It's more trouble to replace the whole stack than spend a little extra time and money for overkill in the beginning.   3A diodes are a good idea too. I use 6A 1KV for all my HV projects, even the smaller linear amps, just cuz they are almost as cheap and even more bullet-proof. (Frank at Silicon Alley)


T

I always go 4 times the ac voltage of the transformer with each leg of the diode bridge. I also use 6 amp 1kv diodes from MCM Electronics.
 "6A10 - Rectifier 6A 1000v SI R-6" http://www.mcmelectronics.com/search.aspx?K=6a%201000&T=R&T=R  It's worth a little extra money for heavy diodes to never have to worry about them. On real heavy stuff I double them up.      

John N8QPC
 

Just use the k2aw high voltage modules.  So simple to install. You only need a couple. Here is a picture.



http://w2dtc.com/w2dtc-hv-power-supply-page.htm

I use the same diodes (5408).  About 4-5 per KV is good.  Keep in mind that the 1KV PIV is the peak voltage rating.  So, every 700 volt RMS is about 1KV peak.

I still use the 470K resistors and the .01 1KV caps across each.  Never had a failure.

Fred

that is the way to go!! They are reasonably priced, and rugged. Stacking diodes has always been a pain in the arse!! Also using a step-start circuit really helps prolong the life of ur diodes by limiting the initial inrush current.

You can also find some of the hamfest parts dealers selling similar diode blocks for reasonable prices. You see a lot of 1.5A / 6 or 8kV diodes from them. Trust me , they are extreemly under rated. I use them in all of my HV applications without ever having a problem.

5 per leg of the 1N5408's is more than sufficient as that will give you 10KV PIV in each pair of a FWB. Always leave at least 1/2" of lead exposed for heat dissipation, I use 1".  The 6A10 is fine for serious power but also overkill here. The same holds with lead lengths.

Caps and resistors across each diode is completely obsolete, even the ARRL finally woke up to that a decade or so ago and explain why in the HB. It wouldnt hurt to place a .0047 6KV or greater disc cap to ground at the output of the bridge and 1 across each full leg of diodes to snub any line spikes that get stretched and thru the transformer. The one to ground doubles to bypass any RF that may be floating around the wiring, electrolytics respond poorly to RF and often go bang.

While the K2AW blocks are convenient the price is ridiculous for what you get which is a bunch of microwave oven diodes of far less performance than the 1N5408. I fried them in my 3CX1000A7 2M amp about a week after it was in use and the 1N5408's have been fine for 25 years.

Use a glitch resistor in the HV lead at the base of the plate choke, about 15-25 Ohms at 20-50W will current limit a tube arc or short circuit until the PS line fuse or breaker activates. Use some more .0047's at each side to ground one of which is also the choke bypass.

A thin wire as mentioned can be a good idea but be careful that it wont create a plasma arc at the failure point or blow at one end and wave the wire around like a snake and arcing to any place it can find. Once a plasma starts it wont stop until power is removed.

Carl

Carl,
       Isnt a 5408 rated 3A / 1kV ?? If so you need more than 5 per leg of a FWB for 10kV PIV, or am I having a brain fart??

Yes, the supply HV resistor is a good idea. I added them to all rigs here a coupla years ago when it was suggested on the forum. Mine are about 10 ohms at 100w for the big supply.

As for the plasma arc problem,  I do see evidence of a sooty plasma event even when using a 2" wide set of stand-offs. However, I know it does help to protect the bridge because I have run it without the safety fuse wire and the power supply stayed on for at least 2 full seconds during a short condix before the 240V breaker popped. Unfortunately, if I use a smaller breaker, it drops out under normal operation. But when the wire is melted, the supply drops out in a fraction of a second. Good, quick protection. The key is to use wide spacing on the stand offs so that the plasma has a better chance of quenching quickly as it rises up and lengthens.

T

Both conducting sections are used so you have 10 diodes in the active path and 10 more in the reverse path.

Yeah Baby - First, is your full wave circuit a full wave BRIDGE or a full wave CENTER TAPPED?

For a bridge, the peak reverse diode voltage must be at LEAST 1.4 x the RMS voltage of the transformer secondary - PLUS A SAFETY FACTOR (should really be a 100% safety factor).

For a full wave center-tapped setup, the peak reverse voltage must be at LEAST 2.8 x the RMS voltage (plus the safety factor).

So, for your say 4500 volts seconday with a bridge, your diodes are going to see about 6400 V reverse on a repetitive, ongoing basis.  *EACH LEG* of the bridge must be able to withstand this voltage MINIMUM.

If I were building this as a bridge, EACH LEG would be 12kV.  If a center-tapped full wave, EACH LEG should be twice this value, or around 24kV.  This gives a nice, 100% (more or less) headroom safety factor to account for spikes, switching transients and other anomalies that are part of normal life.

You won't need equalizing resistors or caps with modern rectifier diodes.

The last such high voltage system I built was a 10kV DC, capacitor input supply.  Each leg was 25kV.  I never had any problems, ever, with that bridge.

Diodes are inexpensive.  Repairs - well they pretty much suck, so do it right the first time  ;)

Regards,

Steve

it is a Full wave (CT) rectifier.  So I am going to run ten diodes in each leg.


Rich

A friend called there recently and K2AW ( Frank) said he stopped making them. so now it's the surplus or used market to get his product. I purchased some about 8 years ago and he was in his eighty's then. He was born in 1914. Whenever I purchased anything from him, the product was here before I wrote the check. I didn't know he operated like that ;D ;D

Found this years ago while reading some forum:
1. K2AW Modules are made by hand by K2AW.

2. Each module has a string of General Instrument 1N5408 diodes, personally matched by Frank before potting in epoxy. The 6kv module has 6 1N5408's, the 14kv module has 14 1N5408's etc.

3. Although the diodes are rated at 3 amps, Frank derates the module to 1 amp to make them "idiot proof".

Yep, Frank's a good guy. He always sent me the goods before a check arrived too.


I wonder how well those diodes are heatsunk to the bottom of the epoxy mold? Most guys use thermal paste and mount them on a heatsink.  ie, Would the diodes work better in open air or sealed in that epoxy unit? I remember  heatsinking mine too. The K2AW instructions say to, IIRC.

Too bad he may be retiring, but at his age, God bless him... Maybe someone will take over the biz.

T

I just talked to Frank’s daughter a few days ago. Frank is suffering from Dementia. For the time being she is handling the business Frank hasn’t made the diodes for some time. He was unable to get the cases that he built them in. They are just off the shelf block diodes that they rebrand. I’ve bought diodes off of Frank for years. I liked them for three phase mobile work but they are too wimpy for base stuff.

John N8QPC

Thats totally wrong. The way you make it out nobody could use 866A or 872A's.

Read an ARRL HB for a primer on power supplies.

Wow, you seem aweful confident ;)  There is an ambiguity in this discussion... we don't actually know if it's 4600VCT or 4600V either side of center.

Whatever the voltage is, one thing is absolutely sure:  The Peak Inverse Voltage rating of the diodes in a full-wave center tapped rectifier MUST be AT LEAST 2.8 x the RMS voltage plus a SAFETY FACTOR. 

If the transformer is indeed 4600VCT, the peak voltage across the diodes will be 6440, so 10 1kv diodes in each leg will most likely afford a sufficient safety factor, although I personally would use 12 diodes in each leg.

If it's 4600 either side of center, 24 diodes would be needed if a good safety factor is to be had.

I'm guessing this is probably 4600VCT - 2300 V either side of center - 10 or or more diodes per leg is required.  No way will 5 1kv diodes work (at least not for long!).

If there is any question about the accuracy of this information, I will be happy to go through the proof, step by step if necessary  8)

(http://www.classeradio.com/ps.jpg)

The secondary is 3650VCT and 4680VCT. I am starting with the 3650.

73

Rich

Sound like a good start !  The 813 works quite well with around 2000-2200V on it. Used to use a pair myself, back in the day.  A pair of 813s modulated by a pair of 810s, run at 2000V at 400mA - common power supply for the modulator and RF amplifier.

Maybe we'll work you over the winter !


Regards,

Steve

Just FWIW I have a pair of Varo VC-80 diode blocks in my 4X1 transmitter. They look like the K2AW diodes except they are black in color. From what I have found they are 1.4A @ 8KV. The current plate iron is 3200v EACH SIDE OF CENTER. and my final sucks down around 400mA + at resting carrier.

I am currently running FWCT with cap input (CLC) filter. (I want the extra voltage) 8kv doesnt leave much if any safety factor room, but..........................
I have been running these same diodes for 10 years now without any problems.
they have survived 4 plate transformer and 1 choke failure and are still kicking along fine. They have also survived several nasty arc-overs and a few hard dead shorts without any problems. I know these things are very conservatively rated, but I have found them to be just about indestructable. They are so much nicer and easier to use / install than perfboards lined with stacks of 5408s. I do however use a step-start on the plate supply.

There used to be a parts vendor at local hamfests who used to sell used "pulls" for a few bucks each, and I'm glad I stocked up on them!!

Mouser had some difficulty with 6A10 diodes some time back and got them listed as 6Amp 100piV ratings .... a phone conversation with Mouser customer support seemed to go like "We have BOTH ratings in stock" .... I tested both types obtained from Mouser with a hypot tester and found the avalanch point for both at approx 1200V inverse ....

It seems to me that however many diodes you choose to use in your stack that I would test the piV before ya use 'em

I am confident.... their is no ambiguity as the voltage was established earlier.



Bring it on but Id suggest you at least read what you posted earlier first.

Carl

I think this horse is dead  ;D  But, as Ross Perot said "I'm all ears" in hearing an explanation of what was said above.