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voltage doubler filtering and ripple




 
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Author Topic: voltage doubler filtering and ripple  (Read 9079 times)
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Patrick J. / KD5OEI
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« on: August 27, 2011, 01:51:21 AM »

The NCL-2000 here is like any other ham type 2KW PEP input, 1KW continuous input amplifier. The power supply has a voltage doubler and a big stack of electrolytic capacitors and is very similar to the SB-220 except for a difference in filter capacitance (and bleeder implementation). The relationship between plate voltage drop and DC power seems straightforward.

Running an unmodulated carrier, Up to about 500W out, the RF output seems unaffected by hum as seen on a scope used with an attenuator. When tuning it and loading it up using a carrier, once the output gets more like 800-1000 watts, there is some obvious ripple modulating the RF output during those short tuning sessions. The ripple at 1KW output is about 8%, but the amp is not really intended for what they'd call "single tone" use at that level, only for 1KW average input. No one has said they ever heard any hum, but I suppose they would not because all they ever hear is a voice signal and all tuning is done into a load. The capacitors all look OK, but are originals. I have not taken them out to check the capacitance. This is more of a technical annoyance than anything else and I should be using a pulser but I don't like them because it gives a falsely high RF output because of the low duty cycle and light load on the power supply.

I have a question about this ripple issue based on what others may have found when deciding to replace the filters in a capacitor input doubler circuit.
The total is eight 80uF/450V caps in each half of the doubler, which are in series, making a 10uF/3600V filter. This capacitance seems pretty small for something with a 2200V400mA average load on it. By contrast the SB-221 has eight 200uF/450V caps, for 25uF total. The only reason I can think of to have such a smallish filter in the 1964 vintage NCL-2000 was space requirements or capacitor technology considering the SB-221 was available in 1978.

Is this kind of ripple (8%) at high DC currents normal for a power supply in this kind of amp with the caps in good condition?

Is it an indication that I should replace the capacitors because they are dry and much less than the stated value? Or just replace them with bigger ones anyway?
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« Reply #1 on: August 27, 2011, 02:12:31 AM »

Most caps made then had a large positive tolerance, sometimes 100%.  As they age, the leakage usually increases, along with lessening capacitance.

I'd double up on the uF value, and go for a bit higher voltage rating as well.  This ought to be easily done physically, depending on your choices of uF value.  Be sure to calculate the surge current needed to charge the new caps up without exceeding your diode peaK ratings.

Remember, a doubler circuit really makes the caps grunt, as without extra 'flywheel' effects covering the power depletion under load, things will show up as a rapidly falling voltage value without even nearing design demand. 

A regular FW or bridge PS asks to act as an 'averaging' device, never going below a critical charge whilst operating.  Doublers can actually ask a cap to go from 0 to full value at a line freq multiple as it 'fills up and dumps again and again and again...

73DG
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« Reply #2 on: August 27, 2011, 06:13:32 AM »

hi Patrick ... the ESR (equivalent series resistance) of a capacitor has more to do with power supply performance than most hams realize .... most modern electrolytic caps have lower ESR and leakage values than their predessors .... this is a very good thing in a doubling or multiplying supply ... I replaced the stock 125Uf caps in a SB200 with modern 80Uf caps and actually saw an improvement in supply performance ....this can only be explained by lower ESR values of the replacements
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« Reply #3 on: August 27, 2011, 09:01:25 AM »


Patrick,

   It has been my experience that a linear amplifier running within ratings offers some power supply rejection ratio (PSRR). When we drive the amp to overload the PSRR goes to zero, and then you see the power supply ripple come through modulating the output. This does not necessarily mean the filter caps are shot.

   Want to sched again for tonight? PM me.

Jim
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« Reply #4 on: August 27, 2011, 11:55:37 AM »

I see,.. I don't think they are shot, but maybe getting old. I don't remember having this much ripple 8 years ago when I bought it. Maybe it's that finding a total filter of only 10uF in there bugs me. The work to take them out to test is the same as to replace them.

Am I right to think that a higher voltage rating means more electrolyte and better durability in heat and higher current?

I admit I am overloading the power supply by 50% during these exercises. I can rig up something to avoid this and see what things look like. New caps can't hurt anything. They are the usual 1" diameter type. Unfortunately the effort to remove them to get at them for testing is as much as to replace them.

I find 100uF 450V caps but they are short er than the originals, about 40mm. The originals are about 5" long. I could not find higher capacitance than 150uF in the long case, but two 100/450's would fit end to end so they could be paralleled.

The rectifiers can be replaced pretty easily as well. Might as well upgrade, it's a good amp and can last me as long as I need it. The next thing it could use is a bigger blower but there's little room and its a bottom feeder so its not like I can just bolt a hose to the back panel.
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« Reply #5 on: August 28, 2011, 10:37:37 AM »

Ive run NCL-2000's since the 60's at 1200W out on CW with no hum complaints either with the original long black caps or later  Sprague TVA Atoms.

What I did do recently to the 6M version is convert to 220uF CDE 105C Snap In;s which fit the clamps perfectly. While rewiring I changed the equalizers to 100K 3W MOX. I drilled a row of small holes years ago in the tube compartment wall of all the amps to get some cooling in that cramped area, there is more than enough for the tubes.

Cheap caps cant handle the high ripple current from a doubler and the Atoms were getting too pricy.

Carl
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« Reply #6 on: August 28, 2011, 03:10:17 PM »

Been thinking of picking up one of those cheap 12V to 230 VAC 50Hz 800-1000W inverters and putting a voltage doubler on it for 600VDC to make a nifty mobile supply for the ARC-5 station to replace the old HEATH. I have the Heath strapped with a bridge to do half the voltage and twice the current but I only get 400VDC. The Heath runs at an annoying but efficient high frequency that drives me nuts because it is on an aluminum plate on the back seat floor. The 50Hz would require larger caps but it would be loafing at 100 Watts draw.

Mike WU2D
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« Reply #7 on: August 28, 2011, 06:56:36 PM »

Carl, do you still have the part number on those caps?

Mike, that's a good idea, just find a way to keep inrush current during charge-up to a low value or the inverter might shut down or not start with an empty capacitor bank on it. I tried that same thing and it happened when the capacitor bank was large.
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« Reply #8 on: August 30, 2011, 09:54:32 AM »

Mouser part 5985-450V220

CDE 221M450K052
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« Reply #9 on: August 30, 2011, 10:08:21 PM »

Thanks! They seem wide at 30mm, but if they fit then so be it! I do not remember clamps in mine but I will have to have it out of the rack and look again. Now that this was identified, I took a closer look from Mouser's page to the datasheet. The part is not on the sheet but Mouser has it. There are a few choices there but I will likely order the given PN from Mouser tomorrow as it is in stock and less than $7 each. I picked some of that size and the next down (25mm) as well as a couple three voltages on the basis that 450V x8 is fine but the real need is a bit less and maybe a higher capacity will fit in the same space. I don't know the secondary voltage of the transformer but have never seen more than 2800V on the unit with the tubes cut off. The caps also have a surge rating quite a bit higher than the working voltage.

* CDE for NCL-2000 Y2011.xls (414.5 KB - downloaded 141 times.)

* CDE for NCL-2000 (Y2011).png (176.83 KB, 522x810 - viewed 362 times.)
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« Reply #10 on: August 31, 2011, 11:52:12 AM »

The original caps measure about 27.5 mm, and the 30 mm are a nice fit in the spring clips.
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« Reply #11 on: August 31, 2011, 10:49:57 PM »

Perfect!
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« Reply #12 on: September 01, 2011, 12:50:59 AM »

While not cheap. I would love to try some of these new caps.

http://www.digikey.com/us/en/ph/Cornell/944.html
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« Reply #13 on: September 01, 2011, 03:49:44 PM »

They will work better according to my friend at Illinois Capacitor. They are still larger and 3x the cost of electrolytics, joules per dollar. I like the plastic casea and I wonder about just bolting them to a chassis and putting in series? The test voltage to case is 4KVAC for 60 seconds. Maybe best to keep to the lexan.

Those are good ones but I'd go with more energy per can:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=478-6452-ND
860uF/1200V $162.66 each
Stacked 5 tall, a 172uF 6KV filter. $813.00




The smaller size, CDE
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=338-1399-ND
390uF/1KV $90.56 each
stacked x6- a more reasonable 65uF/6KV. $543.00


I'd do it to try them out but already have oil capacitors.
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« Reply #14 on: September 01, 2011, 05:12:31 PM »

I've been using these they make oil filled look stupid.http://www.ebay.com/itm/HIGH-VOLTAGE-PULSE-POWER-CAPACITOR-60uF-4kV-New-/200647481080?pt=LH_DefaultDomain_0&hash=item2eb78596f8
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« Reply #15 on: September 01, 2011, 11:16:22 PM »

Clark,

Those are nothing more than polypro caps you can get at RS...  BUT those are on steroids.

The caps that Detroit47 posted are the real deal.  Even though being advert as peak storage caps, they work fine as Cfilter.  I've run 4 of them in series parallel on a pair of 15s.


--Shane
KD6VXI


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« Reply #16 on: September 01, 2011, 11:41:50 PM »


I have seen those Russian caps but wondered if anyone had used them at rated voltages without incident. I always derate pulse cap volts to 75%, maybe an old wives tale for old capacitors, but those have the specs.
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« Reply #17 on: September 03, 2011, 01:15:36 PM »

When the 53uF at 2500V or higher photo flash and laser discharger caps first started showing surplus in the 80's hams jumped on them. Soon the stories of ruptured cases or just plain overheating started and the consensus was to run them at 60-75% so they could handle the ripple current. They were not made to be filters.

Ive had a series pair of 53 @ 2500 in use for about 25 years in a PS for a few of the VHF/UHF amps and the unloaded DC voltage never exceeds 3200V. The PS can select several different voltages using a BC-375 TU switch for the tapped transformer secondaries. Yes its fully insulated from ground Grin
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