HV Transformers(2)

[The Slab Bacon]

some guy in Memphis has new Maxwell oil caps on eBay 5 KV 47 uF wants around $79 each, shipping another $25.

If you look at the pix closely, the tag is blurry enough that you cannot read it. That cap looks awful small for 47uF @ 5K.    Also the terminal insulators look a little wimpy for that kind of voltage rating.

the 10s @ 4Kv in my 4X1 transmitter are the same size, if not larger.

[KM1H]

Those storage caps work OK up to about 50-60% of voltage ratings in a ham style PS which is usually a doubler with lots of 60 Hz ripple. In a FWB they should run cool but always feel the case once in awhile until you get comfortable it will not run hot.

Carl

Carl,

Thanks for the information.

Are you suggesting that in a FWB (or possibly a FW CT configuration), the cap need not be derated 50 to 60%, and can run up to it's normal working voltage in an amateur-type PSU?

Perhaps you can clarify this for me, as I have one of these energy storage caps that I am considering using in a FW CT HV DC PSU for a 2KW class AB1 linear amplifier I am considering building.

Thanks & 73,

Bruce

Going by what Ive "heard" is that a FWB or CT will smoke them at rated voltage and be OK at around 70-75% due to the 120 hZ ripple. OTOH, I dont care to have any cap get much above room temp which should be your benchmark.

I have a pair of 53.5 uF @ 5KV discharge caps which Ive run here in a FWB in the past at 2400-2600V and they get moderately warm (very comfortable to the touch)after several hours of use (typically the PS were on 6-8 hours at a stretch) on a prior AM amp and modulator; 3 x 813 with 572B audio.

some guy in Memphis has new Maxwell oil caps on eBay 5 KV 47 uF wants around $79 each, shipping another $25.
42 uF filter cap on full wave rectifier 4.5 KV DC give about 1.5% ripple.

Would you trust a guy selling the item below???
290419274224

Carl

[John K5PRO]

Saw this testing method for a plate power supply being tested at Universal Voltronics Corp:
15 kV at 6 Amps.
They had a big plywood sheet, standing upright, with over a hundred light sockets, the porcelan types. Screwed into
there were those little conical nichrome wire heating elements that used to be in every hardware store. All wired in series/parallel
to get the 2500 ohms needed, and then a bunch of window fans blowing across this thing. Simple, fairly cheap, but don't let
your cat walk next to it.

I had a photograph somewhere of this setup but can't find it today.

[WD5JKO]

Update:

   My need to re-do this HV supply has taken a back seat since I fixed the original power supply in the Dentron MLA-2500. I cannot believe they (Dentron) used fiber insulators under the electrolytics all the way up the voltage ladder. The cap at the highest voltage had the metal can return about 1/10" away from the metal 6-32 screw. So the B+ (2500 - about 400v) had a 1/10" gap to a 6-32 screw head ground. It was arcing big time. I just replaced that screw with a 6-32 nylon screw. So I'm back up.

   It is good that others have used that same Aerovox 54 MFD 4400v capacitor with success. I called Aerovox and requested a data sheet. Still waiting, but if they send it, I will post it here.

    As far as that Tranny goes, I still think it is useful for this application. I never mentioned it, but I have two of them. If I put them in parallel, then I should have the basis of one heck of a brute supply. This supply will need to be in it's own chassis however which is probably best anyway.

   Thanks for all the suggestions.

Jim
WD5JKO

[K5UJ]

some guy in Memphis has new Maxwell oil caps on eBay 5 KV 47 uF wants around $79 each, shipping another $25.

If you look at the pix closely, the tag is blurry enough that you cannot read it. That cap looks awful small for 47uF @ 5K.    Also the terminal insulators look a little wimpy for that kind of voltage rating.

the 10s @ 4Kv in my 4X1 transmitter are the same size, if not larger.

Frank, good point.  Yeah, I bet those are discharge caps.  I will use 3 GE 12 uF 5.2 KV oil caps (that's 5.2 KVDCW) along with the TOBE cap and get 46 uF 5.2 KV for the peak 4.5 kv from the transformer.  These all have large insulated threaded lugs; they are the real deal. those discharge caps would be okay in a 3 kv supply though.

[WQ9E]

To add to what Carl said about heating, realize that the heat will increase as the supply is actually delivering current.  So sitting at idle is not a sufficient test to see if the capacitors are going to get too hot so monitor them also during the first few hours of use in your application. 

Of course our class C finals running continuous carrier and class B modulators with a high resting current are going to stress the supply a lot more than one used for normal SSB usage.   So be careful in extrapolating web reports of performance over to the different environment of AM.

[K5UJ]

Yeah I wondered about that.  Gee, now I'm wondering if 700 v. margin (5200 - 4500) is enough.

[WQ9E]

Rob,

Are your GE caps energy storage or are they originally designed for filter work? 

[K5UJ]

Rob,

Are your GE caps energy storage or are they originally designed for filter work? 

Well, here they are:  http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250536218736

I hope you're not gg to tell me they're no good.  I've blown over $400 on capacitors now 

Rob

[WQ9E]

Rob,

Those will probably be OK but in circuit testing will tell you for sure.  Do you have the actual GE part # from them?  The ebay picture wasn't detailed enough for me to see.  One of my colleagues came here after several years in GE engineering and he can probably get the product specs.

[WD5JKO]

  I am a little confused about the two categories of capacitors mentioned on this thread, 'energy storage', and 'filter capacitors'.  Looking at some 'energy storage' capacitor data sheets, they have very high peak current capabilities, often in the thousands of amperes. This suggests to me that the ESR of these capacitors must be very low.
   I am thinking of the difference where the 'energy storage' cap is charged up, and then suddenly discharged into a near short circuit (thousands of amps into a single pulse) where the repetition rate is very low. Then compare this to a 60HZ power supply filter where the 'filter capacitor' might see an RMS current of  maybe an ampere where the current is periodic at a 120 hz rate.
   So wouldn't a low ESR 'energy storage' capacitor also make them suitable as a 'filter capacitor' whereas the reverse may not be true? Maybe someone can clarify the issue for the benefit of the group.

Jim
WD5JKO

[WQ9E]

There has been a lot of discussion of this on the Amps reflector at contesting.com.  There is a searchable list archive at:  http://lists.contesting.com/archives//html/Amps/

Like any place on the internet, there is a mix of fact and opinion and it is not always clear in which category any given post falls.  But you will find some good discussion and additional links on a range of topics from tubes and power supplies through esoteric topics like ampliphase modulation along with a lot of "refuse" on parasitics.

[K5UJ]

Rob,

Those will probably be OK but in circuit testing will tell you for sure.  Do you have the actual GE part # from them?  The ebay picture wasn't detailed enough for me to see.  One of my colleagues came here after several years in GE engineering and he can probably get the product specs.

Well I had a really nice reply with everything I had to say and then I accidentally deleted it somehow.  In disgust over that I now offer an attempt at the same content but condensed:

I didn't know until recently that there were differences among oil caps.  It might help me and others if you guys who know how to get good caps for filtering in amp power supplies for AM service would say where you find yours for purchase. 

I've looked at a lot of oil h.v. caps on eBay and none of them say "filter cap."  They may say "energy discharge" which I now know means they are for photo flash units or science fair projects but usually you just get N uF and N KV with no indication the voltage is for working volts or peak volts.  This last time I went by size/uF and the big insulators on the lugs.  They also look like regular oil caps you'd see in a p.s. and not something goofy with spade lugs.  Of course I could just by new and pay $$$ or go to Henry Radio or Surplus Sales ($$$ there too).  What else is out there?  Hamfests do not seem to turn anything up, or else you find 1 uF at 10 KV or 50 uF at 500 v.  I got these three 12 uF 5.2 KV caps for around $160  and figure paralleled with my 10 uF 8 KV cap will get me 46 uF 5.2 KV for a 4.5 KV DC supply.  I guess when I finally fire that baby up with a pair 4x1 I'll check them after 30 minutes operating and see if they're hot but if you guys could give some pointers to sources for good used filter caps other than ePay I'd appreciate it.    tnx

Rob

[WQ9E]

Rob,

Old vintage oil caps are going to be standard filter service caps but some of them will contain PCB if that concerns you.  Hamfests and parted broadcast rigs are both good sources for these older caps.  I used new GE caps in my HB amp (8 times 7.1 uf @ 5 KV) but they are only exposed to 2,200 volts.  They have been in use for heavy CW and SSB contest use for about 8 years without failure and with no detectable temperature rise but I have no data on what happens when the input voltage approaches the nameplate rating.

With the energy storage caps you either need to get the manufacturers specs or test them under load and check temperature rise.  There are a lot of opinions out there but they are generally based on one ham's experience with one or two types of caps and extrapolating that experience out to "energy storage" or "pulse" caps in particular would be dangerous.    For example High Energy Corp. shows that ripple voltage needs to be reduced as frequency increases with a maximum of 20% at 60 hz and dropping to 10% at 400 hz indicating that with a full wave bridge (120 hz ripple) some de-rating is needed.

[K5UJ]

Okay Rodger tnx, i'll use these and see what happens.   Broadcast rigs (or at least the one I have) come up short in the cap department.  AM tube tx used filter chokes b/c regulation pertained more to constant current with v. swings so the caps are fewer and relatively low v.  I think the most I got out of my bc tx was 4 kv and 4, or maybe 8 uF.    These ones I'm getting don't have PCB.  Now, the one that's already in there, the 10 uF @ 8 KV...     Okay you are running low v. with a lot of paralleled caps--I see; well, Handbook says a few hundred v. over the peak DC p.s. v. should be okay.  Anyway, we'll see.  I think they will probably be okay.   

Rob

[KM1H]

For example High Energy Corp. shows that ripple voltage needs to be reduced as frequency increases with a maximum of 20% at 60 hz and dropping to 10% at 400 hz indicating that with a full wave bridge (120 hz ripple) some de-rating is needed.

As I alluded to earlier in this thread.

Dont put all discharage type capacitors in one bucket either, they vary considerably depending upon application. Its a lot like comparing peak power out of a radar tube; it all depends upon the PRF and pulse width. You can get hundreds of KW out of a tube that fits in your palm if the duty cycle is very low. If something goes wrong then it become PIR....Premature Involuntary Retirement

A filter cap is designed with heavy foil and insulation material plus cooling oil and takes up a lot of volume per uF. Discharge caps get their high uF by reducing all of the above.

Since there are so many varieties of discharge caps, ratings and terminology vary by manufacturer and application, then the only real test for filter use is reports by others and the heat test. If it runs cool its OK. If it runs quite warm then you take the Dirty Harry quiz

Carl
KM1H

[K5UJ]

Premature Involuntary Retirement...that's a good one hi hi.   Another thing I've learned to not do is jam the caps all up against each other; instead leave some room between each one to let air circulate around them.  That was a great tip-I have seen them packed in and never thought about heat until now.

Rob

[W2VW]

For example High Energy Corp. shows that ripple voltage needs to be reduced as frequency increases with a maximum of 20% at 60 hz and dropping to 10% at 400 hz indicating that with a full wave bridge (120 hz ripple) some de-rating is needed.

As I alluded to earlier in this thread.

Dont put all discharage type capacitors in one bucket either, they vary considerably depending upon application. Its a lot like comparing peak power out of a radar tube; it all depends upon the PRF and pulse width. You can get hundreds of KW out of a tube that fits in your palm if the duty cycle is very low. If something goes wrong then it become PIR....Premature Involuntary Retirement

A filter cap is designed with heavy foil and insulation material plus cooling oil and takes up a lot of volume per uF. Discharge caps get their high uF by reducing all of the above.

Since there are so many varieties of discharge caps, ratings and terminology vary by manufacturer and application, then the only real test for filter use is reports by others and the heat test. If it runs cool its OK. If it runs quite warm then you take the Dirty Harry quiz

Carl
KM1H

Sounds like voltage doubler service would really ask for trouble.

[KM1H]

dupe, deleted

[KE6DF]

Buy using the same logic in reverse, one would think motor-run AC capacitors would be particularly good when used as filters in a HV supply.

They are designed for AC -- all ripple all the time.

[W2VW]

Buy using the same logic in reverse, one would think motor-run AC capacitors would be particularly good when used as filters in a HV supply.

They are designed for AC -- all ripple all the time.

All ripple until the motor gets up to speed. Usually pretty fast.

[WB3JOK]

that's the motor-start cap, not the motor-run cap? 

[WQ9E]

Rob,

I looked through my Radio Masters catalogs (I have most of them from the late 50's through early 70's) but they were not of much use.  Although the catalogs are very large there were not a lot of listings for oil filled capacitors and the GE advertisements did not provide much information.  I did find that in addition to energy storage caps GE also made another group of oil filled caps that were designed for light duty coupling and buffering usage where the ripple is low and are known as the CP-70 series so make sure yours isn't in that model series.

On a side note, these catalogs are a generally great reference source (they run from 1,000-2,000 pages in length) and provided detailed specs on a lot of components.  If you see them hanging around at a hamfest they are worth picking up.  Mine came as a free bonus with my $75 Mosley CM-1 receiver and I have used them more than the receiver.

[KM1H]

Im under the impression that the motor run cap is mostly along for the ride unless it needs to provide a shot to maintaining the 90* phase relation with the current. Even then its sole duty is voltage with the windings providing the current. Sort of like a PEP PS but not quite....see below..

In many motors the same cap is used for starting except in high torque applications where a seperate cap with higher current ratings is used and now we are approaching a real PS.

Motors can be more confusing than antennas

Carl
KM1H

[WB3JOK]

true, I don't know much about them... the one-cap motors I am familiar with have a centrifugal switch to cut the start cap out of the circuit once the motor is up to speed... I know there are two-cap motors, one for start and one for run though.