HV Transformers(2)

[WD5JKO]

OK, following the other 'hv transformers' thread it was clear about how using a FW bridge with a transformer rated for a FW with C.T. results in the rated current needing to be divided by two since the DC output voltage doubles. This way the same V*A product exists with either rectification type.

But what about keeping the load current with FW bridge the same as FW-CT when the duty cycle is low? Now for short periods we deliver twice the rated power. The applications of powering a class B modulator, or SSB linear come to mind.

So, I got my 8877 shoe-horned into a Dentron MLA-2500 amplifier. With the stock power supply, I could easily do 300 watts AM, sometimes 400, and legal limit SSB. The power tranny got pretty hot though with AM. Now I need to recap, and instead I want to clear out the power supply and start over. The attached picture shows what I am contemplating. 

That Collins transformer rated at 2250VCT @ 0.565A. The secondary DC resistance is 90 ohms, and each primary 115v winding is 1 ohms DC. The current rating is CCS and at 10,000 feet. Any idea what the ICAS 50% duty cycle rating might be?

I figure with 230vac in, I will get about 3200V unloaded, and about 2750 loaded (~1.2*rms). For 375W AM linear at 33% efficiency, I need 1125 watts DC input, which could be 409ma @ 2750v. Is this doable?

For SSB unprocessed, I will let it rip....

Jim
WD5JKO

[KE6DF]

Here is something I posted a while back related to this topic.

Take a look at the link at the end. (but remember the guy was taking about linears for SSB).

Many of the old UTC CG plate transformers are designed to use with the center tap grounded -- you are warned about this issue in the catalogs. The reason is that the center tap is not insulated well enough to withstand the voltage that the center tap would see in a bridge circult.

In some cases, they have some versions of the transformer, that ARE designed to work in a bridge circuit -- this is indicted by a W as the final letter in the part number (e.g., a CG-303 vs. a CG-303W.

I belive that to run a transformer in a bridge circuit rather than a full wave circuit with the center tap grounded you can draw about 70% of the rated current rather than 1/2. In other words, if your transformer is rated at 1000v CT at 200ma that means you can get 500v at 200ma with a full wave circuit with the CT grounded, or 1000v at 140ma with a bridge circuit. (the output voltage depends on the type of filter used of course)

If you look at the old UTC catalogs, you will find some transformers are rated for either rectification circuit, and indeed, the current rating for the bridge circuit is 70% of the current for the grounded CT full wave circuit.

I posted an analysis a while back of this, but it went over like a lead balloon, but I still feel it's right.

The reason is that when you use a grounded CT circuit, only 1/2 the secondary is conducting at a time. When using the bridge circuit, the whole secondary is conducting all the time.

So if you run the transformer at the same output current in a bridge circuit you would have double the heat generated in the resistance loses in the secondary. But, if you decrease the current to 70%, then you have the same heat loses. The reason is that heat generated is I squared R. So you to get half the heating you need to derate by dividing the output current by the square root of two which is approximately the same as multiplying by 0.7.

Here is interesting link for info on using CCS transformers in various ICAS applications:

http://wb0nni.dakotamade.com/xfmrpwr.html

[WD5JKO]

David,

    Ok on that post. Doing 70% rated current as you say would allow me to run AM at 375W out.

So I just put the tranny on the scale, and it's 25 pounds. The Orr chart then says:

.6KW CCS
1.2KW CW
1.4KW IVS
2KW+ IVS min

I find it interesting that the tranny says 200 Celsius on it.

Jim
WD5JKO

[KM1H]

Be careful with what transformer model you are using. The commercial duty C  UTC version is OK to abuse as mentioned if it was designed for a FWB. The economy models are already running ICAS specs and others are sort of in between. The internet is full of old catalogs and this is one of the better sites. There are also plenty of documents on calculating the KVA by measuring the cores cross section if its not a sealed unit. In that case the weight minus a correction guesstimate for potting and sheetmetal has to be used; also they retain heat.

http://www.bunkerofdoom.com/

Old timers will remember the Ameritran brand that was popular surplus in the 50's. Those could take a real beating.

I ran a pair of 250TH's in the amp on a BC-610 transformer and 810 modulators on another. No problems but I didnt go into monologues on AM either. Mostly used the amp on CW chasing DX.

From what Ive read of the commercial BC TX wars in the industry magazines is that Collins transformers were considered light weights. Id suggest load testing before installing.

Carl
KM1H

[KD6VXI]

David,

    Ok on that post. Doing 70% rated current as you say would allow me to run AM at 375W out.

So I just put the tranny on the scale, and it's 25 pounds. The Orr chart then says:

.6KW CCS
1.2KW CW
1.4KW IVS
2KW+ IVS min

I find it interesting that the tranny says 200 Celsius on it.

Jim
WD5JKO

Jim,

Sounds like an SB220 transformer.  Except for the 200c rating!

That's pretty warm!

--Shane

[KE6DF]

OK, well the transformer you have pictured is 2250 at .565 ma.

So assuming collins speced it like most transformer companies, that means 1125V at .565 for a FWCT circuit.

With a bridge, you would get .565 * .7 = .395 out of it.

So at 2250 you would get 2250 * .395 = 890 VA

Now assuming a linear with 375W output on AM, you would need a power supply that could produce 3 times that for full CCS operation.

That would be 1125 Watts.

And you have 890 so  you are 25% short.

BUT, it looks like that tranny you pictured has a Mil spec number on it. So it's likely conservatively speced.

Plus you aren't going to talk 24 x 7 on it, so the duty cycle would be 50% or so.

So it looks like it would work in a legal limit (375 W) linear for AM - barely.

It would probably work OK if you didn't deside to do a 1hr monologue. :-)

But it's none to heavy for the task.

I agree with Carl on my comments about UTC transformers. The comments I made apply to the CG and LS series power transformers. Those were commercial units. The S series where "amateur" and intermittant service and were speced at ICAS ratings.  

If you could jury rig up the supply and try it before cutting metal to install it, that might be a good idea.

Dave

[WA1GFZ]

Anybody have any information on the CG310. I read they were rated for 600 ma at 4KVDC CCS on the high tap.

[KE6DF]

CG310 specs:

4600-4050-3500 - 0 - 3500-4050-4600 at 600ma weight 150 lbs.

That's a big boy.

I have a CG308, but it's weighs a mere 125 lbs.

[WD5JKO]

If you could jury rig up the supply and try it before cutting metal to install it, that might be a good idea.

  Hmmm, I got all the stuff at my work. We have a nice power supply test station with a variac controlled 0-250 vac with V & I meters. To test the supply I need a dummy load, like a 5000-7000 ohm >1000 watt resistor. This makes me think of two probes in a large bucket of salt water. Anybody do this? Could use some advice on how to do that. Don't want to make a mess, or kill myself. 

Jim
WD5JKO

[KE6DF]

I was thinking more like hooking it up on a bread board and then attaching it to the amplifier and seeing how it runs on the air.

But it is dangerous to have such a high voltage supply bread boarded with lots of exposed connections.

Perhaps more trouble than it's worth.

[KD6VXI]

If you could jury rig up the supply and try it before cutting metal to install it, that might be a good idea.

  Hmmm, I got all the stuff at my work. We have a nice power supply test station with a variac controlled 0-250 vac with V & I meters. To test the supply I need a dummy load, like a 5000-7000 ohm >1000 watt resistor. This makes me think of two probes in a large bucket of salt water. Anybody do this? Could use some advice on how to do that. Don't want to make a mess, or kill myself. 

Jim
WD5JKO

Jim,

Search Calorimeter on google, or even better, there is an article on building one in the ARRL manual, circa 1995 that's sitting on the bookshelf.

--Shane

[KM1H]

I regularly safely load test up to 7500V PS using resistive loads and a few times a big tube added in with a lot of air runing idle current only. All done on a wood covered raised basement floor, haywire style (no clip leads  ) but with proper insulation, etc. The load resistors also get lots of air blowing on them. Filtering is by one of the large oil caps I have on hand.

The 13A or 28A 240V Variac and meters are far enough away for safety.

Ive let them run for several hours to bake the iron or monitor temperature rise. No kids or pets allowed in the basement!

Carl

[W2VW]

Last time I wanted to check some large iron I went from mains to the iron and used a second transformer back to mains. Put a few electric heaters on as a load.

The transformer under test was a modulation transformer with a 1:1 ratio. I ran 6kv through it and stuck 4kw worth of heaters on it. The thing didn't even get warm!

[The Slab Bacon]

Last time I wanted to check some large iron I went from mains to the iron and used a second transformer back to mains. Put a few electric heaters on as a load.

The transformer under test was a modulation transformer with a 1:1 ratio. I ran 6kv through it and stuck 4kw worth of heaters on it. The thing didn't even get warm!

IIRC, Timmy did something like that years ago trying to send power to a remote location "up the hill" with ladder line. I seem to remember that the ladder line broke down and the whole mess went up in flames.

Dave, that must have been a STRAPPING mod transformer!!

[w3jn]

THat's prolly the mod transformer Dave lugged up to Tom's place in the trunk of his Cadillac and it busted the concrete steps on Tom's porch 

[WD8BIL]

So at 2250 you would get 2250 * .395 = 890 VA

But it'll do a 500 watt input class c plate modulated rig halfway decent!

[W2VW]

THat's prolly the mod transformer Dave lugged up to Tom's place in the trunk of his Cadillac and it busted the concrete steps on Tom's porch 

6 X 12 Uhaul. I backed into the porch while leaving.

[KD6VXI]

THat's prolly the mod transformer Dave lugged up to Tom's place in the trunk of his Cadillac and it busted the concrete steps on Tom's porch 

6 X 12 Uhaul. I backed into the porch while leaving.

The sign of REAL iron.  Shows up in a UHaul and causes damage while getting it into the house.



--Shane

[W2XR]

OK, following the other 'hv transformers' thread it was clear about how using a FW bridge with a transformer rated for a FW with C.T. results in the rated current needing to be divided by two since the DC output voltage doubles. This way the same V*A product exists with either rectification type.

But what about keeping the load current with FW bridge the same as FW-CT when the duty cycle is low? Now for short periods we deliver twice the rated power. The applications of powering a class B modulator, or SSB linear come to mind.

So, I got my 8877 shoe-horned into a Dentron MLA-2500 amplifier. With the stock power supply, I could easily do 300 watts AM, sometimes 400, and legal limit SSB. The power tranny got pretty hot though with AM. Now I need to recap, and instead I want to clear out the power supply and start over. The attached picture shows what I am contemplating. 

That Collins transformer rated at 2250VCT @ 0.565A. The secondary DC resistance is 90 ohms, and each primary 115v winding is 1 ohms DC. The current rating is CCS and at 10,000 feet. Any idea what the ICAS 50% duty cycle rating might be?

I figure with 230vac in, I will get about 3200V unloaded, and about 2750 loaded (~1.2*rms). For 375W AM linear at 33% efficiency, I need 1125 watts DC input, which could be 409ma @ 2750v. Is this doable?

For SSB unprocessed, I will let it rip....

Jim
WD5JKO

Jim,

Perhaps this comment is a bit off-topic, but will that 54uf cap shown in your accompanying photo be acceptable for use in this PSU you are considering? I ask this as perhaps that capacitor is designed for energy storage/discharge applications only, and it's ESR (equivalent series resistance) may not permit it's use as a filter cap due to the high ripple currents encountered in a typical HV DC power supply.

Just curious.

73,

Bruce

[WB3JOK]

Perhaps this comment is a bit off-topic, but will that 54uf cap shown in your accompanying photo be acceptable for use in this PSU you are considering? I ask this as perhaps that capacitor is designed for energy storage/discharge applications only, and it's ESR (equivalent series resistance) may not permit it's use as a filter cap due to the high ripple currents encountered in a typical HV DC power supply.

Just curious.

73,

Bruce

I have the same cap filtering my 4x4-125A amp, and it seems to be working fine so far. Doesn't get warm or anything.  No reports of hum on signal. Haven't checked with a scope for ripple voltage though...

[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

[W2XR]

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 thinking of building.

Thanks & 73,

Bruce

[K5UJ]

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.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330419830134

[KB2WIG]

Yeah,  made with Ham Oil.

klc

[KE6DF]

We are having ham for Easter dinner. I'll save some of the oil if anyone wants it.

It might work really well in filter capacitors used in a pole-pig power supply.