The AM Forum

My HV power supply is set up to give 2KV and 3KV.  The other wiring alternative would be 4KV and 6KV which is too high.  I needed 4KV to power a linear amplifier.  I tried every configuration involving full wave, choke input, etc. but no luck.  I didn't want to use a Variac due to sag.

Here's the solution:  A strapping transformer booster in the 240V  primary.  

I use two identical HV transformers that have the primaries in series  and the secondaries in parallel. The result is 120AC across each primary using a 240V source.  The transformers have 240V windings so I could add more voltage in series with their primaries without a problem.

I used an old 100 pound Variac core and wound 70 turns of #6 insulated wire on it. This gave me about 75V out with 240V on the Variac primary. IE, I converted a Variac autotransformer into a pri/sec transformer.  This extra 75V gave me the 4KV I needed out of the HV supply. I used a pair of breakers to turn the booster on or short it out when not used.  At 75V, it only has to handle about 1/3 the total power of the supply.

I ran a current test on the HV supply and found the supply was still very stiff when the booster was in line.  So now I have 2KV, 3KV and 4KV available to power the various rigs. The filter cap is rated at 100 ufd @ 10KV, which helps... ;D

Look at the picture below and notice the cigarette lighter for scale. It really is a big MOFO booster and will handle about 100A in the secondary.  Maybe someone here can use the idea when out of ideas - like I was.

BTW, this is the same booster that got pulled and rammed against the filter choke when the HV supply shorted out. The filter choke acted like an electromagnet. I've seen this happen with big mod iron too. It pays to tie down the iron for this reason when in close proximity.

73,

Tom, K1JJ

The 100 pound Booster:

Hey Tom
That's quite a booster!!
I used to work on TV's and they had a boost circuit in them to get the 160vdc to 700vdc for screen volts on the CRT. Would that be of any use? Probably current would be a serious limitation......dunno

Fred

Ya lost me on the curve there Tom!

You say your booster gives an output of 75vac with 240vac on the primary??
That's less not more voltage?

Or, what is your AC voltage input to the primaries of those HV transformers *after* you apply the booster??

Howz about a schematic?

I don't see how this works, or where it goes... etc... but I am slow anyhow.

                               _-_-bear

Bear,

he's using the variac transformer as a buck-boost transformer.  The 75V out gets added to the 240vac for 315vac input to the primary of the plate tranny. 

Yes, Ed, you have it correct.  Actually, I see the schematic says 68V is added to the 240AC primary giving 308V total when the boost is on.   The sec of the booster transformer MUST be shorted when not in use or else some of the 240 will drop across the booster xfmr. I simply use a breaker for the job.

Here's a handwritten schematic to give an idea:

T

I have found that variacs have an unacceptable sag under full load.  One of my biggest mistakes ever was giving away the tapped kilowatt size 120 v.a.c. auto transformer with built in rotary switch that came with an old x-ray machine I salvaged from the dump. That thing was as stiff as the a.c. mains voltage feeding it.  I couldn't measure any sag at all. I replaced it with a neater and more compact 20A G-R variac, but was never happy with its performance.

I have a couple Signal Transformer step down transformers.

One of them has two 28V CT windings that are intended to be connected in series or parallel.

I was thinking that would make good buck/boost transformers for a high voltage supply.

By connecting the two secondaries in series, it would give you a choice of 4 taps each 14 volts apart.-- plus the direct line connection.

Connecting those up with a high current rotary switch would give a nice variable HV power supply.

My transformers are rated at 340 and 425 VA so would only work for a moderate output HV supply.

But Signal has larger sized similar transformers available.

I'm not sure if using one of these would be any better in terms of sag than using a variac, though.

But it's another idea, FWIW.

Then there is the old Thyratron phase angle variable supply method commonly used on industrial RF applications like inductive heaters...

Thyratrons? That's an interesting concept...Why couldn't one use a heavy-duty light dimmer gadget on a plate supply to adjust its voltage? Use a couple of 50A SCRs or Triacs?
Same effect, no?

I believe that's how Continental adjusts the screen voltage on their 30+ KW FM transmitters.

I agree, Don -  the Variacs I've used, even very large ones sagged under full load. After all, they have to pass the same power as the HV transformer, so should be at least as big.  

The advantage of the boost transformer is that it only has to pass the power represented by the extra voltage it supplies.   So if the circuit is say, 10A, the booster will still supply 10A since it is in series with the main xfmr, but it supplies only a part of the voltage. If it supplies 70V out of 308V, then the main xfmr supplies 2400w and the booster supplies 800w.  I like that vs: a Variac having to handle it all.

Variacs are great for low power supplies like grid, screen, exciter plates, etc, but for high power HV supplies, not.  But one alternative is to use one to slowly ramp up the HV and then set the Variac so it is basically out of the circuit when the tap hits the top of the winding, making a direct connection. But then, the value of a variable variac is lost.

Dave/IYH - Yes, those low voltage transformers that handle lots of amps (like fil xfmrs) work FB.  A pair shud serve you well, esp bucking for more versatility.

T

"Use a couple of 50A SCRs or Triacs?
Same effect, no?"

An SCR is pretty much the solid state equivalent of a thyratron, I believe.

There are thyratron equivalents of all the common MV rectifiers.

A 5557 is a gridded 866, an 873 is a gridded 872a, and there are some that are gridded versions of 575A's also but I forget the numbers.

There is a circuit for this in some of the old "Radio Handbooks" in the power supplies section.

I wonder how well it would work if you did the phase angle "chopping" in the primary circuit?? Perhaps just fine.

Also, I'm not sure how high voltage ratings go on SCRs. If you can get them in the 10KV range, you could use them as rectifiers in a HV supply.

The days of large power contactors in radio transmitters seems to be over...

I recently installed a 50 KW broadcast rig and it's all SCR controlled. Totally quiet and a slow ramp up at turn-on. Primary voltage is 480, 3-phase, fused at 200 amps, the power transformer weighs in at around 700 #.

I don't miss those giant relays with dime-sized contacts at all, and they sure do make some big-time huge SCRs.

" I don't miss those giant relays with dime-sized contacts at all, and they sure do make some big-time huge SCRs "

I still pine for the days of rotary equipment,  knife switches and their bright green arc.

klc

enter another method of obtaining an intermediate HV outpoot. One of the WBCQ transmitters uses two 7200 volt 25Kva pole pigs for it's HV supply. Primaries and secondaries paralleled up . Full wave bridge /ckoke inpoot delivered about 6Kv under load. This proved to be a bit much voltage for the PA (6 -3cx2500A3 tubes in push pull parallel ) There was a need to drop the voltage. You could series up the primaries which would halve the outpoot voltage . The transmitter would not make power this way with 3Kv on the PA tubes. I decided to try something a bit different. I left the HV secondaries in parallel and had the primaries  in series BUT, I connected the 240VAC to the center tap of one of the pole pigs.while the other transformer had the full primary utilized. This resulted in an outpoot voltage of 3800 volts. This setup has been working well for years now. Tim WA1HnyLR

I like how you've made it possible to switch in and out the choke from the HV DC side. Nice!

SCRs generate random broadband swithing noise that is a pita to filter at high power.

But they're on only when you're transmitting.

Hi Tom,
Looks like a buck/boost transformer used frequently in apps that need further voltage adjusting.

Yep, Bill, that's exactly what it is. In my case, I needed an extra 1000 volts to run the linear and had no other way to do it without buying another HV transformer.   Having a switchable 2KV, 3KV  and 4KV is just what I needed for all the rigs.

T

One other option to get two voltage choices out of a single supply when you are using a choke input filter is to add an input capacitor right before the choke.

If you switch the input cap into the circuit, you have a capacitor input filter. When you switch the input capacitor out of the circuit, you have a choke input filter.

So if you have, say a 2KV RMS tranformer, with the input cap switched out, you would get about 1750vdc. If you switch the input capacitor into the circuit, you would get 2500VDC or so.

When the input capacitor is switched out of the circuit, you could connect a high resistance between one side of the input capacitor and ground (the other side stays connected to the input terminal of the choke).

When then you switch to high voltage mode (by shorting across the resistor), the cap is already charged up part way to the DC output value and the current surge on the switch would be lower.

FWIW, just one more random idea that wasn't mentioned.

Yep, that is a good idea.

I'm already using it - look at the schematic I posted back a ways.  By shorting out the choke (in the CT of the bridge diode stack) it becomes a cap input. That's how I go from 2KV to 3KV under load.  Then I add the boost to go to 4KV... ;)

T

Hi Tom

Can this circuit adapted to the for power supply ? the schematic that You have Attached,it also used a booster transformer,and like you this transformer  only supply the needed wattage of the 75 ac output.


Gito

Hi Gito,

Sure, it would work fine.

It is a different way of conecting it and is clever.... ;)  Thanks for the suggestion.


T

Hi

When using this circuit don't forget the polarity /phasing of the transformer.
Because it can boost or buck the output AC voltage.
here's a picture as an example

Gito

That core looks like it is from 30 amp 240 volt variac.
I have one in my rig and never noticed sag but only use it for tuning up then it is set to max.

How do you justify running 315V on a 240V winding? Have you measured the primary current in idle and full load?

Carl
KM1H

Carl,
Nope, it's 315V on a 480V rated  primary. (two 240's in series)

I'm using TWO HV transformers, each with a 240V primary - in series giving 480 V.  The secondaries are in parallel. The sag is negligible under heavy load- about the same as when the booster is out of circuit.  100uf as the filter cap helps... ;D

Yes, Frank, you're right -  I'd estimate the primary of that booster(Variac) IS 30A.  But that's perfect, cuz it's only drawing the power it needs from the source 240V line and that's less than  1/3 the total overall power... way conservative.

T

Hi

I admit ,first I'm a little confused  How the circuit work.
Maybe it's like this ,the primary winding  two transformer is series connected and has  a total "240 AC" volt and series connected with a "75 Ac" volt booster Transformer so it looks like a "315 Ac" volt input.

The secret is how we connect the 75 VAC booster , in phase or out of phase,when connected in phase we got a 315 VAC output( boosting the AC output) and that's to high for the load 220 VAC

If we connected out of phase than we got 165 VAC output (bucking the AC output) About 77% of 240 VAC .
That means the HV VAC is reduced to 77% of it value.
Look at the picture I attached before ,in the picture the load "resistance" is the 240 AC of the series of two 120V transformer

Gito

Gito:   The two transformers each have 240v primaries (not 120v primaries) and are connected in series giving the equivalent of a 480V transformer primary.

The Booster IS in phase and adding 68 V to the 240 line.  So we have 308 volts on a 480V primary.  

It works FB.



T

Hi Tom

Sorry I didn't read Your last Comment to Carl, but actually
like Carl I had a conclusion that You series feed the 120 VAC input,giving an input of 240VAC

But using Boosting or Bucking ,the " booster "transformer ,You got nearly the same result,when You used 240 VAC connection or the 480 VAC connection ,depends on the secondary voltage of the "booster transformer" and the power handling of it.
Right Tom?


Gito

Gito,

Yes, the booster was used as a "fine tuning" to dial in the HV to exactly where I need it.  I had 2KV and 3KV - and the booster gave me 4KV.   Any other configurations without the booster made too much voltage, higher than I could use.

T

I have a nice condition old Thordarson power transformer. 1500 v CT 200ma with some filament windings.

One of the filament windings is 7.5V at 5 amps. There is also a 5v and a 2.5v winding. There is a 150v bias tap on the HV secondary and I think the intention is to use the 2.5v secondary for a half wave rectifier like an 866jr or 2x2.

Anyway, what to do with a 7.5V winding? I could use it for 6.3 with a series resistor, but that wastes a lot of energy as heat.

Then I got the idea that the 7.5V winding was perfect as a bucking winding.

I have 123Vac at my shack and the tranny was designed for 115. So are all my other old transformers. So I will have a problem with filament over-voltages with all of them.

But using the 7.5 V winding for bucking brings me down almost exactly to 115. Plus at 5 amps, I can use it to "buck" for another smaller power transformer used for the low level audio and RF circuits.

I'll probably make the 7.5 V bucking winding switchable so I can switch it out if I ever run the power supply on a 115v service -- if there are any of those still around.

same topic, may as well continue it.

The supply made 2100V before and now makes 3100V. like the OP 4KV was too much so a bridge was ruled out. Its a 500mA max load.

The plate transformer has to be well insulated to do this. There is no problem so far with a pole pig core but that's well insulated and designed for series or parallel connection. It works really well with the same regulation and has 5-6 hours on it so far in each of two identical plate supplies. Not many hours but if it were going to blow up it would have.

Patrick,

  That circuit is interesting. I wonder what KVA rating that pole pig is, and how big is that sucker? I suppose you could reverse the primary on T3 to drop to about 1KV for a low power or 'tune' position. Thanks for posting that idea.

Do you ever get on 75m? Maybe we can make a sched? PM me.

Jim
WD5JKO

PS: Stay away from licking the pig!

Looks like that is no bigger than a 7.5 KVA

Tisha, AA4HA

Tom, were you at Nearfest a few years ago when I grabbed the pair of 7.5KVA Powerstats?
With them ganged there aint no excessive sag even on a YC-156 but it sure takes up a lot of room. I like your idea mo betta. Then you can also configure for buck or boost and with several taps get any voltage needed.

Rummaging thru the trailer out back I came across the 11000-0-11000 V iron that fed some industrial rig was surplused when the shoe factory closed. Never did have the nerve to fire that up!

The one that fed a Amperex water cooled triode industrial amp is bad enough, thats 7500VDC at 2.5A (with 53uF at 10KV in a 100+ Lb Sangamo) and the Powerstats are on that for now until I ship this YC-156 amp.

Jim,

The pole pigs I use are the cores, they have no can of oil. I used to worry about that but these seem to have never been in oil because they have no evidence of it, I don't know what is the deal on that. I think they are at good for least 2KVA each

Have you ever operated one out of the can or opened a can maybe to change the oil?

I didn't think about a tune position in the power supply because the TX has a variac for every voltage and I can just turn the screen voltage down on the 4-1000. That is what I have been doing. I have temporarily removed those small center transformers because they need to be mounted. Before they sat on the floor behind the TX because there was no room yet. I want to mount them to the inside of the cabinet on the walls as close to the large transformers as possible, just a matter of space. I use 15KV outdoor luminous transformer wire for all that stuff.

They look a lot like the one shown in the following article, which is seen on its side in the lower compartment rear view at the right on page 9:
http://bunkerofdoom.com/lit/deluxeam/deluxe_amateur_transmitter.pdf

Sometimes 0730 CST Saturday on 3885 for the Texoma traders net. I just check in, the rest is listening. Weekends are my only time.  I'll PM before the weekend.

OOPS! - Jim, I forgot I have an early Saturday morning appt. I PM'd, if you can suggest a time and place..

Patrick was able to run the two xfmrs in series because the one xfmr had a split secondary instead of just a CT.

The same scheme can be done with xfmrs that don't have split secondaries.

Example,  using a plate xfmr with a CT able to run above ground, you can connect the DC output from a second xfmr directly to the CT of the plate xfmr.  Both xfmrs will need their own set of rectifiers.  The rectified DC pulse from the boost xfmr is connected directly to the CT of the plate xfmr.  Do not use any chokes or caps on the boost xfmr, just the diodes.

You can do the same thing even if the xfmrs are using FWB rectifiers.  Connect the DC pulse from the boost xfmr to the negative (ungrounded) terminal on the FWB diodes of the plate xfmr.

This is a good way to bring up the DC voltage of any supply by whatever added voltage you need.  Both xfmrs will need to handle the same current load.  You can lower the voltage, for whatever reason, by simply shutting off either xfmr without having to disconnect anything.  The one xfmr will need to be well insulated to handle the added boost voltage on its secondary.

Fred