Choke input filter power supply "Critical value of Inductance" questions

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KJ4OLL:
Hi,
Using my last few days of vacation to make some progress on my first attempt on a homebrew amp.
N2ZAB educated me (I'm not an engineer!!) about the flaw in the power supply design, so I am fixing it.

It is fun to learn this technology, even though at my age, there are only about 3 functioning synapses remaining, and two of them are always looking for my glasses.

The power supply uses a choke input filter.
The power supply is a single phase, full-wave rectifier, sine wave source voltage design.

Filter circuit:
- not resonant, no luck finding the components at auction sites and hamfests that would form a resonant circuit, can't afford to have them manufactured.
- not a swinging choke.
- Just using what I could find: Fair Radio had some 10H fixed chokes from some kind of Gates.



The "Critical Value of Inductance" is what I missed.

Searching the forums and the web and found lots of good information.

The 2013 handbook has a couple of paragraphs on choke input filters.

Found many different formulas to calculate the Critical value of Inductance for this application.

I have used two of them, (the one from the handbook, and one from a paper by Henry Pasternack) just to see if they agree,
and if I am getting close.

PLATE VOLTAGE
There are three output taps on the plate transformer.
Using the "2600" to start with.
4000 and 4500 are also available if I feel lucky.



 

If the filter choke can be made to work as it should, I could use the "4000" tap and obtain a B+ of about 3600 VDC, which is probably OK for the two 6MFD filter caps, as they are rated at 5kv.

The PA is a 3CX3000A7, which the EIMAC spec sheet says is OK up to 5kv.
 

ARRL HANDBOOK INDUCTANCE CALCULATION
7.8.6 in the 2013 handbook has a chart for finding a constant, based on line frequency.
So I used 60Hz, which gives a "choke inductor constant" of 1100.

The formula in paragraph 7.8.6 is L =R/A

The existing 225 watt bleeder resistors yield R=25k ohms.

So if I am doing this right, L= 25000/1100

L=23H

I have two 10H chokes I can connect in series, so this is pretty close.



ALTERNATIVE INDUCTANCE CALCULATION
From a paper by Henry Pasternack in 2009:

L = vdc/ima
L = minimum choke inductance
vdc = dc voltage output of rectifier
ima = load current in milliamp

ima: I am still experimenting with the Zener circuit that sets the idle current for the PA.
Currently a 5.1v Zener yields 100ma.
It may be that I can just ground the CT of the filament transformer, and delete the Zener, to obtain the correct PA idle current.
The EIMAC 3CX3000A7 specs for a "Radio frequency linear amplifier cathode driven class AB2" call for 250 mills "zero signal plate current" @ 4kv.

So my assumption is for about 200-300ma, (bleeder plus PA idle current), for purposes of this calculation.

2500VDC is what the volt meter actually reads from the rectifier output, using the 2600vac output tap from the plate transformer. I think the higher reading (should be about 2300) is due to the utility presenting 245vac to the plate transformer primary.

2500/200 = 12.5H choke
2500/300 = 8H choke

If I dare to use the 4000VAC tap on the plate transformer x .9 for a choke-input design:

3600/200 = 18H
3600/300 = 12H


- Seems like if I use the two 10H chokes, I will have enough inductance for either of the two plate transformer taps (2600 or 4000)
- Planning to keep experimenting w/ the PA idle current to get close to 250ma @ zero signal.
- Planning to experiment with the 225 watt bleeders to get a bit more current.

Thoughts? Ideas? Am I on the right path, or is it time to drag out the fire extinguisher?

73
Frank
KJ4OLL

N2DTS:
I have built many power supplies and never did any sort of calculation.
I use chokes that are good for the current, and caps that are good for the voltage and whatever bleeders I can get that put up with the wattage.
I do not like burning up a lot of power in bleeders in big supplies.

I thought many amps did not even have chokes in the power supply.

I would use one choke and as much capacitance as you can, with the voltage ratings on everything higher then the supply could get to.

Are you going to leave the supply on or key it with the amp?

KA2DZT:
Don't over think the power supply.  That choke will handle 4500 VDC probably more.  Don't worry about critical inductance, a 10Hy choke is good for the input choke.  What happens if the input choke does not have enough inductance, is the  filter capacitors will charge up closer to the peak voltage.  The more bleeder current you use and in addition the xmtr is drawing current the supply won't charge up close to the peak voltage.


The thing you need to use is filter capacitors that have a high enough voltage rating.  If you are going to key the HV supply on and off with xmit and receive you don't to need to have caps with a voltage rating much higher than the DC voltage.  OTOH if you are going to leave the HV supply on during receive and key off the xmtr some other way then the HV may rise closer to the peak voltage at which point the only current pull from the HV supply would be the bleeder current.  Critical inductance depends on the minimum amount of current load on the supply.

Fred

I just re-read you post.  You have two 10Hy chokes.  Two 10Hys in series is more than enough inductance.  You can use a LLC filter or a LCLC filter.  You'll get better filtering with a LCLC supply.  The first cap needs to have the higher voltage rating and it doesn't have to be a lot of capacitance 6-10mfd is good.  The second cap in the LCLC filter can have much more capacitance.

I would suggest keying the HV supply on and off with xmit and receive, it solves a lot of issues and is much safer.

gerry_w1id:
For a full wave rectifier the frequency is 120 Hz, not 60 Hz. Also keep in mind the DC resistance of the choke as this will result in a voltage drop under load. However with the choke you  show I don't think it will be very much but just keep it in mind.

John K5PRO:
IN the olden days, critical inductance was important factor to keep regulation from no load to full load good. We had to pay attention to it in commercial gear as FCC regs in part 73 suggested keeping the final stage voltmeter operating in the upper third of scale. If you ran it this way, and removed the excitation of a class C amplifier, voltage would soar to the point of pegging the meter offscale. I've seen this is some old Gates rigs. Its always the first choke between the rectifier output and the capacitor that cause this. As long as your bleeder and the combined effect of modulator bias or RF PA idling current are presenting the minimum load for the calcs (the 1100 x) at 120 Hz, then it would only soar hundreds of volts when RF is removed. If you completely ignore this, be prepared to design some overhead in the capacitor voltage ratings. I wouldn't worry about the 3CX3000A7 handling the voltage of sloppy power supply voltage. By removing the first choke entirely and using a big big capacitor as some do (40-80 uF) you can still get effective ripple reduction, and the voltage stays closer to the peak of the rectifier output. In this case, the variation is due to rectifier Vdrop, transformer impedance, and value of cap/maximum load current. The disadvantage of doing this is that now you have a lot of stored energy (joules=1/2 x C x V^2) in that capacitor. Like 250 Joules with 40 uF and 3600 VDC. Eimac recommends limiting any tube fault current to 5-10 joules, to prevent destroying the grid inside the tube. You have to account for this with either am active crowbar (don't go there) or a dissipative series resistor like 10-20 ohms. This is good practice even with an LC filter and smaller capacitor.

Something else to watch for (power supplies are not trivial, despite what some say!) is the LC resonance in the supply. This is seen in two places. When you switch on the HV first time, you may get a very large transient voltage spike, hard to see on a meter but on a scope there it is. This ringing can be 50% of your DCV or more. The other time is when keying the rig with CW, at some rates you find the voltage hunting up and down. It also was a concern in old TV transmitters with tubes, the visual signal was AM and had a lot of regular modulation repetition in it (sync and dark part of picture for example). You can read about this is some of the papers on this site, old copies of GE Ham News that describe it.

So yes, you should at least calculate the critical inductance and improvement from lower bleeder R and idling current, and compare to your ratings for capacitor voltage, meter scales, voltage divider ratings, bleeder voltage rating. To ignore this and tempt fate would be foolish and poor engineering. Going to a dual section filter LCLC isn't recommended with intermittent loads like AM, just put all the L up front like you planned, 2 x 10 Hy in series.

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