Valiant II PS

[W9BHI]

I need the opinions of the power supply gurus here.
The Valiant uses two 80Mfd caps and two 20K 10 watt bleeders in the HV plate supply.
Would there be any noticable difference in performance if the caps were increased to 220Mfd and the bleeders to 100K to reduce heat dissapation?
I see that they are using the bleeders as a voltage divider for the clamp cicuit but it is going thru a 470K resistor so I think there will still be enough current using the 100K's.
Please correct me if I am wrong.
Thanks,
Don W9BHI

[W2VW]

There needs to be enough current draw for the choke to behave as a choke input supply. The HV might land up a lot higher in voltage until cathode current is drawn.

The extra capacitance may not be kind to whatever switches the xfmr on. A current limiting MOV in series might help there.

[W9BHI]

I kinda thought that the 20K bleeders were there to draw enough current to make the choke input circuit work.
The plate transformer has power applied by a relay.
What value of current limiter would be best?

Thanks,
W9BHI

[W2VW]

Current limiter either a CL-6 or CL-8 from R.F. Parts.

Maybe someone has tried one in this exact app.

Repeated short spaced keying of the supply is worst case for the CL.

[W3GMS]

I kinda thought that the 20K bleeders were there to draw enough current to make the choke input circuit work.
The plate transformer has power applied by a relay.
What value of current limiter would be best?

Thanks,
W9BHI

If the HV supply is keyed from the primary supply of the transmitter and the final is on pulling suggest plate current that will be more than enough current to keep the choke input filter happy.  On CW, you would then have an issue since the P.S. would likely be loaded very lightly when the key is open.  That is not so bad as long as the voltage rating of the capacitors can handle the extra HV.  I would go with something around 10 ohms for the thermistor in the primary of the TX.  Here again, for rapid break in you will likely run into a situation where the HV caps become discharged and the thermistor is still hot so you will loose the inrush protection on the next key up.  Other soft start circuits use time delay components to short out the inrush protection after turn-on, so even in a break in condition the Thermistor never gets hot since it gets shorted out 50 or so milliseconds after you key the primary of the plate xmfr. 

Lots of choices so based on how you operate the rig some questions can be answered.

Joe, W3GMS   

[W9BHI]

I understand now that the choke input circuit needs some loading when very little current is drawn by the final in the CW mode.
I don't run this rig in CW mode.
Now comes the question will larger capacitors after the choke help with regulation? (going from 40Mfd to 100Mfd)

Thanks,
Don W9BHI

[W3GMS]

Don,
With the larger filter capacitor you will likely have slightly more inrush at turn on.  The largest current flows when the capacitor is being charged from its discharged state.  I have 220 ufd of capacitance in my Ranger but I do have an inrush current limiting circuit in the primary.  In my Ranger the HV stays on all the time.  When in standby, I remove the screen voltage from both the final and modulators and un-key the VFO line otherwise you will hear the oscillator in your receiver.  This has worked out great for over 20 years with no component failure.  The extra capacitance will give you better dynamic regulation, meaning for rapid short duration load changes the larger "C" will be able to supply the energy.  The steady state regulation will be the same but the ripple voltage will be less. 
Joe, W3GMS

[KM1H]

The higher value caps after the choke will have almost no effect on regulation and do very little for surge current since the choke is doing the current limiting along with the bleeders. The weak link is the rectifiers if using tubes.

I use a pair of 100uF with 100K equalizers and the 50W 25K bleeder in a Viking I and II CDC simply because I had them on hand and the stock 8uF oil caps were running hot. There was almost no change in regulation or PEP that I could repeatedly measure tho the ripple was a hair less. Im using the original 5R4's in the II (very low hours) and NOS in the I (which was run hard and put away wet) and calculations show that the peak inrush current is well within their spec.

Leave things alone and especially dont use MOV's; they have strict requirements for Minimum as well as Maximum currents and on either side become resistors that get verry verrry hot before failing as dead shorts and causing damage. I dont believe the run of the mill CL series are suitable.

Carl

[W3GMS]

For extremely fast instantaneous current demands the output capacitor in a choke input filter network does supply the initial energy.  These are current demands that are extremely short in duration.  This is referred to as dynamic load regulation and has nothing to do with the chokes contribution when discussing steady state load regulation.  Regulation in supplies is express with 3 terms.  Line regulation, Load Regulation and Dynamic regulation.  Due to the extremely low output ESR and ESL of the cap right across the output, when an instantaneous demand for current is expected the current is delivered from that capacitor since that is the lowest impedance point in the filter.  Following that, the contributions of the choke come into play.   A switched electronic load placed on the output of the supply will easily show this.  Look at the rising edge of the dynamic current pulse and with a current probe look at where that rising edge current demand is sourced from.  Line impedance, rectifier impedance, winding resistance are factors on static load regulation but have little to do with supplying current during the rising edge of an extremely fast current demand.  Its really no different in a regulated supply.  The loop response crosses over at about 1/10 than of the switching frequency and therefore is not fast enough to deliver rising edge current demands of a switched load. That is specifically the reason why multiple output capacitors are used to reduce the ESR and ESL.  As the capacitors runs out of current delivery capability that is when the loop response must be good enough to deliver more energy to the output filter network. 

We are not talking about using an MOV for inrush protection.  MOV's thresholds reduce as voltage transients hit them and ultimately they will destroy themselves.  That component is completely unsuitable for inrush protection.  Transorb's are much better in that application.

Special thermistors approved by all the safety agencies are rated for inrush current protection when placed on the AC mains.  There are required mandates which dictate the placement of these components because they do get extremely hot during steady state condition in order to drive the resistance down.  Most switching power supply do use thermistors for inrush current protection.         

Joe, W3GMS 

[W2VW]

We are not talking about using an MOV for inrush protection.  MOV's thresholds reduce as voltage transients hit them and ultimately they will destroy themselves.  That component is completely unsuitable for inrush protection.  Transorb's are much better in that application.

        

Joe, W3GMS 

I said MOV and it was in error. I meant thermistor. Even got the part number worng. Can't spell either 

http://www.rfparts.com/inrush.html

[W3GMS]

Hi Dave,

I knew what you meant ! 

Hope all is well....

73,
Joe, W3GMS

[W9BHI]

There are two 100Mfd caps in series in there now and the rig seems to be working ok, so I will leave it alone.
I'm always tinkering with someting.

Don W9BHI

[KM1H]

Those CL parts are exactly the ones Im saying do not use.

Read the specs and make a sensible decision. RF Parts sheet says almost nothing. Here is the real one.
http://www.ge-mcs.com/download/temperature/920-325C-LR.PDF

Also show me where it says the C in a choke input supply is contributing much to the peak current and not exceeding the peak ratings of the rectifier tubes.

Carl

[W3GMS]

You may not like the application but Thermistors are a very common method used in the industry to control inrush current. 

Take a capacitor and charge it from the inductor, then disconnect the capacitor from the inductor and see how much instantaneous current you can draw from the capacitor by shorting it.  You will see that the instantaneous current being removed from the capacitor is much more than the current flowing through the choke.  The choke charges the capacitor and replenishes its charge but you can pull more instantaneous current out of the capacitor for a very short period of time.  The maximum instantaneous current through the capacitor is limited by its internal ESR and ESL. 

Joe, W3GMS

[Tim WA1HnyLR]

 A surge limiter I use in various transmitters  is done by ,in the case of the Valiant. Place a 10 OHM 25 watt resistor in series with the primary . Connect a 120 VAC 10 A minimum rating relay , N.O. contacts across the resistor. Connect the relay coil directly across the primary of the plate transformer. I use a pair of 325 MFD caps in my Valiant's HV supply. I have yet to install this circuit  in my Valiant but have used it in other applications. I have had no issues with inrush current as yet. The filter reactor limits the  surge anyway. The step start would reduce or eliminate the tell tale "thunk" that comes from the intense magnetic field from the HV power supply filter reactor that is induced into the steel cabinet of the Valiant transmitter. De Tim WA1HnyLR

[ke7trp]

Gotta try that, Thanks Tim!

C