PSU designer understanding.

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pre65
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#1 PSU designer understanding.

Post by pre65 »

I'm doing a simulation in PSU designer, trying to see if the Hammond 735A transformers I have will do what I need for the 833a project.They are 3,000V CT.

What was giving me concern is that the Hammond 193Q 10H choke I have is rated at 1,000V max, and the HT was going to be around 1,300V.

BUT, when I ask for the voltage at L1 (the 193Q), it shows, after 0.5 seconds a rapidly fluctuating voltage from (approx) +300V to -300V.

Is that right, and if so I don't understand. :?
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Nick
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#2 Re: PSU designer understanding.

Post by Nick »

Thats likely to be the voltage across the inductor not the voltage to ground.
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#3 Re: PSU designer understanding.

Post by Nick »

If you are going choke input then:

1. At those voltages the choke is going to be working very hard
2. Watch for off load start-up voltages.
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#4 Re: PSU designer understanding.

Post by pre65 »

Nick wrote: Tue Apr 28, 2020 1:07 pm Thats likely to be the voltage across the inductor not the voltage to ground.
So that is within the 1,000 V rating from Hammond ?

On the sim, I've put a 1K resistor and 2.5uf capacitor (2 x 5uf in series) before the choke, to get the voltage down from 1,600v ish to 1,300v ish.
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#5 Re: PSU designer understanding.

Post by Nick »

So that is within the 1,000 V rating from Hammond ?
No idea, but I would think not.
On the sim, I've put a 1K resistor and 2.5uf capacitor (2 x 5uf in series) before the choke, to get the voltage down from 1,600v ish to 1,300v ish.
That 1k resistor is going to make it a crap power supply, and also a good room heater.
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pre65
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#6 Re: PSU designer understanding.

Post by pre65 »

Nick wrote: Tue Apr 28, 2020 1:57 pm
So that is within the 1,000 V rating from Hammond ?
No idea, but I would think not.
On the sim, I've put a 1K resistor and 2.5uf capacitor (2 x 5uf in series) before the choke, to get the voltage down from 1,600v ish to 1,300v ish.
That 1k resistor is going to make it a crap power supply, and also a good room heater.
I've ditched the room heater, but still playing with PSUD. :)
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#7 Re: PSU designer understanding.

Post by Mike H »

Just a thought, I'd think about having 2 or more PSU's stacked in series. The components in each one then have less Voltage to contend with.
 
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#8 Re: PSU designer understanding.

Post by Mike H »

Oh wait you've already got the 3,000V TX's .....

:oops:
 
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#9 Re: PSU designer understanding.

Post by pre65 »

Mike H wrote: Tue Apr 28, 2020 7:45 pm Oh wait you've already got the 3,000V TX's .....

:oops:
Yes, had them for several years.

When I started PSU designing today I got some horrendous voltage figures, until I remembered you only enter one half voltage (ie 1500v) for full wave rectifiers.
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#10 Re: PSU designer understanding.

Post by simon »

I know all this was said 10+ years ago Phil but please be careful with those voltages :shock:

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#11 Re: PSU designer understanding.

Post by pre65 »

Cheers Simon.

It's now obvious to me that those 3,000V CT transformers are not the ones to use for the 833a project at the HT I want to use, and with the chokes I have.

BUM.

So, 833a goes on the back burner-again.
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#12 Re: PSU designer understanding.

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:thumbright:
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#13 Re: PSU designer understanding.

Post by Paul Barker »

You’re biggest risk is the choke input choke; don’t exceed its voltage tolerance which you’ve contemplated. Don’t it will end up arcing, and if it’s potted you won’t know until it’s too late. You can use the 1500v (3kvct) but drop it to 1kv with big variac or a suitable sized power transformer and only using the secondary voltage chosen to absorb a proportion of the mains. Leave primary disconnected, but measure the primary voltage (which is now secondary as the additional transformer is back to front.). In case it provides a second B- source for creating a Minus dc supply for the negative rail of you’re mosfet for example. You could calculate it from the turns ratio but empirical testing will be necessary . Empirical means the actual measured findings.

So, a calculation : add a (voltage to be added to picture when I know main transformer power rating)v secondary on you’re dropping transformer, which will give reduced ac to original transformer due to Copper and iron losses and accounting for the primary under wind the manufacturer builds in to account for losses in conventional use and works against you back to front. Hence I’ll knock it down 10% (5% underwind and 5% losses.) But on tets it may require a tweak which you can do in youre filtering.

Use the main b+ to supply the positive rail of mosfet (taken down with a shunt reg if you know how. Bring negative rail down likewise after basic smoothing).

You don’t really need a separate bias supply you place a voltage divider between -100v and ground to level the gate of the mosfet at youre 833 operating bias calculating the drop across mosfet gate to source or just apply probes between Centre point of filament of 833 And grid/sorce node. If no centre as it’s dc then adjust desired outcome by 1/2 filament voltage added or subtracted depending which side you test. This presumes you are cap coupling the voltage amplifier valve to the mosfet. If you use direct coupling at this stage it’s more complexed and I suggest a step too far in youre case as it requires level shifting. It’s all easily doable but I don’t believe you should build a high voltage amp you don’t know intimately and comprehend without doubt the full picture of level hsifting and how to account and provide for the other demands other than geting dc level right, such as the resistance the VA is driving. Similarly initial stabiliy of dc level to final level and whether any protective circuitry required etc.

The above drops 3kvct to 2kvct so from you’re b+ you’ll get 900v dc before filtering and probably end up with 850. Adjust back up with tweaking first cap. Not sure what the 1k resister is doing, I’d go with caps in series and voltage devider resistors parallel one across each cap.

I’ll scribble it out for you as it’s a lot easier to demonstrate pictorally.
Last edited by Paul Barker on Wed Apr 29, 2020 9:56 am, edited 3 times in total.
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#14 Re: PSU designer understanding.

Post by Paul Barker »

So I can scribble it appropriately; What’s the wattage of youre 3kv transformer? I would think you’d require a thread bare minimum 400mA secondary for two 833 power amp integrated supply, because the primary voltage seen by the 3kv transformer is reduced the primary wattage requirement for 400mA secondary reduces to <1kva. So if transformer is at least 1kva you have whta you need, and youve probbaly got a series transformer in stock but when I get the wattage from you I’ll calculate voltage and wattage required for the series transformer. Then I’ll draw it so it’s easy to follow.
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#15 Re: PSU designer understanding.

Post by Paul Barker »

Image 
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