[AMPS] Re: Surge voltage, HV connectors.

[Wilbert Knol]

> > Measured:
> > 2093 VRMS (no load)
> > 2038 VRMS @ 400 mARMS (projected amplifier load)
<snip>
> I suspect you are measuring RMS voltage, and not peak voltage. 
Yes.
> 
> Excessive ESR in the transformer does not show up on an RMS 
> meter.

Perhaps I should have clarified. There is no capacitor or rectifiers. All 
waveforms are sinewave AC. The test load is a string of lightbulbs. The 
magnetic shunts have been removed.

Transformer ESR=(2093-2038)/0.4 = 137.5 ohms.

The transformer sags 2.7 % under projected full load (which includes line sag)

> A good 2000 volt transformer 500 mA transformer for capacitor 
> input supplies should have under 15-20 ohms ESR, which is about 
> ten ohms secondary dc resistance in a typical transformer design.

The oven transfomer measures: 
Rdc,sec = 82 ohms
Rdc,prim=1.6 ohms.

...that is nowhere near what it should be.

> 
> A choke input will help, because the supply will draw current over 
> virtually all of the cycle with the correct size choke. Peak current 
> can be reduced by a factor of 10 or more.

It seems to me that the oven transfomer itself is good enough. After all, it 
handles the load with little sag. 

The problem is with the demands placed upon the transformer by the 
capacitor input network. To hold B+ to within 10 % of the unloaded surge 
value (equal to Vsec,pk) demands these extremely low values of transformer 
ESR, as well as a large capacitor.

The conclusion will have to be that I could press the oven transformers into 
service. By increasing the bleeder load and relying on amplifier idle current I 
could probably improve regulation somewhat. But it wouldn't be a great 
supply.

Adding a choke won't work in my particular case. I would wind up with even 
less B+ (although better regulated...). Another way would be to add proper 
shunt regulation...the supply would have to dissipate in standby what it 
provides to the amplifier during transmission...

Hmm...Perhaps I should ditch the oven transformers and get something with 
the correct ESR made up...I will have to think about this.

> both sides of the line float with no CT reference to earth, although 
> you have a safety ground. I had a specific conversation about that 
> with two ZL's. Was I misinformed?

Very much so. I hope those guys aren't practising electricians in ZL2.... The 
mains here is multiple earthed neutral, typically 235 V, 50 Hz.

> 
> If it isn't, you need to isolate the HV lead from the mains with some 
> form of guard area that is always well grounded, so there is no 
> possible path.

Fair enough...good idea.

> The shack smelled like burnt Ham for a month.
That would have put me off home-brewing for good. You are lucky to be alive 
Tom!

I will have to modify the Ig and Ip meters on the amplifier for peak holding if I 
am to tune up at 50 WPM dots.

Also I now have some info on the Kings HV connector/socket pair. They are 
bayonet style, look a bit like BNCs, are properly voltage rated with recessed 
centre contacs etc. Looks promising, I will try to get a pair.

Thanks for all the good 'feedback' (an unfortunate expression....) from 
everybody so far.

Wilbert, ZL2BSJ.


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