[Milsurplus] Negative Lead Filtering

[Jim Whartenby]

MilesI do not understand your safety concern.  
If the filter choke opens then there is no current flow in the HV transformer secondary so the rectifiers will not be forward biased.  Any B+ voltages measured to ground are the result of small leakage currents and can only be measured with very high input resistance voltmeters.  An analog voltmeter with 20k ohms/volt should measure a much lower, if any, voltage since it will load the circuit being measured.  
If the center tap of the HV transformer does have a hard short to ground, the filter inductor will then be out of the circuit.  Then the power supply will operate normally with the exception that the power supply filter will now be a capacitor input, not inductor input, with the resulting higher B+ voltage seen with capacitor input filters.
If the center tap of the power transformer, in the normal filter inductor configuration, has a high resistance contact to the chassis, then a problem similar to the open filter inductor in the negative side of the power supply will also exist.  This will limit the current available from the power supply and will cause the HV B+ to be much lower then normal.
Regards,Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: k2cby at optonline.net
To: milsurplus at mailman.qth.net
Sent: Sun, Sep 4, 2022 10:28 am
Subject: [Milsurplus] Negative Lead Filtering

<!--#yiv2682924818 filtered {}#yiv2682924818 filtered {}#yiv2682924818 p.yiv2682924818MsoNormal, #yiv2682924818 li.yiv2682924818MsoNormal, #yiv2682924818 div.yiv2682924818MsoNormal {margin:0in;font-size:11.0pt;font-family:"Calibri", sans-serif;}#yiv2682924818 span.yiv2682924818EmailStyle17 {font-family:"Calibri", sans-serif;color:windowtext;}#yiv2682924818 .yiv2682924818MsoChpDefault {font-family:"Calibri", sans-serif;}#yiv2682924818 filtered {}#yiv2682924818 div.yiv2682924818WordSection1 {}-->I assume from Mark’s post that he is talking about a high voltage power supply.So far as I know, the only reason to use negative lead filtering is to avoid the need for a choke with a high voltage rating.Operationally, the power supply will be no different from one where the choke is in the positive leg.There is a safety concern, however. If the choke should fail, creating an open circuit in the negative leg, everything between the input side of the failed choke and the transformer center tap will immediately rise to negative high voltage. The negative poles of the filter capacitors – and everything else in the negative leg -- should therefore be insulated. What’s worse, the center tap lead of the power transformer (which was never designed to see high voltage) may short to the frame, making the entire chassis lethal. /Miles, K2CBY  ______________________________________________________________
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