[R-390] FL-101 replacement Corcom IEC power Filters

[Mike Carroll]

Hello list:
I agree that this topic has been adequately addressed (beaten to death) 
but I'm in need of an education re: the alternate; isolation 
transformers.   If getting zapped is the result of a fault within the 
device,  what does an iso transformer do to address that issue? I'm 
assuming that the transformer is installed at the mains, i.e., before 
the plug.
Thanks in advance
Mike


On 5/13/2014 2:22 AM, Charles Steinmetz wrote:
> Bill wrote:
>
>> So when someone says a filter is leaky, they are not talking about the
>> leakage that comes with age, yes?
>>
>> They are talking about the reactive leakage current that occurs with
>> capacitive reactance at line voltage and frequency, no?
>
> Correct, generally speaking, although it would not be unthinkable to 
> find one with excessive ohmic leakage at this age.  That said, I have 
> yet to see one with ohmic leakage.
>
>> In other words, a brand new R-390 class filter will trip a GFCI even if
>> it shows infinite DC leakage resistance.
>
> Correct, if the GFCI is operating properly.
>
>> In other, other words, a filter that does not trip a GFCI has to be less
>> effective than the original because it has less capacitance to ground.
>> And it is that capacitive reactance at RF that makes the filter work,
>> with the aid of some series inductance.
>
> It's more complicated than that.  Modern line filters use "common-mode 
> chokes" -- two equal windings on the same core, each placed in series 
> with one leg of the supply, in phase opposition. So, differential mode 
> current flows unimpeded, but common-mode current is opposed by the 
> choke's magnetic field.  Theoretically, this eliminates the need for 
> capacitors to chassis (or at least makes smaller caps do a bigger job 
> than they would otherwise do). Indeed, that is precisely how "medical 
> grade" line filters operate -- they rely solely on the common-mode 
> chokes to suppress common-mode currents, without the benefit of any 
> capacitors to chassis.
>
> Using two common-mode chokes in series makes the medical-grade filters 
> somewhat effective, but the best hospital-grade filters do not have 
> nearly as much common-mode attenuation as comparable filters that have 
> two chokes and DO have capacitors to chassis. Why is this?  
> Common-mode chokes work because any common-mode voltage creates equal 
> currents of opposite phase, which oppose and cancel each other.  
> Therefore, they are very sensitive to any imbalance in the magnitude 
> or phase of the currents generated in the common-mode choke.  As 
> anyone who has tried to make a wideband RF transformer or balun can 
> attest, getting good magnitude and phase matching over a wide 
> frequency range is difficult.  The capacitors to chassis make up for 
> the limited performance of common-mode chokes by themselves.
>
> Consequently, "medical-grade" line filters are substantially inferior 
> to standard line filters in all respects except leakage current, which 
> you can easily see if you look at the attenuation vs. frequency data.  
> For example, compare the attenuation of a 3 amp Corcom medical-grade 
> filter (3EHQ) to the Corcom 3VAQ (or even the 3EP or 3EMC).
>
> Since standard filters have leakage currents well below 1mA, there is 
> absolutely no need for medical grade line filters in the radio room 
> and they are clearly not recommended.  Luckily, line filters do not do 
> anything useful for the way we operate radios, so they are really 
> nothing but decoration in any case and there is no penalty for using 
> one that doesn't work very well.  Indeed, as I suggested the other 
> day, you can remove it entirely and you will never notice that it is 
> gone (except that it won't trip a GFCI).
>
> Best regards,
>
> Charles
>
>
>
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