[ARC5] Collins xfmr ratings

[Dennis Monticelli]

Geoff,

If only swinging chokes were more plentiful....sigh...

I see you concern regarding potential warm-up issues, however no problem
with the VR tubes being slow to come on.  They ionize right away.  I have
solid state rectifiers in my PS and there is no soaring due to a tardy
bleed path.  It works.

Dennis AE6C


On Fri, Aug 15, 2014 at 11:00 AM, Geoff <geoffrey at jeremy.mv.com> wrote:

> Even for some constant loads a critical inductance and load at the PS
> bleeder is needed.
> Example is a SS rectifier that is instant on and the load takes awhile
> until the filaments are up to the job.
>
> Even with AM the Class C stage is cutoff in some designs until the PTT is
> activated and if drive goes away in other cases the HV soars.
>
> Swinging chokes are ideal for maintaining CI under all conditions.
>
> Carl
>
>
>
>
> ----- Original Message ----- From: "Kenneth G. Gordon" <
> kgordon2006 at frontier.com>
> To: "ARC-5 Mail List" <arc5 at mailman.qth.net>
> Sent: Friday, August 15, 2014 12:43 PM
>
> Subject: Re: [ARC5] Collins xfmr ratings
>
>
>  On 15 Aug 2014 at 10:23, D C _Mac_ Macdonald wrote:
>>
>>  I'm just WAGing that a bleeder load of 10% or maybe even 5%
>>> of the total current would do that, Ken.
>>>
>>
>> It might, Mac. Yes.
>>
>> But since I have the opportunity, let me try to explain for the rest of
>> the crew,
>> if any of us don't know about it.
>>
>> "Critical Inductance" only really becomes an important issue when the load
>> on an AC power supply is varying a lot....like in a CW transmitter.
>>
>> Dynamotor and regulated power supplies never have this sort of problem.
>>
>> For AC power supplies in which the load is fairly constant, it is not a
>> concern,
>> since the constant load makes this issue moot.
>>
>> The "critical inductance", defined as that inductance needed to prevent
>> output voltage from soaring, needed for any power supply is approximately
>> the "load resistance", divided by 1000.
>>
>> For instance, for an ARC-5 level transmitter, if the DC voltage on the
>> rig is
>> 500 VDC, and we are loading it to 200 mA at key-down, the load resistance
>> is then 500/.2=2500 ohms. Therefore, in this case, critical inductance
>> would
>> be 2.5 H.
>>
>> But, at key up, the load might drop to 20 mA. Then the critical inductance
>> would need to be 10 times as much, or 25 H. For this case, load resistance
>> has risen to 25000 ohms due to the much lower current being drawn at key
>> up.
>>
>> Since we have only installed, say, a 4 H choke, the output voltage would
>> rise
>> WAY up, in this case to almost 800 VDC, and in addition to the dynamic
>> voltage regulation being terrible, contributing to chirp and other
>> distressing
>> noises, the voltage might be high enough to severely stress some
>> components in the transmitter, like blocking capacitors, etc.
>>
>> So, we add a bleeder resistor to the power supply which will draw enough
>> current at key UP to keep the critical inductance required to no more than
>> our installed choke.
>>
>> In this case, we would want the key-UP load resistance to be 4000 ohms or
>> less. So, we would need a 4800 ohm resistor at 100 watts as a bleeder.
>> This
>> bleeder would draw 52 watts at key up from our power supply and would
>> keep the voltage from soaring.
>>
>> Of course, you can increase the needed resistance value of the bleeder
>> resistor, thereby reducing the power dissipated in it, by increasing the
>> size of
>> the choke.
>>
>> Although I have not done the math, I would think that if you used an 8 H
>> choke, you could use an almost 10K bleeder, which would then only have to
>> be 50 watts, and the bleeder would be drawing only 25 watts.
>>
>> Electric Radio Magazine, in their August issue, beginning on page 3, has a
>> reasonable discussion of this, although for our purposes, it isn't quite
>> complete.
>>
>> Also, I hope all my math above is correct. If it isn't, I am sure someone
>> will
>> correct me. :-)
>>
>> Ken W7EKB
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