[ARC5] Collins xfmr ratings

[Geoff]

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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