Nearly there with IBM 029 KeyPunch

Roger Holmes roger.holmes at
Thu Jun 11 13:55:20 CDT 2009

> From: Brent Hilpert <hilpert at>
> Roger Holmes wrote:
>> The theory said to change the resonant frequency of my keypunch's
>> ferro-resonant power regulator from 60Hz to 50Hz I should up the 15uF
>> capacitor to 21.6 uF. Well I tried 21.5, and the voltage went up by
>> ONE volt. Hmm, scratch head, at least it went in the right direction.
>> Search around for AC capacitors and find one in a defunct large
>> electric lawnmower. Its 30uF, I think about replacing the 15 with it
>> but if an extra 6.5uF only gained one volt, and I need 6 then it
>> seemed reasonable to combine it and try 45 uF. That gives me 46volts,
>> only 2 volts short. I throw in the 6.5uF too and I get 47 volts. I  
>> try
> I had a niggling concern about this when the suggestion to change  
> the C was
> first profferred.
> I don't have a thorough enough understanding of the ferro-resonant  
> principle
> to categorically say what the problem is but I can half-think of a few
> possible issues:
>  - Because the ferroresonance principle involves the inductor working
>  in the core saturation region, the standard resonance equation with
>  which you calculated the new C may not be applicable. (I see you got
>  the square of f proportion.)
>  - If the changed LC relationship changes the circulating current
>  in the LC circuit then the core magnetic field will also be affected,
>  which would upset the rest of the transformer design targets.
>  - Changing the C changes the resonant frequency in an LC circuit,
>  as you desire to accomplish. However, the Q factor of the resonant
>  circuit is dependant upon the ratio of L/C, so you have also changed
>  the Q factor. I'm less sure if this would matter, as the circuit
>  is operating at a well-fixed frequency.
> My remaining concern might be that even with the new C bringing the  
> V up,
> the regulation function of the supply may have been lost if the  
> transformer
> is no longer functioning in the regions it was designed to.

Yes, but relays are unlikely to be damaged by a few volts too much.

My concern is the current in the intermediate winding could damage  

So far so good but I am charging a much larger capacitance for a  
slightly longer period so I would think the current would be higher.  
To get 48 volts I am going to have to add about another 6 uF, so about  
48 F instead of 15, so very roughly three times the current, assuming  
(and its a big assumption I can't justify), that the voltage is the  

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