[Boatanchors] OK Smart People: Capacitive Voltage Divider?

[Geoff]

Have you actually tried this into an inductive load with varying current?

Carl
KM1H


----- Original Message ----- 
From: "Brian Clarke" <brianclarke01 at optusnet.com.au>
To: "Phil" <ko6bb1 at gmail.com>; "David Stinson" <arc5 at ix.netcom.com>
Cc: <boatanchors at mailman.qth.net>
Sent: Saturday, March 17, 2012 11:39 PM
Subject: Re: [Boatanchors] OK Smart People: Capacitive Voltage Divider?


> Hello Dave,
>
> No need for a Voltage divider. Here are two methods - the mathematical way
> and the suck-it-and-see way.
>
> The complex way is to measure the current that flows into your NC-100A at
> 110 Vac (use a Variac). Then draw a triangle showing 125 V on the
> hypotenuse, 110 V on another side, and calculate the third side (= sq rt
> [125^2 - 100^2] = 59 V. Now calculate the capacitor whose reactance gives 
> a
> 59 V drop at your rated current.
>
> Say the radio draws 0.25 A at 110 Vac; then the required reactance is 59 V 
> /
> 0.25 A = 236 Ohm. Now calculate the capacitance that gives this reactance 
> at
> 60 Hz = 11.2 uF. Now, I have only given an example. You need to plug in 
> your
> value of mains current and recalculate. If you have difficulty with the
> maths, get back to me.
>
> The suck-it-and-see way is to just keep putting capacitors in series with
> the mains input till you get 110 Vac across the NC-100A's mains 
> transformer.
> To be safe, start with a smaller value capacitor - which will have a 
> higher
> reactance - then you won't cook your radio.
>
> I have used both these methods to run American (110 - 125 Vac) equipment
> quite safely off our 240 Vac mains for years. I also use it for European
> stuff rated at 220 Vac.
>
> You must use capacitors rated for 125 Vac at 60 Hz. Electrolytics will 
> give
> off an awful aroma. A good place to look is for motor-start capacitors.
>
> Back to your impossibility of using a bucking transformer - it only needs 
> to
> handle the primary current at 15 V. So, if your set draws 0.25 A at 110 
> Vac,
> your 15 Vac transformer only needs to have a power rating of 15 x 0.25 = 
> ~4
> W. This is tiny and will probably be smaller than a motor-start capacitor.
>
> 73 de Brian, VK2GCE.
>
> ----- Original Message ----- 
> From: "Phil" <ko6bb1 at gmail.com>
> To: "David Stinson" <arc5 at ix.netcom.com>
> Cc: <boatanchors at mailman.qth.net>
> Sent: Sunday, March 18, 2012 1:25 PM
> Subject: Re: [Boatanchors] OK Smart People: Capacitive Voltage Divider?
>
>
>> I'd suggest making an external box with a 12 Volt bucking transformer
>> connected to an electrical outlet on the box.  Then just plug the box
>> into the wall and the radio into the box.  (A 12 Volt drop would
>> probably be sufficient).
>>
>> 73 de Phil,  KO6BB
>> http://ko6bb1.multiply.com/ (OTR Blog)
>> http://www.qsl.net/ko6bb/  (Web Page)
>>
>> On 3/18/2012 2:15 AM, David Stinson wrote:
>>> I'm resurrecting a National NC-100A.
>>> Sounds beautiful.  Nice AM, BCB and SWL rig.
>>> One problem:  our blasted 125 Volt AC power lines.
>>> This rig wants 110 volts and 125 is very tough on it.
>>> There is no room to hog-wire in a bucking transformer,
>>> so that's out.   Niether can I dedicate a variac to running
>>> this receiver.
>>> I understand one can use capacitors as an
>>> AC voltage divider by placing one in series with
>>> a leg of the transformer primary and one across the
>>> transformer primary.
>>>
>>> How can a dummy who can barely do the math
>>> needed to number book pages determine what
>>> values to use to drop this primary voltage from
>>> 125 to 110? Or even if it will work?
>>>   Easy!  He askes the dozens of people
>>> smarter than him on the mailing lists.
>>> Ain't that cool?  ;-)
>>>
>>> TNX ES 73 DE Dave AB5S
>>>
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>
>
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