[Boatanchors] OK Smart People: Capacitive Voltage Divider?

[Brian Clarke]

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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