[ARC5] Vibrator Power Supplies

[Leslie Smith]

Hello Bob,
I can explain this in an empirical sense easily.
Consider a power transformer, with an iron core.  The inductance of the
primary winding may be 10 to 15 henries.
Consider an IF transformer, with a ferrite core.  The inductance of a
winding may be 100 to 300 micro-Henries.
Consider the inductor in the tank circuit of a 6m receiver.  It may have
an inductance of 1uH (or half that).
As the frequency increases the inductance decreases to provide the
"correct" reactance.
We swap the iron core for a ferrite core and finally for an air core.

Now take your 60Hz 10 Henry choke.  The reactance (at 60Hz) will be
about 3770 ohms. 
 If the primary resistance is 200 ohms and there is no load, the
 impedance should be (sqrt(200^2 + (3770)^2) = (about) 3775 ohms.  
Since I = E/Z so I = (about) 30mA.  (Ohms law for AC).

But at 25Hz, X(l) will be 25/60 the value at 60 Hz so 1570 ohms, and Z
will be 1582 ohms.
The "secondary disconnected" current will be I = E/Z, so 69.5mA.
That's a lot more current at the lower frequency!

It's necessary to figure the phase angle to calculate the power
dissipated in the core (as IR losses), and there are also magentic
losses, but I don't know anything about that subject.  As the reactance
of the circuit is MUCH higher than the resistance the current will be
well out of "sync" with the voltage, so the IR loss in the copper
winding will be small, but less at the higher frequency.  Also, magnetic
losses will go "up" as frequency goes "down" - that is, if you believe
frequency has the attribute of "direction".  

73 from Australia, where everything is up-side down.


  Les
  vk2bcu at operamail.com


On Sat, Dec 8, 2012, at 8:15, Robert Nickels wrote:
> On 12/7/2012 1:51 PM, GDM wrote:
> > converted 6 volt DC car radios to AC, by feeding 6 volts AC from a 
> > filament xformer into the 6 volt primary winding
> I've done that too for old commercial FM radios back when in the early 
> days of VHF-FM.   Maybe they ran hot - but since I can't remember one 
> burning up it must not have been a problem ;-)   Like a lot of stuff you 
> do when the stuff is easy to get and cheap.    I'd like to understand 
> what characteristics of the transformer result in this not working as 
> expected as I wasn't aware that it was a problem.
> 
> One commercial ham rig I am aware of, the Gonset G-77, was a two-part 
> unit with the very small (by 1950s standards) RF unit under the dash and 
> the modulator and vibrator power supply in a separate box (in the trunk, 
> normally).   They sold two power supplies that were identical except for 
> one thing:  one just had the DC input for operation from vehicle 
> batteries, and the other had an additional primary winding so it could 
> run from DC or AC by simply plugging in a cheater-type cord.  I had one 
> of the former before trading around to get one of the latter, and it 
> works great.    But maybe there are some changes to the transformer that 
> I'm not aware of.    Sure makes it nice to have that option.
> 
> 73, Bob W9RAN
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