[ARC5] Vibrator Power Supplies

[Kenneth G. Gordon]

On 7 Dec 2012 at 15:15, 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.

As Sandy pointed out, the switching-frequency of a normal vibrator is 115 
Hz.

Our power line frequency is 60 Hz.

The amount and kind of iron in a transformer is determined by (among other 
things) the AC input frequency: the lower the frequency, the more iron is 
required. 

Therefore, a transformer designed for an input frequency of 115 Hz would 
have LESS iron in it than one designed for 60 Hz, and if 60 Hz is applied to a 
transformer designed for 115 Hz, there will be a lot of heat produced.

The amount of iron required in a transformer vs the input frequency is the 
reason that aircraft use 400 Hz power supplies: the iron required is much 
less than 1/2 of that required at 60 Hz, and thus the weight and bulk are 
reduced.

This is also why the Navy tried some systems at 800 Hz. They are real 
screamers because of it too. 

If you find a transformer designed for 25 Hz AC, the transformers will be at 
least 1/3 heavier and bulkier than those designed for 60 Hz., yet they will 
work fine at 60 Hz.

A transformer designed for a lower frequency will generally work fine at a 
higher frequency, but not always.

Others here can tell you exactly why all the above is so. It has to do with the 
"support" the magnetic field needs, and that is dependent on frequency.

> 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.

Yes. It probably was designed to operate from 60 Hz, and since a HIGHER 
frequency will work fine in a transformer designed for a lower one, they could 
easily use the same transformer.

There are also Handbook vibrator-power supplies that were designed the 
same way: to use DC through a vibrator, or 110 VAC 60 Hz. input, 
depending on how the jumpers were set, and they used 60 Hz transformers.

Ken W7EKB