Replacing chokes with resistors in power supplies

Replacing chokes with resistors in power supplies

Post by David Bar » Wed, 28 Jan 1998 04:00:00



Reason I'm asking this is that I have a general-coverage HF receiver[0]
which originally came with an external power supply, which was long
missing[1] by the time I acquired it.  I can get close enough to the top
B+ voltage[2] by using a 120:220 voltage adapter transformer and a silicon
full-wave bridge rectifier.  (Everything in the AES catalog either has
an insufficient current rating, or is criminally expensive.  Even a
low-end adapter transformer can handle 100 VA, which is plenty enough
for my purposes and costs a fraction as much as an official "B+"
transformer.  Getting the "A" and "C" voltages is no problem so I'm
not discussing them here.)

[0] Hammarlund SP-400-X in case you're interested.
[1] Yes, I've put out feelers for an original replacement, but haven't
    had any luck.
[2] 350 volts.  I should be able to get 300 volts out of the
    arrangement above, and when the set is operated from a (large!)
    stack of B batteries, it made do with a B+ of 225v.  So I think it
    should be plenty happy with 300.

I have the schematic for the original supply, which uses a multi-stage
filter and draws out successively lower B voltages behind subsequent
chokes.  I plan on substituting power resistors for these chokes, and
it would be nice to get a rough idea of what values I should initially
try.  I suppose I am going to have to experiment a bit, but it even a
ballpark figure will help give me an idea of what values I should have
handy before I start experimenting.

--

      "Rats and roaches live by competition under the laws of supply and
     demand; it is the privilege of human beings to live under the laws of
                     justice and mercy." -- Wendell Berry

 
 
 

Replacing chokes with resistors in power supplies

Post by Jim Muell » Fri, 30 Jan 1998 04:00:00


Quote:

>Reason I'm asking this is that I have a general-coverage HF receiver[0]
>which originally came with an external power supply, which was long
>missing[1] by the time I acquired it.  I can get close enough to the top
>B+ voltage[2] by using a 120:220 voltage adapter transformer and a silicon
>full-wave bridge rectifier.  (Everything in the AES catalog either has
>an insufficient current rating, or is criminally expensive.  Even a
>low-end adapter transformer can handle 100 VA, which is plenty enough
>for my purposes and costs a fraction as much as an official "B+"
>transformer.  Getting the "A" and "C" voltages is no problem so I'm
>not discussing them here.)

>--

>      "Rats and roaches live by competition under the laws of supply and
>     demand; it is the privilege of human beings to live under the laws of
>                     justice and mercy." -- Wendell Berry

You have a MUCH more serious problem here than what to do about the
chokes.  The problem is the transformer.  Most 120 to 240 Volt
adapters are autotransformers.  That means that there is no isolation
between the input and output.  They do this because they can get about
twice as much power out of the same core and winding than with an
isolated winding design.  That is why they are so much cheaper.  If
you use one of these transformers with a full wave bridge rectifier,
you will place the chassis of your radio at a high voltage (even if
you reverse the plug).  You need to use a proper transformer with
isolation between the primary and secondary.  If you can't find one
surplus or at a ham fest, you might try*** around a s***metal
yard.  Industrial control transformers usually have isolated windings
and you just might find one with appropriate ratings.  Then again, you
might not.

Good luck,

Jim Mueller

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