[ARC5] Transformers in low-voltage to B+ supplies.

[n3lll --]

If it helps, our Chinese friends make nice inverters 12 - 24 in 90 - 400
volts about 90 milliamp current capability.  I have been building them into
a dynamotor package that plugs on the back of an ARC 5 receiver, but I have
also used them stand alone for some audio projects.   Twenty years ago I
used to use the TRIAD TY-88, which used germanium transistors.  I think it
was specifically made for the ARC5 sers 28 in, 250 out.  I bought them from
what was then Cleveland Electronics, they were about $10 back then
https://www.electronicsurplus.com/triad-utrad-ty-88-transformer-inverter-out-250vdc-60ma
<https://www.electronicsurplus.com/triad-utrad-ty-88-transformer-inverter-out-250vdc-60ma>
only one left and the price is now $30.  I have the inverters in stock if
you want to try one contact me off line ... tom, N3LLL


On Wed, Oct 26, 2022 at 11:34 AM Leslie Smith <lnsmith99 at gmail.com> wrote:

> Hello Bob/Jeep:
>
> In the end I believe my only solution is to build.  But discussion before
> and after will assist my understanding.  On the British VAMRS web-site I
> found two or three very functional circuits that replaced a
> (not-now-common) vibrator with transistors or FETs.  So they built a - 12V
> to B+ PSU around a reverse-wired 240 .. 9-0-9V transformer.  That's one
> option.
>
> However, some year ago I saw a hand-book on OpAmp design with a statment
> to the effect that a small modification to some particular circuit gave a
> completely different result.  "Only a dull and unimatinative mind could
> fail" - the book said - "to investigate the possibilities available from a
> few modifications".  Thinking about this - adding a couple of transistors
> to form an "H" bridge inverter seems to be a circuit that my dull an
> unimaginative mind must contemplate.
>
> First, consider the circuit driving the main "H" bridge transistors.
> Switching the transformer primary (i.e. the low voltage winding) with
> reverse-phase square waves suggests a problem at the moment the wave-form
> switches.  A short 'off' interval between each alternate power cycles would
> be 'useful'.  Consider the venerable CD4017 decade counter IC from the CMOS
> logic family.  Feeding this device with a square wave produces a sequence
> of pulses, each 1/10 the duration of the input signal.  If I leave outputs
> 0 and 5 unconnected and OR-wire output 1, 2, 3 & 4 to switch one arm of the
> bridge and OR-wire outputs 6, 7, 8 & 9 to switch the opposite arm then I
> have a convenient 'off' interval between alternate cycles.
>
> Drew up the circuit last night.  I need 4 signal diodes for each 'OR'
> circuit; x8 1N4148 diodes in all.  I read that many vibrators run at about
> 80 .. 90Hz.  Clearly this is a frequency that a common E-I or toroidal
> transformer will 'accept'.
>
> This particular PSU is for a small receiver - specifically the 3-6MHz
> BC-454.  Many inverter circuits are free-running.  Birdies that 'get thru'
> appear at no specific frequency because the switching frequency changes
> with the load.  Crystals are cheap.  What if the PSU oscillator ran at a
> fixed frequency?  A crystal controlled switching frequency would put the
> 'birdies' I can't eliminate at regular intervals (frequencies) on the
> dial.  Annoying, certainly, but not so annoying as a birdie that appears
> anywhere on the dial.  With careful choice of frequency  I may even get a
> free 'spot' frequency generator from the PSU.  Some harmonic of the final
> switching frequency.  Say 50kHz.  Even if I can filter out every harmonic
> from the switching frequency - I get a bonus.  I'm thinking about driving
> the CD4017 with a CD4040 now.
>
> At this point my power supply circuit is still simple and practical.
> That's where the "Q" about highest operating frequency I can use to switch
> the power transformer originated.  At the moment I'm guessing it will
> handle 200Hz - maybe even 400 or 800Hz.  I reckon (I don't know) iron core
> losses will be high at 1kHz.   I know nothing about the iron in transformer
> cores - hence my "Q".   I'm confident some-one on this list will have
> experience in switching a conventional transformer core at some frequency
> above 50/60Hz.
>
> Leslie.
>
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