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AC PSU: 0 - 40V in 1V steps

 
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AC PSU: 0 - 40V in 1V steps

Post by Terrykc » Fri Feb 05, 2016 12:42 pm

In this thread I said:
Terrykc wrote:I first came across this idea very many years ago when someone published a low voltage transformer design which, by altering the order of several secondaries and connecting them in or out of phase, could derive a wide range of output voltages adjustable in one volt steps!
This got me thinking about how easy - or otherwise - it might be. I doubt anybody has a practical use for such a device these days, so I present it as historical curiosity.

Obviously we need to start with a one volt secondary to give ±1V steps. The next one will be three volts which will provide outputs of 2, 3 or 4 volts dependant on whether the 1V winding is connected and in which phase. For five volts, if we add 5 and 4 we get nine so, conversely, we can subtract the first two windings from a 9V winding to get 5V and work the precious sequence backwards to get 1V steps up to 8 then, after 9, start adding them again until we reach 9 + 4 = 13V.

Continuing this process gives a massive jump for the next secondary to 27 volts! When added to the other nine, three and one volt secondaries in phase we reach a grand total of forty volts and can now generate any voltage up to 40 in one volt steps. The table below shows the sequence:

Volts1-1011-2021-3031-40
119 + 3 - 127 - 9 + 327 + 3 + 1
23 - 19 + 327 - 9 + 3 + 127 + 9 - 3 - 1
339 + 3 + 127 - 3 - 127 + 9 - 3
43 + 127 - 9 - 3 - 127 - 327 + 9 - 3 + 1
59 - 3 - 127 - 9 - 327 - 3 + 127 + 9 - 8
69 - 327 - 9 - 3 + 127 - 127 + 9
79 - 3 + 127 - 9 - 12727 + 9 + 1
89 - 127 - 927 + 127 + 9 + 3 - 1
9927 - 9 + 127 + 3 - 127 + 9 + 3
109 + 127 - 9 + 3 - 127 + 327 + 9 + 3 + 1


From memory, the original article suggested taking a battery charger transformer of suitable rating, dismantling it and removing the secondary winding(s), counting the turns at the same time in order to derive a figure for the turns per volt - typically four.

Then wind on the four new secondaries and reassemble.

What eludes me at this point is how the output voltages were selected. This was essentially a hardware project but I don't think a 40-way rotary switch would have been an option! 

The only thing that I can come up with is to use eight DPDT toggle switches and a look-up table like the one above. The configuration is like this for each secondary:  
AC Switch step.png
Switching for one of the four secondaries.
AC Switch step.png (5.43 KiB) Viewed 830 times

One switch determines whether that particular secondary is in use or not and the other selects in/anti phase connection.

If anybody can come up with a simpler solution I would be interested to see it!

 
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Re: AC PSU: 0 - 40V in 1V steps

Post by Ed Dinning » Fri Feb 05, 2016 9:21 pm

Hi Terry, in the depths of time I'm sure RS did a transformer like this the had a max tap of 27v, and at about 2 A as well,


Ed

 
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Re: AC PSU: 0 - 40V in 1V steps

Post by Terrykc » Fri Feb 05, 2016 9:34 pm

This wouldn't be taps, Ed. It has to be four completely separate windings.

 
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Re: AC PSU: 0 - 40V in 1V steps

Post by jimmc101 » Fri Feb 05, 2016 11:35 pm

From 1970 Radiospares catalogue (also in 1973 cat. as L.T. Tr.)

Universal L.T. Trans.

PRI. 10-5-0-200-220-240V

SEC. 1V_9A, 2V_9A, 2V_9A, 5V_9A, 10V_3A, 10V_3A, 10V_4.5A

Provides 1 - 40V in 1V steps Max Load 90W

Start and finish of windings indicated to permit correct series and parallel connections.

=====================
From 1973 RS Catalogue

Univ. Rec. Tr.

PRIM. 0-115-205-225-245

Sec. Separate windings of 1V, 1V, 3V, 3V, 9V, 9V, 27V, 27V all at 1.1A

For rectifier supply typical ratings are 40-0-40V rms 1A capacitive load.

Provides 1 - 80V in 1V steps. Max load 90W

=====================

I have one of the first type and have been meaning to box it up for many years.
I was thinking along the lines of using two rows of 4mm terminals (one for starts and one for ends) and using a set of patch leads with stackable plugs to provide an easy way of making the series / parallel connections.

It seems to me that for the sake of three extra secondaries (7 vs 4) the 1, 2, 2, 5, 10, 10, 10 system makes far better use of the current rating of the windings. (and simplifies switching)

I wonder why it wasn't 1, 2, 2, 5, 10, 20?


Jim

 
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Re: AC PSU: 0 - 40V in 1V steps

Post by Terrykc » Sat Feb 06, 2016 5:29 pm

Interesting ...

The first one uses seven secondaries rather than four but selecting any required voltage is easy without resorting to a look-up table whereas the second one uses the four secondary principle, doubled up to provide any voltage from 1V-0-1V  to 40V-0-40V for a full wave rectifier in addition to a single 0-80V series.

 
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Re: AC PSU: 0 - 40V in 1V steps

Post by Terrykc » Mon Feb 08, 2016 7:13 pm

By sheer coincidence, someone has posted details of this transformer on eBay over at the Golborne forum:

http://www.ebay.co.uk/itm/vintage-PARTR ... 2108a8252d

It has 1, 3, 9 and 27V secondaries so ideal for anybody who wants to recreate the original PSU.

I think its a tad overpriced, though - and its just been re listed after closing with a 'winning' bid which was considerably higher!

Presumably the successful bidder suffered a serious outbreak of common sense ...


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