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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Oct 2018, 2:32 pm   #1
M0FYA Andy
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Default How does a constant voltage transformer work?

As a matter of curiosity, how does a Constant Voltage Transformer work, such that even the heater outputs are stabilised?

Andy
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Old 8th Oct 2018, 2:37 pm   #2
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Default Re: How does a constant voltage transformer work?

They use a saturating core with a tuned winding. They can be had up to several kW capacity. Although very good at stabilising voltage, they are very susceptible to frequency changes so caution is needed. I used to buy from Advance.

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Old 8th Oct 2018, 3:43 pm   #3
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Default Re: How does a constant voltage transformer work?

The one I encountered had a bathtub capacitor in series with the primary.
It saturated and then relied on the capacitor working as a dropper.
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Old 8th Oct 2018, 4:47 pm   #4
M0FYA Andy
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Default Re: How does a constant voltage transformer work?

Many thanks.
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Old 8th Oct 2018, 6:07 pm   #5
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Default Re: How does a constant voltage transformer work?

The Advance ones are resonated and are very picky about mains frequency. Never run them from a generator.

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Old 8th Oct 2018, 7:25 pm   #6
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Default Re: How does a constant voltage transformer work?

I think that the ones I used had three windings - primary, secondary and a parallel tuned one. They were used on a generator supply but a very smart system with standbys and extensive monitoring. The main problem with the generators was the number of seagulls gathering nearby for the warmth. This was in very remote part of the uk but we were constantly surprised by people "passing by" and cadging cups of tea !

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Old 8th Oct 2018, 10:30 pm   #7
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Default Re: How does a constant voltage transformer work?

There's a bit of a technical description on the Claude Lyons website, one of those old companies who have and still are making this sort of thing. It mentions the resonating capacitors and the i/o specs they will acheive.

PS just reading an article about the closure of Claude Lyons in 2016!!!
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Old 9th Oct 2018, 4:02 pm   #8
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Default Re: How does a constant voltage transformer work?

Many moons ago... early 80's..we were tasked with finding a replacement for the "constant voltage transformers" used in the "back woods" in Turkey, as they were vastly in efficient........ well the Turkish ones were....these transformers were used to stabilize the mains voltage for the TV's in use at that time, as the mains voltage varied widely.
My colleague did the maths...... and we had a chip made by Ferranti, which was basically a number of voltage comparators and logic, which switched transformer taps with thyristors. This was hugely successful and the Turkish government were well pleased.
I have no idea how many were made and sold in Turkey. We also designed and made higher power transformers for fridges and other domestic items.
That was basically how NEI.. Network Electronic Industries came into being,
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Old 11th Oct 2018, 11:20 am   #9
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Default Re: How does a constant voltage transformer work?

I'm just starting to power up test a 1966'ish British Physical Laboratories RM175 LZ II Megohmeter that I'm restoring. It uses a two transformer scheme to generate a psuedo constant 240VAC to then supply two transformers (for instrumentation and HV supply).

It looks like a step-up auto-transformer is used to reduce the size of the resonant capacitor, and the sum of the saturating voltage across T1 terminal 3, plus the secondary of T2, is the regulated voltage passed to T3 and T4. T2 has a 4.66:1 turns ratio. T1 has terminal 2 tap at 45%, and terminal 3 at 67%. I haven't checked the inductance levels.

The resonant cap C1 had been removed at some stage - and I'm presently substituting a 4.4uF, so it will be interesting to see what the regulation is like over mains voltage and load variations. I'll see if I can find a nice reference that describes the operation, and take some operating measurements soon.
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Old 11th Oct 2018, 12:13 pm   #10
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Default Re: How does a constant voltage transformer work?

A description from Lee, 1955 (Electronic Transformers and Circuits) attached.
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Old 11th Oct 2018, 4:01 pm   #11
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Default Re: How does a constant voltage transformer work?

The ones I remember were fitted to some 1970s ICL computer gear: they used a 'third winding' on the transformer, resonated by a big non-polarised capacitor in a can. These ones had a tapped third-winding to allow 50 or 60Hz operation; output was 12 to 14V at about 30A.

They made one hell of a thump when switched-on!
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