EVS 2KVA AC mains voltage regulator with thyristor ( inside view ) by STB Greece

[Kiriakos GR]

This is a 2 KVA AC mains voltage regulator with thyristor, and is a custom build, designed to serve as protection of sensitive medical equipment.

STB brand is probably the naming of a small Greek industry or the logo of a maker which has skills in assembling such devices.

This AC mains voltage regulator belongs now to me and I received it as gift, it job is now handled by several  Battery Backups/UPS Units.

[giorgos]

A complex construction, that remids past years when Greek industry was more active than nowdays...

[Kiriakos GR]

When I got this item this came with zero documentation, but today I did test it by adding as input my new Variac Transformer.

Now I am aware that this stabilizer it does precision work at delivering an output of 230V with input voltage of 190V up to 250V. 
Internal circuit this acts as constant active inverter and it does an annoying buzzing sound all times.

This stabilizer it does a good job at stabilizing, it is clean of harmonics noise, but it does that by it self consuming significant power even with out load at all.

Worst consumption condition this is at 190V input and consumption get lower at higher volts.
When input this is 230V and output 230V at that moment there is a minimum of 575 VA of consumption with out load but with Power Factor 0.04 actual Watts are approximately 28W Power meter TS-836A
 
Here are values of recorded consumption according to input voltage: (Variac Panel meters)
6A  ---->  190V AC Input
2.8A  ----> 225V
2.5A  ----> 230V
2.1A  ----> 235V 
1.6A  ----> 240
1.2A  ----> 245
0.5A  ----> 250

In the positive side of things and what makes me extremely happy, this is that specific stabilizer it handles very well load conditions.
Voltage stability at 230V this is superb. 
295W lamps load: 230V, 1.27A, PF 1.0 (at the side of the output). 
Total load at the Mains input this is 326W (prior the stabilizer, and at Variac output).
What is interesting this is Power Factor between Variac and stabilizer (between two transformers), this measured 0.43 PF
 
IEC standards require the nominal voltage (230v) +/- 1 % and frequency (50Hz) +/- 1% with a Crest Factor between 1.34 - 1.49 and a Total Harmonic Content (first 13 harmonics) under 2%.
Therefore this stabilizer it seems excellent for testing of power meters and of power quality analyzers,  especial when voltage stability after several confirmation measurements this is at 229.5V ~ 230.5 V   

Programmable ACV sources today comes for a price of 2500 ~ 3000 or more Euros.
My stabilizer this is fixed at 230V but either way, it is fixed exactly were it must be so this equipment to have great value for me and ITTSB Blog.   

[Kiriakos GR]

Update -- Five years later

I did take apart this voltage regulator, mostly to inspect electrolytic capacitors health and now I know much more about this treasure.

The most popular name for this technology is: AC Power Control Using SCR (silicon-controlled rectifier)

Thyristor No1, this is in control of the positive wave of sinusoidal.
Thyristor No2, this is in control of the negative wave of sinusoidal.
AC voltage output this is adjustable by potentiometer.

Voltage active stabilization this deliver almost five digits of stability  =  220.00 V AC  +/- 10 counts
Actual measurement example of output:
220.08 V
219.98 V

By adjusting the output at 220V ( with 235V as Input from Mains / Grid), overall consumption of this power source came down to 500mA.
Power consumption this is always correlated with switching noise caused due the two thyristor.
By adjusting it at 220V output, the switching noise was reduced down to minimum and its now much less annoying. 
Voltage stabilization this in now effective down to 180V AC of input voltage.       

Details of Parts list:
All transistors at the control circuit are from Motorola.
Thyristor : Two 10RIA120  (Made in Italy)
Motor start capacitors (two of them): (Made in Italy)
All capacitors at the control circuit are made by Philips. 

Large size transformer = output filter
Medium size transformer (three) these carry different AC voltages at several frequencies ( 50Hz up to 150Hz as highest)

I did take several pictures of the PCB, that I will keep fro personal use.
The specific product design and circuit this is nowhere to be found over the internet, and I do not plan helping others about giving them a chance for reverse engineering.
     
In summary:
AC Power Control Using SCR = full AC wave control circuit = phase control method due two anti-parallel SCR's.
The number of degrees from the beginning of the cycle when SCR is switched on, this is firing angle.
And or ....
Firing angle = control of SCR/Thyristor due applied gate pulse for adjustment of phase degrees.
The earlier in the cycle the SCR is gated ON, the greater will be the voltage applied to the load.

Basic electrical specifications
The switching technique used in SCR supplies results in an efficiency of 50~60% at no-load up to 80~90% at full load.
SCR (thyristor) this is limited to operation at the line frequency (50 ~ 60 Hz).
SCR regulators cost per Watt was high in comparison to modern MOSFET ... MOSFET operation this is not limited at the line frequency.
SCR Output voltage control range: 0 to 97% of input voltage
Output voltage characteristics: Linear output by voltage feedback
Degree of output stability: Output fluctuation ±2% maximum as against input fluctuation ±10%   

Therefore this is a Power source of highly stabilized power output that are ideal for the product development and verification even of AC to DC switching power supply.
Its also ideal source for electrical power efficiency measurements, when AC power meter this get connected between the source and the PSU / DUT (Device under test).

Modern AC Power Sources (of our times) they are programmable, they can vary voltage output along frequency, and they also include a fancy measuring panel meter.
Price per Watt, this is also very high today, because nowadays the software and features (large combination of select-able testing conditions) these are of what you are paying for.
   

[Andrew-88]

Voltage active stabilization this deliver almost five digits of stability  =  220.00 V AC  +/- 10 counts
Actual measurement example of output:
220.08 V
219.98 V

So this old monster its a great performer after all. 
Modern made AC power sources, cost more than 6.000 EUR.

I bet that you will use it at Seasonic PSU product's review. 
Such move will elevates product review quality by huge margin, young PSU reviewers does not have access at such tools.