# Regulating VAC transformer voltage

Good day all,

I was wondering if there are ways to stabalize the AC voltage between 220 - 230VAC (standard mains voltage in Europe) if the mains output voltage drops say for example 200VAC?

I am asking because I have a 12V Voltage Regulator and when the VAC voltage is below 220V the voltage regulator does not output 12V.

• Yes, buy a UPS, but what are the details of your voltage reg? Computer power supplies put out a regulated 12, 19 or 22V with an input of 110 or 220 with no issue. – Solar Mike May 6 '19 at 7:48
• +1 on what @SolarMike said. Otherwise there are motorized variacs with feedback if you perfer heavy and expensive. – winny May 6 '19 at 7:56
• Well I have a AC go DC circuit. The transformer is rated input is 220VAC and then outputs about 15VAC which is then rectified then smooth capacitor then voltage regulator of 12V. Could this mean I need a large output transformer or larger capacitor to how higher voltage when the input AC is below 220VAC? – M2T156 May 6 '19 at 7:59
• Idealy I would prefer to not have an external device – M2T156 May 6 '19 at 8:00
• A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. – Transistor May 6 '19 at 8:21

Well I have a AC go DC circuit. The transformer is rated input is 220VAC and then outputs about 15VAC which is then rectified then smooth capacitor then voltage regulator of 12V.

Since you have a 220Vac-to-15Vac transformer (so n = 14.67), the secondary voltage will be about 13.5Vac if the input voltage drops to 200Vac.

I am asking because I have a 12V Voltage Regulator and when the VAC voltage is below 220V the voltage regulator does not output 12V.

If you rectify the secondary and use a sufficiently large smoothing capacitor then you'll have a DC bus voltage of at least VDC = 13.5 x 1.4 = 18.9VDC which is still enough to regulate to 12VDC.

You didn't provide any info about the load but I think the load draws high current. So the smoothing capacitor cannot filter out the excessive ripple thus the DC bus voltage drops a lot. If you are using a linear regulator (e.g. LM7812) then the input voltage of the regulator should be at least 2V higher than the desired output. So, if the DC bus voltage (i.e. the voltage across the smoothing capacitor) drops to 13VDC or lower (due to the unsufficient smoothing capacitor) then you cannot get 12VDC output.

I'd suggest you to use either paralleled smoothing capacitors or a DC-DC buck converter (better choice) instead of regulating the line voltage.

• Hi there thanks for the feedback. This is a photo of my design for the power – M2T156 May 6 '19 at 8:38
• @M2T156 Ok, but what about the load? What are you driving with this regulator? Or in other words, what is connected to the output of this regulator? – Rohat Kılıç May 6 '19 at 8:50

You can use an autotransformer to raise or lower the AC voltage some margin. Some uninterruptible power supplies (UPS) include this.
However, at some point an UPS will decide it's mains loss and switch to battery.

However, if your product cannot handle 200V, and you want it to, changing mains is not the best way.
Instead, replace the transformer for an AC/DC drop in power supply that is capable of 90-240 Vac 50-60 Hz. These are sold by Traco Power or Vigortronix for example.

470uF is way to small for LM7812 and 1A of load. LM7812 is rated at 1A and calculating for capasitance I got 3000 uF is sufficient for 1A load when mains is 10% lower than nominal (220V*(100% -10%)=198V). 1700 uF is enough if mains is 220V. 470 uF can be used if load is within 270 mA if mains stays at 220V or 150 mA if mains dips down 10%. This web page http://www.skillbank.co.uk/psu/thumb.htm recommends even more: 5000 uF per 1A of load in typical regulated lineal power supply design