# Why does the voltage regulator not regulate when ac is around 200V AC?

So the voltage regulator works well when the input of the primary winding transformer is at 220V or greater and the 12V regulator does output around 12V. Why is it that when I turn down the variable transformer to say 203V the voltage regulator does not output around 12V and in fact outputs less?

The transformer I have is VC 16/1/9 https://catalog.block-trafo.de/prodvardatasheet/393257-524506_EN and the voltage regulator is an L78S12CV

The transformer you've linked is rated for 9VAC at 230VAC input so you're already running under spec if you're trying to get 12V. The transformer ratio is around 25:1 so 203V would give you around 8VAC.

The 12V variant of the L78S series of regulators has a minimum output voltage of 11.5V and a dropout voltage at 25°C between 1.5-2.5V depending on current draw which means your minimum input voltage would have to be at least 13.5VDC. This doesn't take into account whatever rectification/smoothing you're doing from the transformer.

• You can use some HTML markup for symbols: &deg; for degrees, &Omega; and &mu; can also be useful. They don't render in the comments. – Transistor Apr 23 at 7:27
• Good to know! Will include. – Rob Apr 23 at 7:29
• Thank for your answer and that makes sense. So how would I be able to still outpout 12V if the input is lower than 220 - 230? Would I need another type of transformer or another way of dropping the voltage such as a buck converter? – M2T156 Apr 23 at 7:37
• Do you know what the load size is? Also what type of rectification are you doing between the transformer and regulator? – Rob Apr 23 at 7:42

If we continue your logic, why does the voltage not give out 12 V when the AC voltage is zero?

The answer is that the 78xx series are voltage dropping regulators and need a certain amount of "headroom" to operate. When you reduce the AC voltage then you also reduce the input voltage to the regulator below that required for correct operation.

• What's more surprising is that how OP is getting 12V at 220 VAC. Transformer datasheet says 230 in and 9 out. 220 in will give 8.6 vac out. Putting a diode and cap will further reduce it. No way that regulator is going to work with this voltage. – Whiskeyjack Apr 23 at 7:11
• The no load voltage is 1.24 times as high. And yhen multiply that by $\sqrt{2}$ and the peaks are about 14.5. Drops a volt on the rectifier and another 1.5 in the 7812. – Oskar Skog Apr 23 at 7:27
• That is why I chose this transformer because then there is little power drop due to no load is around 14. The voltage regulator worked fine but doesn't when the input AC is lower than around 220 - 230V AC. – M2T156 Apr 23 at 7:39