# Dropout Voltage for Linear Regulators

I am new to this electronics/PCB stuff and was wondering about Dropout Voltages for a Linear Regulator. If I supply a 12V Output Linear Voltage Regulator with 9V, what will happen (firstly if the regulator's dropout voltage is under 9V and secondly if it is not).

Thanks for the help in advance!

• Nothing will happen. You must supply it with at least 14V to have an output of 12V. The input voltage can't be lower than the output, Dec 1, 2020 at 21:52
• @Fredled Not true: some designs will simply put out something like 8-9V in that condition, unregulated. Regulation improves above 12V until you hit the ~14V range, where the regulator is in full control of the output voltage. Dec 1, 2020 at 21:54
• Yes, but that 8-9V output will still be less than the input voltage. Dec 1, 2020 at 22:21
• @Warren Young The "some design" is not specified in the question. As The Photon wrote below, the output can be anything, including zero volt. I think that recent L78xx models output zero volts or close to it when the input is too low. But I must check. Dec 2, 2020 at 18:27
• The AS78LXX datasheets show output zero or near zero volt until +-2.5V input. Above that, the output is linearly 2V below input voltage. Other datasheets not clear, but likely the same. Dec 2, 2020 at 18:45

The dropout voltage is the minimum difference in voltage between the input and output voltages of the regulator required for proper operation.

A linear regulator can only reduce the voltage, so if you supply 9 volts to a 12 volt regulator it will output somewhat less than 9 volts. For the 78xx family of linear regulators, the dropout voltage is 2 - 2.5 volts, so, for your example, I'd expect the regulator to output under 7 volts.

To get 12 volts out of a 7812 regulator, you would have to supply at least 14 volts.

• Thanks - that is much appreciated! Dec 1, 2020 at 21:59

If I supply a 12V Output Linear Voltage Regulator with 9V, what will happen

It depends on the design. Most typically you will get an output voltage somewhat below the input voltage, so perhaps 7 or 8 V. In a "fancier" design the output might be totally suppressed by a brown-out detection circuit. Or, really, anything can happen because operation in these conditions is not specified in the datasheet.

firstly if the regulator's dropout voltage is under 9V and secondly if it is not

The drop-out voltage is normally specified as a difference between the input voltage and the output voltage. So if the drop-out voltage is 9 V, you will need to provide at least 21 V at the input in order to get 12 V at the output.

But 9 V is a very high value for drop-out voltage. 3 V or so (requiring ~15 V at the input for 12 V output) was common in old designs, and 0.5 - 1 V (requiring 12.5 or 13 V at the input for 12 V output) is common in new, "low drop-out", designs. Even lower drop-out voltages may be available, but watch out that the quality of regulation may drop as your input-output difference voltage nears the drop-out limit.

If you have a 12V linear regulator with a 2V dropout (i.e., a 7812), then if you supply it with 14 or more volts, it'll output 12V. If you supply it with 14 or less volts, it'll output (supply V) - 2V.

If you have a 12V linear regulator with a 0.2V dropout (I'm not going to look up a part number -- there's too many), then if you supply it with 12.2 or more volts, it'll output 12V. If you supply it with 12.2V or less, it'll output (supply V) - 0.2V.

Read the datasheet -- any voltage regulator you want to buy will tell you the dropout voltage. Design your circuit to work with the maximum rated dropout voltage, not the typical.