# Using more than one voltage regulator in series

Is there any problem (in addition to a likely loss of current) in using more than one voltage regulator (L7812CV) in series? And is there a real gain in stability?

I ask this because I 'm going to use some high precision components in a car, using it's battery.

Edit.: My bad, I'm new to electrical engineering. I was thinking that voltage regulators are more like a stabilizer. For ex: inputing either 14v or 9v, it will force 12v in output by varying its current. So i guess that this question doesn't make sense.

There's any other way then to stabilize the voltage of a car battery to guarantee 12v?

• A voltage regulator needs some voltage to "work with", an L7812 regulator needs at least 15 V. It's output voltage will be 12 V, that is no use to a second L7812 as it needs at least 15 V. What IS possible is an L7812 followed by a L7805 but then you would get 5 V at the output. – Bimpelrekkie Nov 17 '15 at 16:12
• Show how you intend to connect them. Your "in series" might be different than mine (or someone else's..) – Eugene Sh. Nov 17 '15 at 16:12
• Two 7812's in series is 24 volts, so if your car's battery is 12 volts, how do you propose to make the scheme work? – EM Fields Nov 17 '15 at 16:15
• @FakeMoustache Please see edits – Guilherme Nov 17 '15 at 16:21
• @EugeneSh. Please see edits – Guilherme Nov 17 '15 at 16:22

To add to the other answers, here is some general information about getting a low noise power supply from a car:

• The voltage from your car will normally be between 12V (engine off) and 14.5V (engine on) but may have drop-outs, glitches and spikes up to about 30V. So you are correct to think about smoothing it for precision use.
• The simplest starting point is a DC-DC converter, such as one described in the other answers. On its own, it'll give you the set voltage with probably 100mV or ripple, depending on how much you paid for it.
• If you need a cleaner power supply, you can pay for a better DC-DC converter, or you can add a filtering stage. A low-ESR ceramic capacitor connected across the output is a good start, and a LC pi filter will do better, but be careful to select low resistance parts to make the filter.
• If that's still not good enough, you can set your DC-DC converter to 14V and use a linear regulator to drop the voltage to 12V. That will usually produce much lower noise on the supply, but make sure the linear regulator has a good PSRR at the switching frequency of the regulator.
• For absolute best results, use a DC-DC converter, then an LC filter, then a linear regulator, and finally a large bypass capacitor near each IC you are powering. Put the DC-DC converter in a metal box, the regulator in another box and your actual circuit in a third box, so they are all shielded from each other, and use shielded cable to connect the 2nd and 3rd box.

There are very few applications where you'll need to go beyond a decent DC-DC converter and maybe a capacitor. Taking all of the steps above would only be necessary if you were doing something like powering an ultra-low-noise instrumentation amplifier to make sub-nV measurements.

The L78* series of parts are linear voltage regulators. What that means is they essentially work like this:

simulate this circuit – Schematic created using CircuitLab

The potentiometer R1 can be adjusted to achieve some target output voltage. A voltage regulator isn't exactly an automated means of adjusting a potentiometer, but by considering it as such you can have a good intuitive understanding of the basic physical limitations of any linear voltage regulator:

• the output voltage can not be higher than the input voltage, and
• the drop in voltage ($V_\text{in} - V_\text{out}$) is achieved by converting some of the input electrical power into heat. A higher voltage drop or more current though the regulator will mean a hotter regulator.

By "in series", I assume you mean to connect the input of one voltage regulator into the output of another.

I don't think this will have the advantages you hope to get. Since a linear voltage regulator's output must be a lower voltage than its input, you can't take a 9V input and make it 12V, no matter how many regulators you have.

I also suspect just one voltage regulator, properly used, will do a quite sufficient job of giving you a steady output voltage. I suggest you read the datasheet if you need to know "how good" the voltage regulation is, but unless you are making some precision lab equipment, you can probably bet it's good enough.

If you must be able to boost the input voltage, then you need some kind of nonlinear voltage regulator, like a SEPIC.

Furthermore, since this is for an automotive application, you should be aware that a car's electrical system is pretty hostile. You'll want to consider conditions like load dumps and include appropriate protection. I'm hardly an automotive engineer so I couldn't even begin to give you advice, but you might read some application reports for ideas.

So you want something which will take an input of 9 - 14 V and will output 12 V regardless. Luckily such modules exist. Here's an example: Boost Buck DC adjustable step up down Converter XL6009 Module Voltage

There's a trimmer on it to adjust the output voltage to your desired 12 V. Optionally you could also set it to 15 V and add the L7812 regulator to have some extra stable voltage.

You can use a SEPIC DC DC Converter to guarantee your DC output output even if car battery goes above or below your operating voltage. Give yourself a little margin on the top end and then you can use a low-noise high PSRR ratio linear regulator to give you a nice clean 12V.

Linear Tech DC-DC SEPIC calculator

TI LDO article

• For people with experience this is a solution. But for less experienced people (like OP) and lazy ones like myself I'd just buy a ready-to-use module from ebay. – Bimpelrekkie Nov 17 '15 at 20:33

Me too learning this stuff. here is simple solution.

Put 5V(6v would be nice) battery in series with 9v DC input.

• That's not really a solution unless the power consumed is very low. One battery costs the same as the module I suggested. Also it does stabilize the voltage at all. Then you'd need a voltage regulator which burns more power. – Bimpelrekkie Nov 17 '15 at 20:32