Can I connect two voltage regulators in series, to reduce power consumption?

dendad

Joined Feb 20, 2016
4,449
Somehow this strikes me a a great way to blow up a phone. It may work but why not use a switch mode supply as has been suggested?
Car cigarette lighter to 5V USB switch mode supplies are readily available for a couple of dollars.
https://www.ebay.com.au/itm/Cheap-U...hash=item2ef6b9c957:m:mHtYQkXu_DCoq1FtwnjmMPA

Or use a 3Amp buck converter board like...
https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=5V+3A+buck&_sacat=0
3Amp Buck.jpg
Smaller and a lot less heat and probably cheaper as there is no extra heat sinking needed.
 
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AnalogKid

Joined Aug 1, 2013
10,971
Somehow this strikes me a a great way to blow up a phone.
Nah, it's not as dangerous as it sounds. Picture a 5.1 V zener diode with a resistor in series to a 12 V source, and the phone in parallel with the zener. As long as the series resistor is small enough to feed the phone without starving the zener, everything is fine. Replace the resistor with a 7805 and another phone in series, and you're there.

To be clear, there is no way that this will work *well*. But the TS's original idea of somehow reducing the total dissipated power is neither impossible nor dangerous.

Note: This is a concept drawing. It needs decoupling capacitors, transient protection, and a much larger zener diode (or an active zener circuit).

ak
PhoneCharger-Dual-1.gif
 
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Thread Starter

onlyonce18

Joined May 8, 2016
48
Thank you all, learning a lot from your ideas (still have to test the proposed circuits).

I understand that a switching regulator is the best option in this situation, but I was very curious about the possible outcome of connecting the linear regulators "in series".

As some commented it should work (with limitations), and there is the alternative of connecting the phones in series that seems to be a better option.

Would like to make the test for the first circuit (already looking for spare parts), will post what I find although I have no access to an oscilloscope so my measures will be limited.

Thank you all again!
 

dendad

Joined Feb 20, 2016
4,449
If you do try this and blow up your phone, just know that you were advised against it.
A question just for interest sake is one thing, doing it for real, why would you, as the other option is so much better in every way.
 

Tonyr1084

Joined Sep 24, 2015
7,829
12 volt auto power operates at 13.8 volts typical float voltage and rarely drops below 13.6 volts. Usually just after you start the car/truck/whatever, the voltage goes up to around 14.4 volts. That's significantly more power to be wasted somewhere. And that's what linear regulators do - waste excess power.

Energy can not be created or destroyed. It can only be converted from one form to another. In the case of a linear regulator, it's turning the power (i.e. wattage from a 12 volt source (13.8 to 14.4)) into a 5 volt power source. That means the extra power has to be converted into something else. Typically heat. Heat is energy. When driving your car is traveling on kinetic energy. When you step on the brakes the car doesn't just magically slow down; the kinetic energy is converted (via friction) into heat at the brake pads. Energy can be wasted but it can not be destroyed. It must be converted.

Someone mentioned a switch mode regulator - a far greater idea for sure. That would waste less energy because the regulator is not always on. It's on for a certain period of time and off the rest of the time. The time it spends ON and OFF equal 100% of its duty cycle (switching on and off). Because when at 50% duty cycle, a 10 volt source will average out to 5 volts. For the period of time its ON energy is building and being used. For the period of time its OFF - no energy is being wasted. Far less heat generation and a much more efficient way to regulate the power.

Here's a novel thought - go buy a second 12 volt outlet similar to the factory cigarette type outlet. Plug two phone chargers in. OR better yet, buy one of those plugs with two outlets. I have one that has the standard cell phone cable and an additional USB outlet. I can charge two phones from that and I don't have to make ANY modifications to my car.
 

BobaMosfet

Joined Jul 1, 2009
2,110
If you don't want to use switching regulators, for sake of experiment, you can use a different LDO regulator. Just know that regulators need a load, otherwise they may not necessarily behave as you expect. The LM7805 is very forgiving, but that is partly because of the higher drop-out overhead of 2V, and how it was engineered.
 
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