What are the disadvantages of using a 7805 (with a heat sink) to use in a car USB phone charger?

Another option: It gives a 1 A current, so could I maybe wire two of those and give a 2 A output?

In past 10 years have tried more than a dozen chargers and they all break and stop working in a month. Have already spent lot of money and now tired.

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    \$\begingroup\$ Perhaps a more reputable make of adapter would be worth trying? E.g., Anker, Ugreen, any adapter sold by a phone manufacturer. Especially if it is rated for 12–24 V and not only 12 V. \$\endgroup\$ May 1 at 13:42
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    \$\begingroup\$ thevikas - Hi, Now that you have received answers, the question should not be changed or extended - otherwise it invalidates the existing answers, or makes them look incomplete. Therefore I have removed the extension to your question, where you are effectively asking how to use a 7805. It might seem like you aren't adding much, but since none of the answers included details on your newly-added topic, your addition made them all look incomplete, and that's unreasonable. See discussions about "chameleon" questions on the main Meta for more on this. Thanks. \$\endgroup\$
    – SamGibson
    May 1 at 14:15
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    \$\begingroup\$ @thevikas - Hi, "I just appended the text I had added in a comment just for context." Thanks for explaining what you were trying to do - adding context & clarification is ok, but adding new questions after answers have already been given is not ok. || This is one way that Stack Exchange (with its Q&A format) differs from typical forums, where topics tend to be more fluid and potentially changing / evolving. On Stack Exchange we try not to have questions evolve, especially after answers have been written. || You can use the site search for previous Q&A about using 78xx regulators. Thanks. \$\endgroup\$
    – SamGibson
    May 1 at 15:27
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    \$\begingroup\$ In the past 20 years I've never had a charger that broke. Form my venerable old Nokia brick phones to iPhones. Apart from car cigarette lighter chargers I only use the original chargers that came with my phones. And my car chargers are all Chinese brands (though I don't buy the cheapest I also avoid the expensive ones) \$\endgroup\$
    – slebetman
    May 3 at 2:48
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    \$\begingroup\$ I installed a homemade iPhone 30-pin dock into the cigarette bin of my mum's old Volvo 940 back in 2007. I used a 7805. The mount was made of wood. I realised it was a bad idea when I saw smoke drifting out of the ash tray! \$\endgroup\$
    – joerick
    May 3 at 13:07

8 Answers 8


Missing component - a heatsink

Whilst you can make a power supply out of a 7805, you need a small difference between the input and output voltages and a small current draw.

Automotive electronics needs to work with around 14V - that's the voltage from the alternator. So you have a 9V drop on the 7805, and 2A. That's 18W. Suppose (round numbers) you could tolerate a 56 degree rise in temperature, that'd need a 3 degC/W heatsink. Looking online, you're going to need a heatsink that's around 10cm x 10cm.

This is going to be very inconvenient to put in your lighter socket! It's also going to cost you around £5 for the heatsink, before you even start thinking about the cost of anything else like an enclosure.

More missing components - protection

Automotive electronics needs to be protected against spikes of high input voltage, reverse input voltages, reverse current draw, output short circuit to 0V, output short circuit to 12V, overtemperature, and those are just the ones I can remember. Linear regulators exist which have all this protection built in for not much more money, but the basic 7805 doesn't. It is a very bad choice of component in a harsh environment like a car, and without all those protection features you could easily kill your phone or the 7805 if the 12V is less than perfect - and in a car it often isn't.

But even with an automotive grade voltage regulator, you still have the problem of needing a huge honking heatsink.


Because it gets too hot. A linear regulator like the 7805 reduces voltage by turning that excess voltage into heat.

So if you feed it 12V from the car to produce 5V@1A, it is turning 7V@1A worth of power into heat. That's 7W which is a lot. Even if you had a huge heatsink that rises 5C per watt, that would still be a 35C rise above ambient. At ambient temperature of 25C, that much temperature rise is starting to burn skin.

With a more reasonable heatsink it would be easily be a rise of more than 100C rise above ambient.

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    \$\begingroup\$ It is probably cheaper to make a 2A switching supply, especially considering the cost of a large heatsink. Also, some chargers and phones may be configured to negotiate the current, which will also need "smart" components and firmware. \$\endgroup\$
    – PStechPaul
    May 1 at 3:32
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    \$\begingroup\$ Not only because it gets too hot. It also requires a large heatsink which both occupies a lot more space in a car, costs more AND wastes a lot of energy. \$\endgroup\$ May 1 at 8:07
  • \$\begingroup\$ Thanks for the info. In past 10 years have tried more than a dozen chargers and they all break and stop working in a month. Have already spent lot of money and now tired. It seems 7805 besides its heating, I guess it is pretty stable and wont spoil the phone and also does not need anything in the circuit. \$\endgroup\$
    – thevikas
    May 1 at 13:32
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    \$\begingroup\$ It gets even worse if you assume that the alternator is running, and outputting anything up to 14.4V. \$\endgroup\$
    – Simon B
    May 1 at 15:45
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    \$\begingroup\$ Not to mention the inefficiency... \$\endgroup\$ May 2 at 9:44

A 7805 is a linear regulator. It is inefficient and can not provide 1A output if you want 12V in and 5V out. For getting that 5W out from a 7805, there will be 12W going in, so 7W wasted as heat by the 7805. The 7805 can't handle that. Also you can't just parallel two linear regulators by connecting them together, so paralleling needs some additional circuitry to be designed, and since one 7805 can't provide 1A, then two of them can't provide 2A.

So for the money to spent on designing how to make it work with a 7805 taking account the power dissipation and cooling, it is cheaper to just design a 3A switching mode power supply that can provide 5V at e.g. 3A and be 95% efficient, so for 15W output, only 15.8W input is required, and only 0.8W is wasted as heat.

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    \$\begingroup\$ 12V auto voltage is more like 14.5V when the car in running. \$\endgroup\$
    – user338146
    May 2 at 0:41
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    \$\begingroup\$ @ronsimpson You are correct, and yet the system is still called a 12V system, and not a 14.5V system. Like many other systems, some sort of nominal voltage is used. Using 12V instead of 14.5V won't change the reason why linear regulators can't be used. \$\endgroup\$
    – Justme
    May 2 at 6:57
  • \$\begingroup\$ @Justme - it looks to me, like the comment is just reinforcing your answer, by saying it will be even worse, in reality. \$\endgroup\$
    – SiHa
    May 3 at 18:51
  • \$\begingroup\$ @SiHa Sure, but it should not matter. It's already way beyond limits at 12V. Even if the input voltage were only 10V, and ambient temperature is 0°C (freezing point of water), the part would overheat approximately to the limit of recommended operating junction temperature of 125°C with 1A current. \$\endgroup\$
    – Justme
    May 3 at 19:42

A cheap-as-possible phone charger will use something more like an MC34063 which does not require a heat sink and is very inexpensive in quantity ($0.05 maybe).

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    \$\begingroup\$ The 7805 needs 3 components. The IC, and 2 caps. The MC34063 needs 9 components. The IC, a diode, a coil, a ceramic cap, 3 resistors, 2 electrolytic caps. The MC34063 solution is still quite "small" and compact as circuits go. \$\endgroup\$ May 1 at 13:45
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    \$\begingroup\$ @thevikas It takes a number of components to protect such chips from the often-unfriendly automobile power bus, and they often skimp on such protection. The 7805 can handle 35V, the 34063 40V, so the latter is actually easier to protect. \$\endgroup\$ May 1 at 13:47
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    \$\begingroup\$ Also note that “can I make a phone charger with an LM7805?” Is a quite different question from the one you asked. \$\endgroup\$ May 1 at 14:10
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    \$\begingroup\$ @SpehroPefhany - Hi, FYI I'm just about to rollback the new part added to the question... \$\endgroup\$
    – SamGibson
    May 1 at 14:11
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    \$\begingroup\$ @SpehroPefhany's comment is probably the answer to the OP's underlying question about short-lived chargers (I had one that lasted 10 years of infrequent use before a similar failure). \$\endgroup\$
    – Chris H
    May 2 at 10:18

As others have noted, there are several reasons why a linear regulator is not a good idea for a car charger.

It boils down to efficiency, which is horrible for linear regulators in general, especially when there's a large input-output voltage difference.

Add to this the cost to provide adequate heat-sinking when the current is above 100-200mA or so. Good heat sinks do cost quite a lot compared to other components.

Moreover, your linear regulator choice is also quite debatable: the 7805 chip is an old dinosaur. A much sturdier beast (almost as old, but much more rugged in my experience) is the LM317, as far as linear regulator goes. But still it can't handle more than 1A without external circuitry.

All in all switching power supplies are the right choice, and they can be quite rugged, if designed correctly, and still remain affordable.

Until now I more or less said what others have already said. However, what I want to point out specifically is the following statement of yours and some possibly related issues nobody has mentioned yet:

In past 10 years have tried more than a dozen chargers and they all break and stop working in a month.

Yes, indeed cars are a harsh environment for electronics connected to the car electrical system, but more than a charger a year is quite suspicious, especially if they lasted just a month or so.

If you purchased low-quality, el-cheapo brands this could be barely justifiable (I have bought some of them in the past, and none failed, although I admit I never used them for very long periods). However, if you purchased quality ones I would start questioning your specific situation regarding "electrical cleanliness".

Maybe the electrical system of your car puts out particularly nasty spikes from the 12DC outlet, and this could cause the damage. If the outlet wiring was routed too near the high voltage circuits that power the spark-plugs, this could happen. You could search the Internet for your car model and see if this is a recurring problem.

Another issue could be ESD (Electro-Static Discharge) problems. Especially if you live in a very dry climate, the synthetic upholstery of the car, together with your clothes may act as a very high voltage generator (thousands of volts) when rubbing together. In turn this could cause yourself to being charged to a substantial electric potential.

Touching the charger while you are so charged-up could potentially damage the internal circuitry, especially if the case of the charger is light plastic and the circuit has little ESD protection. If you ordinarily experience your fingers being "zapped" when you enter or exit your car, that could be a hint of an ESD problem.

Of course the El-chepo charger brands are probably more susceptible to ESD damage, so the two issues may compound.


I know that this is possible, because I did use 7805 as a phone charger. It took breaking 2 devices (one for the cable and the other for the 7805 regulator itself), but it was kind of emergency.

The chip failed to effectively dissipate the heat when mounted on a painted steel sheet (this is what we had available) and its thermal protection triggered before it had some meaningful effect on the phone battery. So I ended up immersing it in a glass of water. The positive leads (in&out) corroded a bit for the few hours of use, but everything worked fine.

Using two 7805 to get 2A output is not going to work - except maybe by using equalizing resistors of ~0.1 ohm at their output terminals and living with some voltage loss. 7805 are not created equal so one of them will always have few mV higher voltage and will bear the whole load. When overloaded, most 7805 don't simply limit the output current to 1A, but almost completely shuts down itself, leaving less than 100 mA output.

This generally doesn't matter because when you miss the circuity that advertises the charger capabilities to the load, phones and other USB-charged devices generally limit themselves to either 0.5A or 0.9A .

Why factory-made chargers get burnt in you car?

I cannot replicate your experience. Even the cheapest, grocery-store sourced chargers work flawlessly in my cars.

I can propose few reasons:

  • You live in an extremely hot climate and regularly leave the charger plugged in the car in a direct sunlight.
  • Your car battery terminals are lose. This can create 80-100V surges in the car's electric circuits.
  • Your car is actually a 24V truck or bus (smaller 24V vehicles do exist as well, but almost all of them are of military descent), probably with somewhat off-the-spec voltage regulator.

Most car chargers are happy to run off either 12V or 24V power.

On the other hand, some of them are 12V-only and fail (not necessarily right away) when the voltage goes 20V or higher.

Even a 24V-capable charger may find itself at its limits when the engine is running, the weather is cold (or the voltage regulator is wonky) and the socket actually gives off 30-33V. 33V (or 16.5V in a 12V system) is bad, but still not fatal for the car batteries so one may not be immediately aware of some problem there.

  • \$\begingroup\$ Putting two in parallel isn't going to work, but you can boost the output current of a regulator by using an external transistor. \$\endgroup\$
    – Hearth
    May 4 at 1:46
  • \$\begingroup\$ @Hearth indeed it is possible, but with the modern switching-mode element base it gets above the complexity of an equivalent buck converter. With the added transistor you lose the various protections of the 7805 IC (overcurrent and overheat come to mind) \$\endgroup\$
    – fraxinus
    May 4 at 8:23
  • \$\begingroup\$ Right. I just wanted to add to your answer, since you implied you can't increase the output current while maintaining regulation in your second section. \$\endgroup\$
    – Hearth
    May 4 at 15:46

In past 10 years have tried more than a dozen chargers

Calculate how much you've spent on them, in total. Then buy a car charger that costs that much. It will last more than 10 years on average I bet :)

Stick to brand names, and claim warranty benefits when needed. Some brands in some markets will offer warranty longer than a year, so that's worth looking into as well.

and they all break and stop working in a month.

I know you're a bit hyperbolic, but even if those chargers lasted a quarter each, that would be 40 chargers. I don't think I have bought that many over the last decade for my entire household, never mind just to keep in a car.

A car charger dying in a month is somewhat typical for bottom of the barrel stuff that, for example, is typically sold in gas station convenience stores, or at the grocery store checkout lanes. Don't waste your time and money on that. It's usually junk made to work long enough that the customer is a long way away by the time the thing fails. It's usable if you're "in a pinch" and absolutely need a charger and there's nothing better available nearby or there's no time to investigate or drive to get it. Otherwise, those cheap things are best avoided.

It's not particularly hard to design a one-off switching power supply charger that will last longer than the USB connector on it. It will cost in the $50-$100 ballpark due to low quantity, if you're experienced in such designs.

As other answers have well argued, a linear regulator is not desirable. It may be "just a 7805" and yet it may not last long anyway. I've seen plenty of 7805s fail due to running hot and heat cycling in office applications, never mind in a car. A 7805 may well be wiped out just by transients in an automotive application - if you don't protect it sufficiently.


This is a bit of an X->Y problem. The purely linear regulators are not suitable for your objective as the other answers have suggested.

However, when I was faced with a similar problem - 5V power from a 12V vehicle with little heat generation - I ended up with switching regulator modules by Recom: https://recom-power.com/en/products/switching-regulators/switching-regulators-sip/rec-s-R-78K-2.0.html?2

I only required 500 mA in my project, but there's a 5V, 2A version of it as well:


You should probably still use a capacitor on the input and outputs though.


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