12V 10A DC-DC to 5V circuit

[misiek303]

Need to step down from a 12V Car battery to Arduino?

I found one but it using large components ( attached ).
I wonder if LM2596 board will do the job safely, but can't find a circuit for it.

Thanks

[JustMeHere]

Cars have a very "toxic" power environment.  They are nominally 14.4 volts while running.  They can drop to 9 volts when starting. There can be large load dumps causing 120 volt spikes.

[Swainster]

Car battery to 5V @10A? Your linear circuit example (at least the 3D modeled part) will definitely burn up as it will be dissipating almost 100W when the car is running. Those pass transistors need to bolted to a big ol' heatsink. Agreed that using a buck regulator is much better, however LM2596 is only specc'd for 3A.

[james_s]

I knew someone would mention the mythical load dump. Well technically it's not a myth, but I've never seen it happen and the vast majority of USB chargers and other devices that plug into a lighter socket will smoke if it happens. If your car was made within the last 40 years or so it is extremely unlikely, load dump protection is something you put on critical hardware like the ECU though even many of those lack it. For an auduino don't even worry about it. Modern car electrical systems are not nearly as hostile as a lot of people think, those warnings are from the era of crude electromechanical voltage regulators.

[Eraldo]

What do you need to power with the 5 volt? If just the arduino and some sensors then a regular 5v linear regulator can handle it.

If you want a lot of power then just buy a powerful buck converter with high current capabilities (depends on your needs   ). They are also constant voltage devices so no need to worry about the battery voltage.

[JustMeHere]

I knew someone would mention the mythical load dump. Well technically it's not a myth, but I've never seen it happen and the vast majority of USB chargers and other devices that plug into a lighter socket will smoke if it happens. If your car was made within the last 40 years or so it is extremely unlikely, load dump protection is something you put on critical hardware like the ECU though even many of those lack it. For an auduino don't even worry about it. Modern car electrical systems are not nearly as hostile as a lot of people think, those warnings are from the era of crude electromechanical voltage regulators.

All it takes is a loose battery cable.  And that's what was probably making the computers reset in my car the other day.  Had to do a quick repair the other night.

[JustMeHere]

Here's a good app note:
https://www.ti.com/lit/snva681

[misiek303]

I did research on LM2596 as I don't need a lot of output power, 3A is sufficient. It will be actually powered MEGA2560. So I draw the attached circuit based on the LM2596 datasheet. I don't need an adjusted regulator. And yes the car ( it is actually for the motorcycle ) can supply 14V. So it must sustain that.

As far as the >100V spikes on the bike shut down. Is there anything I can add to protect it? - not sure if the flyback is for. The are only for reverse current right?

Can you guys have a look at the circuit, please? I added a 5V fuse (F3) and would like to add a fuse (F4) for 12V, no idea where to put it. Can you help? - unless this circuit and components are trash, please tell me that as well.

Big thanks

Cheers.

[james_s]

I've used the LM2596 and it's a good part, easy to use and works really well. Beware the Chinese modules you can get on ebay, they actually work pretty well but they are NOT real LM2596, the frequency they operate at is completely different.

[Jwillis]

The 680uF capacitor is configured wrong.  Negative side of cap should be grounded. The way it is now will block DC voltage.
Feedback must be connected to output.

https://www.ti.com/lit/ds/symlink/lm2596.pdf   Page 1

[james_s]

That's a very weird way of drawing a buck converter too, I would not even have recognized what I was looking at without closer investigation. Draw it the way it is on the datasheet and it will be a lot more clear.

Also the part shown in your drawing is the -ADJ (adjustable) variant which needs an additional pair of resistors to set the output voltage, without that you will have a 1.2V power supply.

[Jwillis]

That's a very weird way of drawing a buck converter too, I would not even have recognized what I was looking at without closer investigation. Draw it the way it is on the datasheet and it will be a lot more clear.

Also the part shown in your drawing is the -ADJ (adjustable) variant which needs an additional pair of resistors to set the output voltage, without that you will have a 1.2V power supply.

Not really. Its a way to emphasize that connections need to to be kept as close to the regulator as possible. 

[misiek303]

680 Cap, I screwed this one up. Thank you, corrected.
Also changed LM to SX-3.3 from -ADJ. Is this correct ?
I redrew the circuit, based on the new pinout locations.

6th pin, is it just a heatsink?

What about the fuse for 12V, if I need it at all. And additional spike protection, can you advise?

Thank you so much

[Bud]

3.3 means it is a 3.3V part. If you need 5V you use the part with index 5.0, or ADJ with a resistor divider, see the datasheet for the adjustable regulator design procedure.

[Jwillis]

What about the fuse for 12V, if I need it at all. And additional spike protection, can you advise?

You can put a fuse on the input side if you like but won't be necessary since you have one on the load side.Arduino is usually supplied with 500mA from a USB so no more than a 500mA fuse would be required. 
Over voltage lock out could be added but really won't be necessary. The 680uF capacitor is probably enough.
Main thing is to try to keep the coil orientated far enough away from the chip to keep EMI from the coil magnetic field interfering with the chip. A board layout is presented on page 32 of data sheet. 

[james_s]

A fuse on the input side is advisable for an automotive application, unless you know the circuit feeding it is fused at a low enough value that the fuse will fail before something catches fire. Under normal circumstances you can rely on the regulator to protect against faults, but if the fault is with the regulator IC itself or the input filter capacitor then the protection features of the IC won't help.

[james_s]

Not much fun in that.

[Eraldo]

I'm  failing to understand op. Does he just want stable 5v 3A or does he just want to try and build his own buck converter.

Just wanna know if what he needs is functionality or a project to do in his free time. If functionality is what you need, just buy a buck converter off amazon or any other store with buck converters. In aliexpress you can get 8A ones for less than 5 euros.

On the other hand if you just want to build your own then you will have to delay the main project where you seem to want to work on (which requires 5v 3A). Or, as Edavid said, just buy a capable car charger (which are buck converters in disguise) and youre done.

You gotta be more specific with your needs op.

[misiek303]

I'm  failing to understand op. Does he just want stable 5v 3A or does he just want to try and build his own buck converter.

Just wanna know if what he needs is functionality or a project to do in his free time. If functionality is what you need, just buy a buck converter off amazon or any other store with buck converters. In aliexpress you can get 8A ones for less than 5 euros.

On the other hand if you just want to build your own then you will have to delay the main project where you seem to want to work on (which requires 5v 3A). Or, as Edavid said, just buy a capable car charger (which are buck converters in disguise) and youre done.

You gotta be more specific with your needs op.

I am designing a board for my motorcycle that controls all lights and starts the bike. I want it all on one board. The board will use Mega2560 which will control ProFets. I will connect it to the 12V motorcycle battery to power it up, therefore the DC-DC buck converter implementation that I want to put on the board. Is this DC-DC buck overkill or should I look for a different approach?, like using an optocoupler with resistors?

I started the other post to add one more feature. I need to sense another 12V input that comes from the Ignition Key

[] GND
[] 12V Battery constant power
[] 12V Ignition Key

Somebody responded that an optocoupler is sufficient. It must just sense the 12V ignition Key and trigger an IO on Mega to wake it up. So if I can do that safely with Optocoupler, why I can't use another one to power it up?

Does this make sense?

Thank you so much for helping out?

[james_s]

An optocoupler is an isolated switch, you power the LED on one side and it causes the transistor to switch on and conduct on the other side. You can't power something through an optocoupler, each side needs its own power source.

[misiek303]

Can optocoupler share the GND with input and output ?

[james_s]

Sure, the two sides are totally isolated, they can share anything that doesn't need to be isolated. They're essentially a relay, except optical rather than mechanical.

[misiek303]

I did research and draw this. It is an inverted connection for the optocoupler. I wonder if bc817 should do it as well.

As far as I understood, the inverted is pull-down and the non-inverted is the pull-up?
I don't know why the attached drawing is inverted actually, I don't understand the physics here yet.

Regardless, is this circuit correct?

[misiek303]

ok, regardless.

If I have input IO with PULL-UP enabled, Do I need to pull it down to change the IO state?

if so. Do I need an inverted or non-inverted option and what does it actually mean, I get different information when I read things and this is so confusing.

Thanks

[james_s]

I'm not sure I understand what you're asking. Input pull-up is used for active low inputs, ie the default state is high and then you assert it by pulling it low. This is the most common way buttons and such are interfaced to a microcontroller.