How to achieve a safe 5v supply in an automotive environment?

Hi all,

I am building a new dash instrument cluster for my car and need a good and safe 5v feed with which to power 3 arduinos, servos and LEDs etc. Looking around there doesn’t seem to be a definitive best practice on how to do this. The best I have found is the attached picture, but I copied the circuit layout but forgot to take all of the values. When I tried to find it again to get the zenner & schottky values I had no luck so 3 questions I now have:

  1. is this the best way to do it?
  2. if so, which values should I use for each component (particularly the zenner & schottkys), and ideally which PNs should I use (I’m trying to build it in Eagle so need actual components to add to my BOM
  3. if not, is there a better way with known components?

Thanks,

Nick

The OPs diagram:

BTW: 450nick, you, too,, can master such wizardry by reading the following: How To Insert Images In a Post

Before any component values can be determined, or even a determination given as to the veracity of this approach, we will need to know more about peak current demand, and perhaps, more about the actual components you plan to power.

Thanks very much, that's much easier to work with... In terms of current demand, I will be running 3 Teensy 3.2s, which I believe max out at 250mA, and up to 30 backlight LEDs, probably running at some state of dimness using PWM. So I would estimate around 1A should be ok - does that sound reasonable? Maybe a bit more for some contingency in the design?

Just buy a quality automotive USB phone charger? Note the word "quality", not cheap unbranded ones!

Hi Mark, I guess I could but I'm trying to implement it all onto one PCB to fit within a housing I'm designing so I'd be happier with it all integrated. Taking apart a USB charger and using it feels a bit like cheating too :slight_smile:

450nick:
Taking apart a USB charger and using it feels a bit like cheating too :slight_smile:

Depends on the temper of your Overlords.

450nick:
Hi all,

I am building a new dash instrument cluster for my car and need a good and safe 5v feed with which to power 3 arduinos, servos and LEDs etc. Looking around there doesn't seem to be a definitive best practice on how to do this. The best I have found is the attached picture, but I copied the circuit layout but forgot to take all of the values. When I tried to find it again to get the zenner & schottky values I had no luck so 3 questions I now have:

  1. is this the best way to do it?
  2. if so, which values should I use for each component (particularly the zenner & schottkys), and ideally which PNs should I use (I'm trying to build it in Eagle so need actual components to add to my BOM
  3. if not, is there a better way with known components?

Thanks,

Nick

With my experience of running electronics sensitive to electrical noise (old TTL is ever more sensitive!) in an automobile, my comments on this circuit are as follows:

  • The diodes look like Zeners. If so, they should not be Zeners, but ordinary silicon rectifiers or maybe Shottky diodes (lower forward drop, faster switching time). Ordinary rectifier diodes, however, are more than good enough here.
  • D6 should be a hefty 3 amp part (1N540x family) to handle the momentary inrush current when the 12 volts is applied and the input cap is at zero.
  • D10 is a TVS diode. Probably not needed, but if used then there should be a small (1 ohm or, 1 or 2 watt) resistor between the raw 12 volts and the TVS to absorb powerful spikes that could exceed the specs of the TVS and short it.
  • In any event there should be a small resistor (fireproof) between the raw 12 volts and the regulator or an appropriate fuse (or both). In fact, a fuse is (IMHO) mandatory. There are a lot of fire starting amperes available in a car 12 volt bus!
  • C2 should be much larger... 1000 uF or more. The purpose for it is if there is a sudden high drain on the 12 volt line (starting the car, electric power steering, etc..) the circuit will not "hiccup" with a momentary loss of power.
  • C4 should be a nice, low ESR ceramic capacitor (and maybe put a few in parallel for an even lower ESR). Keep the leads as short as possible. A bypass cap with long leads is a capacitor in series with an inductor (not good for bypassing!).
  • There should be a diode between the regulator input and output so that, if somehow, the regulator input abruptly goes to ground, the regulator will not be destroyed by current back-flowing from the output pin to ground (although the chances of an input side short to ground are minimal, 3 terminal regulator datasheets recommend them nonetheless). Here, a Shottky diode might be preferable due to it's forward drop being lower than any parasitic junctions in the regulator (thereby keeping ANY unwanted current spike from barfing back into the regulator).
  • D9 at the output is not necessary.
  • The 3 terminal regulator (LM2940) is a Low Dropout (LDO) part. A less expensive LM7805 will do the job. Going from 12 to 5 does not need an LDO regulator.
  • If you DO use a 2940, be aware that the part can be unstable without an OUTPUT bypass capacitor. Datasheet says: "Cout must be at least 22 uF to maintain stability. May be increased without bound to maintain regulation during transients. Locate as close as possible to the regulator. This capacitor must be rated over the same operating temperature range as the regulator and the ESR is critical".
  • If you use a linear regulator, you will need to heat sink it sufficiently. 12 to 14 volts to 5 volts at 1 amp max (the regulator's rating) means that you have to dissipate 9 volts at 1 amp or 9 watts of heat. A decent heat sink and some airflow will be mandatory.
  • I suggest that you either use a small switching regulator in place of the 2940, or if you need cleaner power, use a switcher as a pre-regulator to drop the 12..14 volts to maybe 6 or 7, then use a linear regulator to finish the job. There ARE neat little regulator modules that have the TO-220 form factor of a 7805 but instead are switchers. Drop in place, and no heat! Plus, some of them are rated for better than 1 amp output.

Hope this helps.

krupski:
The diodes look like Zeners.

Thanks Krupski! Ok I have binned the lot now and started again with one of THESE as the basis for the circuit on your recommendation.

So the new circuit now looks like this... How are we looking...??

For the schematic in post #8, pin 2 "common (ground)" needs to go to ground and the plus side of C2 wants to go to pin 1 "POSITIVE_INPUT"

Karma to "krupski" for a great post. :slight_smile:

Karma indeed! Yes sorry that was a bit of a silly mistake... Here's my new updated circuit... Any advances..??

Paul__B:

Well, you managed to find an oddball image. Google "zener diode symbol" and 99 percent of the images are the correct symbol...

When I went to "EE school", a zener symbol looked like a diode with half a hakenkreuz on top...

450nick:
Thanks Krupski! Ok I have binned the lot now and started again with one of THESE as the basis for the circuit on your recommendation.

So the new circuit now looks like this... How are we looking...??

That looks like exactly the right way to go. Don't forget the fuse though!

Oops.. looked more carefully.

Pin 1 (the input) should have the diode between 12v and the 1000uF cap, and the cap should go from pin 1 to ground (plus side up of course!).

The fuse and maybe a 1 ohm, 2 watt FIREPROOF resistor should go between the 12 v and pin 1.

Pin 3 (output) goes to your Arduino etc.. and be sure to bypass it to ground as the datasheet says.

To make it more clear:

12v ----- fuse ----- 1 ohm ----- diode ----- 1000uF to gnd ----- regulator pin 1

hope this helps.

Thanks again I will update, I'm struggling to find a fireproof pcb mount resistor in Eagle.. any idea what kind of product I'm looking for? So many options!!

Why build when you can buy cheaper?
Here is one on Amazon that will convert 7.5 to 28V to 5V, 3A.

Because my ultimate goal is something a bit bigger, and unusually for me, price is not really a factor in this build as I want quality and functionality to be as high as possible as it is going in my classic car which I want to work (ideally) forever!

To give a bit of background, the instrument cluster assembly I'm working on currently looks like this:

And like this with the decal removed (the PCB I'm now working on):

The plan is to house all of the electronics for this unit within the housing, ideally on one board for simplicity, with one I/O connector as small as possible. This will be connected to a fused source from the cars solid state Power Distribution Module (not a physical fuse). So hopefully the signal will be relatively stable before it gets to this unit.

Perhaps the topic for another thread, but I've attached my current Schematic and board designs (there is a separate tiny PCB for the 4th servo as it must sit 7mm offset from the main PCB to get the servo shafts close enough. I was going to ask as my next question if anyone on this forum was able to look it over for me to see if it makes sense. The pinouts between the Teensy and the servo controller I'm happy with and have tested on my breadboard, then the serial connection between the two teensy's will pass information between. One controls the servos alone, the other does the processing and OLED screen and other LED functions. The last bit I think I need to add is the LED backlighting which may be another ring shaped PCB with just LEDs and resistors, plugging into the connector on the main board. Still some more to do but as a first go I think it's looking ok?? Can post eagle files if anyone is interested in helping me check the board :slight_smile: