Running a microcontroller in car - power supply aspects

[stj]

Re: Running a microcontroller in car - power supply aspects

hmm...
you shouldnt have zeners on the opto's

you shouldnt have 5v1 across the regulator output either - it will be destroyed.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

they will conduct earlier than you expect.

I see. I'm guessing I do not need to tweak those 1k'ers though.

[stj]

Re: Running a microcontroller in car - power supply aspects

yes, the zener value is too low - they will conduct earlier than you expect.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

So you're saying the zener value is too LOW and you suggested a higher value ? 5.1 is lower than 5.6, isn't it ?

[stj]

Re: Running a microcontroller in car - power supply aspects

well zeners - good ones are 5%
so they will start conducting at 5.05v and your atmel has internal resistors that may not like that.

as the mcu is o.k. upto atleast 5.5v i would use 5v6

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

5,1v zeners are a bit close, should go up a step.

A bit close to what ? Go up, as in to 6v ?

and why reference the window switches to the battery?

In order to keep the harness as stock as possible - that part is already wired into the car. I COULD start chopping stuff, which was my initial idea, but I wanted to simplify the project.........which is not really simplifying anything, but anyway I want this thing to be as PnP as possible, because I don't have a garage to work in - I have to do it all out in the parking lot with my gas soldering iron, so keeping modifications to the car's harness to a minimum is a must

EDIT: the micro MCU1 in the schematic is a Nano, but I'll be using a ProMini. KiKad didn't have the exact part and it was too trivial to waste time drawing it myself. The pinning also differs from the actual project.

[stj]

Re: Running a microcontroller in car - power supply aspects

hmmm..

5,1v zeners are a bit close, should go up a step.
and why reference the window switches to the battery?
if you reference them to vcc or better yet use the ground, you can dump the opto couplers.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Here I am again. I messed around with KiCad and after a very long time (KiKad not being the most user-friendly out there), I managed to put together the schematic for the project...hope I didn't leave anything out. Not sure about what diode should go across L1, but other than that, it works on the breadboard the way it is, without L1 for now.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

I got myself THESE for the inductor....pretty overkill at 3a, but it's better to have more there than not enough.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

The minimum is a ferrite bead and TVS+reverse diode. The ferrite bead is to stop high frequency >1MHz noise from affecting AM radio of the ECU.

Ok, so no CM choke then. Your last schematic seems to be what I should go for - the one with the 470uH coil and diode across it.

[redwire]

Re: Running a microcontroller in car - power supply aspects

If you put a scope on a car's 12V power, there is a lot of low frequency noise from the ignition system, fuel injectors, fuel pump motor, alternator ripple etc. It is a few hundred to a few kHz, and over a volt pp.
It is worse if you have an old battery or old wiring connections like rusty grounds and battery terminals.

The noise is differential (not common-mode) so it is between 12V and GND. A mains SMPS makes lots of common-mode noise (between LV secondary and earth GND). Rare to see a CM choke in a car.

If it's an old car with a distributor, which makes a lot of RF interference and is one reason car makers went to coil-on-plug. It's hard to read an O2 sensor when you have many kV arcs in the distributor nearby.


An inductor on 12V input helps with noise in either direction (coming in from ignition, or going back from 5V SMPS to car). For analog car stereos it used to be big iron-core mH's and now 22-470uH choke or whatever fits is still enough.

The minimum is a ferrite bead and TVS+reverse diode. The ferrite bead is to stop high frequency >1MHz noise from affecting AM radio of the ECU.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Another idea came to mind, which I've seen on wires and circuits before but don't know for sure how to correctly choose it or IF it should be used at all in my scenario at all: how about a common-mode choke ? Two things come to MY mind when I hear the term: a ferrite ring which has the + and - wires going to the circuit wrapped through it AND the more "advanced" version which already has copper wires properly arranged on it and comes as a standalone device which all SMPSs have on their AC mains inputs. How's that ?

[Curious.George]

Re: Running a microcontroller in car - power supply aspects

Not sure how much ripple/noise a car has on its battery when running. I plan to run wires straight from the battery terminals to run this thing (quite a lot of current required for the power windows), so there's going to be no filtering whatsoever there - what's present at the battery will immediately show up at the input to my circuit exactly.

You're assuming your circuit represents a high enough impedance that it doesn't factor into the distribution characteristics of the "signal" from the battery.

You're also assuming that the battery's output impedance is low enough that "loads" (yours and others) won't affect what you see on it's terminals.

Note that even the resistance between the binding post and the cable clamp can be "significant" in terms of other aspects of the "system" (assuming you aren't drilling into the binding post to secure "your" connection, upstream of the cable clamp).

[I.e., all resistances between the battery and the loads in question appear as parts of the battery's output impedance]

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Not sure how much ripple/noise a car has on its battery when running. I plan to run wires straight from the battery terminals to run this thing (quite a lot of current required for the power windows), so there's going to be no filtering whatsoever there - what's present at the battery will immediately show up at the input to my circuit exactly.

[Retro-Hipster]

Re: Running a microcontroller in car - power supply aspects

I might be wrong but, in a fairly clean dc to dc source, you are mainly going to be choosing an inductor based on the dc resistance the inductor poses and the potential peak current spikes from the source. Essentially, if you don't have visible/measurable switching noise on the dc output on the car, then doing frequency calculations doesn't really work. With that, the main focus would be "how much current will it take to drive the core into saturation?" and "how hot will this inductor get due to it's dc resistance"? You can get both of these statistics from Datasheets and, in this case, would want an inductor with a higher saturation current than what your circuit is going to be using and to know that the total watts that the inductor dissipates would be within tolerable range for the inductor.

I THINK I'm right on this, but I really really need to brush up on inductors/the math needed to do calculations.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Also, why 470uH on the coil there ? Forgot to ask. How would one determine the value of the inductor in such a circuit ?

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

Are you sure the LM2596 can run off your car battery? I thought it uses a lot of current, and more with an Arduino running. You should measure it, or run it off IGN power.

I did measure the overall power consumption since it IS important indeed - don't want to wake up to a flat battery one day, but I can't remember precisely what this was. I THINK like 30mA or something total...

Can't run off accessory power, since the thing needs to be on standby all the time - I added a feature to roll up the windows when the car is locked with the key fob, so the ignition would be off at that point.

[redwire]

Re: Running a microcontroller in car - power supply aspects

I know adding a filter inductor is a good idea but it added new problems.
The inductor back-emf can make a spike worse, so they add a diode across it. This is the circuit I remember. I will check with the people I know that design automotive ECU's.

Are you sure the LM2596 can run off your car battery? I thought it uses a lot of current, and more with an Arduino running. You should measure it, or run it off IGN power.

[Dannyx]

Re: Running a microcontroller in car - power supply aspects

If I have room I add 100uH-1mH with a diode across it, instead of ferrite bead.

So I should have the diode reverse-biased across the coil and place this setup in series with the input to the regulator ? What diode should this be ? A regular 1n4007 or an UF one ?

The DC-DC converter can take 32v max apparently.

[redwire]

Re: Running a microcontroller in car - power supply aspects

This is what I use in cars. For me it's quicker to draw it in KiCad or Altium we use at work. A pencil is more powerful but taking pictures of paper sucks.

If you use an large inductor to filter out ignition and fuel pump EMI, it can add to a spike sometimes and make them worse. So you have to be careful. If I have room I add 100uH-1mH with a diode across it, instead of ferrite bead.

Your 5V regulator needs to take ~30-40V input max. and 6V min. or else the Arduino will reboot when you start the car, cranking gives low voltage.
I think an Arduino Nano needs around 20mA. I use an LDO LP2950 to power it and a sleep command to go down to a few mA.

I can explain each part if you want to make sense of it.