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Automotive +5V
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GigaJoe:
Hello,

I have some device that has to be installed in a car , and powered from +5V from the car source , consumption approx 100-150 mA ( no measure , assumption).  Where the +5 come from i have no clue and possible load on the bus as well.

When I open the box it PIC18F2480, + 3 MCP4912

Device basically do input as a MAF voltage,  and a output skewed data ,   overclocking the engine ....

An electronic design itself are terrible , like basic \ simplifying \ cutting corners design ;  but multi-layer PCB .... 
for DAC:   Vref == Vsuppl == +5 from the car with noise up to 6.5V spikes ...
There is no any input  / output even basic protection for dac\adc,  MCU , dac out  or ADC input  not buffered
I found just one ceramic cap for a Vsupp.  voltage;  2 place where suppose electrolytic caps,  empty.   

I need some advice to make +5V better,  as I cant make an active filter , i need some passive one,  I even has no clue how much capacitance I can load on this +5 bus ....

I probably can cut Vref pins for MCU and DAC  to supply from some better source like 100ohm 1000uf    RC filter -  I dont know ...   I need some advice

Device:


+5V: (not my measurement)
sokoloff:
Trace where the two [presumed] capacitor circles on the right side of your picture go.

If they go across Vcc and GND, consider putting 16V electrolytic caps that will fit into the case on both those footprints.
Andreas:

--- Quote from: GigaJoe on July 01, 2019, 06:44:39 pm ---consumption approx 100-150 mA ( no measure , assumption).

--- End quote ---
never: this is usually the current that 5-10 sensors have together from the 5V sensor supply.


--- Quote from: GigaJoe on July 01, 2019, 06:44:39 pm ---Where the +5 come from i have no clue and possible load on the bus as well.

--- End quote ---
A ECU has usually 1-4 sensor supplies that deliver the 5V for sensors like accelerator pedal some pressure sensors ...
Each can deliver 50-150 mA typically.


--- Quote from: GigaJoe on July 01, 2019, 06:44:39 pm ---When I open the box it PIC18F2480, + 3 MCP4912

--- End quote ---
so in sum about 10 mA.


--- Quote from: GigaJoe on July 01, 2019, 06:44:39 pm ---I need some advice to make +5V better,  as I cant make an active filter , i need some passive one,  I even has no clue how much capacitance I can load on this +5 bus ....

--- End quote ---

shure that the measurement is done with the right ground connection?
(you have to measure directly between sensor ground and sensor supply).

Usually the inrush current has to be limited on the sensor supply.

by the way: using soccets for the processor is a no no in automotive applications (vibrations).

And I fear the cirquit does not withstand a short cirquit of the 5V to the 12V battery voltage.

with best regards

Andreas

james_s:
Perhaps look at some older stock ECUs? I repaired the ignition control module in a 80s VW years ago and it had a bog standard 7805 in it, but the regulator was what had failed.

The ECUs in my 80s and '90 cars have a slightly fancier 3 terminal linear regulator with a couple of electrolytic capacitors, nothing exotic.
GigaJoe:
hook a scope to +5V

in AC mode random spikes ...



Seem better , need a filtering Mhz spikes ....

sooo ... trying to explain in seq, we feeding that +5V   it need filtered:   
10ohm   resistor - ceramic cap 0.1uf - some ferrite  coil  -  ceramic caps 0.1uf -  20uf el. cap -  5.6V zener (crowbar)  ---  device.
  Vref:  from croqbar zener  - 200 Ohm - cer.cap 0.1uf - el. cap 20uf.  (dont know  ... it looks strange )

the main  uncertainty:   will it work , due to RC has some reaction time ,   what happens when engine start ... i dont know ....   as MAF voltage will skewed due to Vref RC till it fully charged ...

can it be that caps was removed by very that reason ....  it basically shorted +5 bus at beginning   ????

do separated psu from 12V  + Vref ....  an ideal situation ,   but ...  it a separate box .... power dissipation, heat ,  ...  will it reliable  ...   
 
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