automotive board, power supply circuit: selecting one of two supplies? - Page 1
Forum section: electronics / projects

author: JPortici
author_rank: Super Contributor
timestamp: November 25, 2016, 11:53:03 am
content: I'm redesigning a product that goes under the hood of a car/tractor/truck.As for now two methods are used:1)the power supply is the 5V line from a sensor2)A regulator is fitted (present on board but not populated) and the board is powered by the batteryBoth have problems: for 1), not all car are equal, some doesn't like the added current and shut down the sensor line. in these cases we use number 2. 2) board is constantly powered. consume very little power but still not the best thingOne partial solution i'm testing is to use a regulator with Enable signal and the enable signal is controlled by the presence of 5V Sensor (I am testing LP2951 for this, which seems suited for the job and has a Q100 variant)selection between the two supply lines is made by soldering a 0 ohm resistor in one of two places.works fine: problem with that is that after the board is potted i can't change the power source anymore, so now i have to find a way to cut the 5VSensor Line if the battery voltage is present..only thing i came up with was a pmos pass transistor which gate is pulled high by the voltage battery, but i am not sure on how to select such transistor.I have attached sketch for a circuit, could it work? does anybody have some other solution?keep in mind: 5V minus a diode voltage drop isNOTacceptable or i would already have used it. 5V has to remain 5V +/- 2%also, i have no space left on the board. the mosfet can be a sot-23, not bigger.

author: jeroen79
author_rank: Frequent Contributor
timestamp: November 25, 2016, 01:38:04 pm
content: You mention that the drawback for option 2 is that it would be constantly powered but wouldn't that apply for option 1 as well?Where would the current drawn from the sensor come from?The simplest solution would be to have it wired into a 12V circuit that is not powered when the car is turned off .

author: JPortici
author_rank: Super Contributor
timestamp: November 25, 2016, 03:05:54 pm
content: if we take power directly from the battery the board is always powered, even when the engine is turned off and we have no need for that.if we use the 5V from a sensor the power is present only when the sensor is active but in some cars the ecu doesn't like the additional current drawn from the sensor. It's a very small (but increasing) percentage of cases and using battery voltage is a PITA in many cases and require modifications to the cabling we otherwise don't have to do (and thus can not be seen) so we are trying to have a single circuit solution. i can live with selecting the source with a resistor then pot but if an universal solution can be had, better.

author: max_torque
author_rank: Super Contributor
timestamp: November 25, 2016, 03:25:58 pm
content: I assume this is some sort of "bodge" to modify the sensor output prior to the ecu input to fool that ecu into making more power or whatever? (ie the typical add on box to the fuel rail pressure sensor on common rail diesel engines etc?)If so, your best bet is to simply reduce consumption of the device to the point where the host ecu doesn't notice the extra load! A modern micro, with careful software design should easily get down to a very small extra parasitic load on the ecu's 5v power line!If you connected to the vehicle battery (either directly or via the ignition feed (google "KL30" and "KL15") then you need to ensure your power conditioning includes the requisite load dump protection (google "automotive load dump protection")

author: SvanGool
author_rank: Regular Contributor
timestamp: November 25, 2016, 04:01:57 pm
content: Couple of comments:Since you are Italian and you are talking about tractors/trucks, I assume you mean European trucks, those have a normal on-board voltage of 24V (opposed to 12V for cars and American trucks)Your design is not CE automotive compliant, it will not pass ISO7637-2 testing because:You are not protected against reverse polarity (0 .. -3xV), U1 will not surviveU1 will probably also not survive because your 30V TVS will only fully clamp above 30V and your regulator is specced just till 30VI am not sure if this will always work when everything is switched on and both voltages are available, you can have both (different) 5V voltages active for a short instance, I would at least put an elco of 470uF at the drain of Q4Beware that you pick a MOSFET that can go higher then 30V on its gate or you have to add a zener thereA linear regulator is not the most efficient way of doing things (especially not at 24V), not knowing what your actual current draw is