Automatic switch-off of 12V receivers after engine shutdown – MAX4374F, LT3008 - Page 1
Forum section: electronics / projects

author: max-bit
author_rank: Frequent Contributor
timestamp: June 01, 2025, 02:16:52 pm
content: Here is such a simple project - battery protection / automatic switch of receivers after turning off the engineSince in a car and not only (e.g. motorcycle, etc.) we have various receivers connected to e.g. the cigarette lighter socket, sometimes it is necessary to automatically disconnect it when the car is turned off, of course some cars have such a function but not from all sockets and not always.So there is a need for something to disconnect our car receivers after turning off the engine.Assumptions:1. Disconnection occurs automatically after the voltage on the socket drops below 13.2 V (with the engine on we usually have 14-15V)2. The current drawn by the protection system will be very small <1mA3. Power of switched devices >50WAs the executive element, the Analog Devices MAX4374F integrated circuit was used as the main executive element.The diagram looks like this: (the original version is based on the diagram published by Maxim/Analog Devices -https://www.analog.com/media/en/technical-documentation/design-notes/overcurrentoverdischarge-protection-for-leadacid-batteries.pdfversion one (before changes, more on that later)Additionally, IC LT3008 was used as a 5V voltage source, the executive transistor is FDS4675.The current in the off state does not exceed 300uA, which basically does not burden the battery, after a month of standstill without the engine running, it will draw about 0.22 Ah from it, we can still reduce this value by increasing the resistor value in the LED1 diode (R8) - (when I have the final soldered version, I will do experiments to what extent the R value can be realistically increased) but simulations have shown that even 22k would give positive effects) then the current would drop to about 200uA.Printed circuit board (also version one):I soldered the system and ... and ... I blew 2 IC MAXInitially everything started working, the response to voltage changes was OK, cutting off the current was also correct.The first time it was killed by playing with currents under the cap and "making short circuits" as if the system did not like sudden current jumps (generating overvoltages?).The second time I tested the "reset" with an exceeded load and it did not survive that either (this is already quite suspicious)I have to assess what exactly cooked it, for now I have nothing to use, I had to order another one.The system should withstand 30 VOnly the COUT outputs are less resistant to it because up to about 6V, did the overvoltage appear through the transistor gate capacitances and blew it?It is true that I powered it from a switching power supply and this one is not exceptional, it could have added something from itselfAs they say, it is not easyAlthough the system is quite delicate, but as they said A, you have to say BSo I made changes to the schematic (and PCB)First, I lowered the resistor values ​​(resistance values ​​that were too high could cause instability in the systemSecond, based on the analysis in multisim,Upper trace without TVS diodesLower trace with built-in TVS diodesI noticed that the system tended to generate overvoltages on the Cout 1 and Cout2 outputs (which could still have values ​​(to be tested) on the completed system) and the Cout outputs have a maximum acceptable voltage value of 6V.Therefore, I built TVS diodes on the Cout outputs.The current schematic after the changes looks like this:Now I have to test the module after the changes.The situation is strange that everything should be OK but the reality turned out to be more brutal.If anyone has dealt with the system or has any comments on the project, please write.