Multiplex the voltage

[elecLear78]

Dear All,

i am facing problem to design the following circuit

i have a hardware board with minimum operating voltage of 10.5 volts. The board is connected to a battery of 12v. In normal operations the 12v input is going to the hardware board but in some cases the battery voltage can fall upto 9v for some 10min and again comes back above 12V. So, under these conditions when the battery voltage is going below 10.5 volts i want to design a circuit such that when the battery voltage falls below 10.5 volts the new board should supply the required 10.5 volts and when the battery comes above 10.5v the battery should supply the voltage again. The new hardware to be designed should operate on the battery and even if the battery voltage going upto 9v the new board should supply 10.5 volts. Current rating is not a problem it requires only few millamps.

somebody please help me on this.

Thanks in advance,
regards

 

[Ian Rogers]

You need a buck boost regulator... https://www.linear.com/products/buck-boost_regulator

 

[electroyas]

what do you mean by multiplex voltage? I think above solution is best, buck boost converter , but what is the load current?

 

[alec_t]

Is it possible to adapt the board to run on, say, 8V?

 

[crutschow]

The simplest would be to add a 1A Schottky diode in series with each supply (two diodes total). Set the backup supply voltage to about 11V. That way, when the battery drops below about 11V, the backup diode will start conducting and keep the voltage at 11V minus the diode drop, which is about 0.2V to 0.3V at low currents.

 

[MikeMl]

If the battery is lead-acid, especially SLA, then you should not be discharging it to 9V. If you do, you greatly shorten the life of the battery. If you were not mistreating the battery, then you would stop discharging it at >10.5V, in which case, your problem is solved...

 

[elecLear78]

I am sorry, I think I have not clearly explained the scenario.

I have only one source of power that is the vehicle battery which is around 12.0 volts to 12.5 volts. Now i have connected one of my hardwares which reads this battery voltage and briefly tells me the voltage of the battery or indirectly the state of charge of the battery, so reading the battery voltage is important for me. My problem is, this board works only upto 10.5 volts and not below it. So, i want to design one more hardware board which would start operating when the battery voltage goes below 10.5 when the engine is cranked. The new hardware board should only provide the voltage when the actual battery voltage falls below 10.5 volts and when the battery voltage is above 10.5 after engine cranking then the battery voltage should be going to the first board as reading the battery voltage is important for me. So, finally there is only single source of supply that is battery and when this voltage falls below 10.5 i should still generate a voltage of minimum 10.5 for some time and then come back to my battery voltage. Hope i have explained clearly. i have attached my idea of implmenting could someone suggest me if it is correct and what components i can use to acheive the same.

thanks in advance,
regards,
Satish

 

[MikeMl]

So you want a voltage-controlled switch which turns on when the input voltage is less than 10.5V?

A TL431/LM431 is ideal for that.

Something I posted here years ago. You would have to tweak it to switch at 10.5V

 

[alec_t]

While the engine is cranking the battery voltage measurement won't, IMO, be of much use in indicating the state of charge (the cranking sound would be a better indicator); so why not just ignore the reading then and wait until cranking stops and the 10.5V is restored?

 

[elecLear78]

While the engine is cranking the battery voltage measurement won't, IMO, be of much use in indicating the state of charge (the cranking sound would be a better indicator); so why not just ignore the reading then and wait until cranking stops and the 10.5V is restored?

The problem is after the engine cranking the voltage is actually taken over by the alternator and does not indicate the battery voltage and does not give me the correct soc and therefore i want to maintain the soc before engine cranking.

So you want a voltage-controlled switch which turns on when the input voltage is less than 10.5V?

A TL431/LM431 is ideal for that.

Thankyou for the circuit. i will try the circuit. But i am so poor in circuit design that i do not know how far i will be successful, but i will try in a simulation software and then post any questions i have.

Thanks and Regards,
Satish