Voltage sense controlled relay

Thread Starter

hartwrenches

Joined Jan 15, 2024
5
Hello, new to forum. Looked around and now I'm confused more. I would like to close a 12v relay when charging is happening and open the relay when on battery power. My though was a zener diode and npn transistor. I have no idea how to size the resistors and would much appreciate where to research and study how to build such a circuit. Thank you.
 

Ya’akov

Joined Jan 27, 2019
9,098
Welcome to AAC.

You‘ll need to supply more information about the charger and the connections. Sharp, well lit photos and schematic(s) or at least wiring diagram(s) are also very important. WIth enough information it should be no problem to get help.
 

Thread Starter

hartwrenches

Joined Jan 15, 2024
5
This is a recommended circuit. The battery voltage is 12.6 vdc, charging voltage is 13.2 + I would like the relay to close when charging, open when not charging to prevent battery from discharging.

I assume the Zener is 11v because the 1k resistor used up 1v. I understand that a resistor is necessary to protect the components, but I don't understand how the size was determined. I'm going to build this and see what actual switching voltage will be and maybe this is finished product?
 

Thread Starter

hartwrenches

Joined Jan 15, 2024
5
This automotive application. All connections are made at the 12vdc AGM battery. Charging is from the stater when engine is running, the voltage is regulated before battery connection.
 

ericgibbs

Joined Jan 29, 2010
18,826
hi hart,
Even when using a12Vzener, your circuit will not work as expected,.
After charging, the battery voltage will be well over 12V
E
EG57_ 1414.png
 

Thread Starter

hartwrenches

Joined Jan 15, 2024
5
That's good information. What size Zener would be recommended so that the circuit will shut off and the battery will maintain 12v when not in a state of charge? Also, how are the size of the resistors determined?
 

crutschow

Joined Mar 14, 2008
34,386
Below is the LTspice sim of a circuit using the cheap TL431 programmable voltage reference as a comparator to turn off the relay at the desired voltage, which should work for you.
The battery will be above 13V when charging, and drop below that when not, so the circuit is set to turn the relay off/on at about 13V.

The TL431 turns off the relay when its REF voltage drops below 2.5V.
This voltage can be adjusted by trimpot U2 to change the trip point.

R4 provides a small amount of hysteresis to the trip point so the circuit doesn't chatter when the battery is going through the trip voltage.

1705434040212.png
 
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