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hello?
We use lm2904 to configure and use battery low voltage cutoff circuit.
In some products, the lm2904 output voltage is 0.6V instead of 12V.
The output of the lm2904 is a method of driving the relay using NPN TR, and when a load is applied to the relay contact terminal,
the above symptoms were found.
Hi Han,
have you installed a freewheeling diode across the coil of relay? Omitting it can destroy the circuit.
I would connect pin of U1-B to pin 2 or pin 3 of U1-A. Otherwise you drive the output of LM2904 into saturation which can make issues from time to time.
I miss the supply voltage decoupling cap of OPAmp.
The "KA431" does not like arbitrary decoupling capacitances. Decrease C1 to 4n7. (Also see the datasheet of LM431.) Or mount an RC filter behind the "KA431".
Kai
Hello Han,
In addition to all Kai's advice. 0.6V is a normal output voltage if pin2 voltage is greater than pin 3.
It is a circuit that uses the output voltage of LM2904 and applies it to the BASE terminal of TR to drive the relay.
Hi Han,
when you turn off the current through a relay, the coil inductance produces a huge inductive kickback voltage. This kickback can destroy the circuit. It can also make the circuit behave weird.
https://www.apogeeweb.net/electron/flyback-diode-flywheel-diode-applications.html
You urgently need to install a freewheeling diode in parallel to the relay and you urgently need to add a decouping cap across the OPAmp's supply voltage pins.
Kai