LM2904 output 0.6V

han gisung

Other Parts Discussed in Thread: LM2904, LM431

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.

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

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

0 Ron Michallick over 3 years ago in reply to kai klaas69

TI__Guru**** 174826 points

Hello Han,

In addition to all Kai's advice. 0.6V is a normal output voltage if pin2 voltage is greater than pin 3.

0 han gisung over 3 years ago in reply to Ron Michallick

Prodigy 10 points

It is a circuit that uses the output voltage of LM2904 and applies it to the BASE terminal of TR to drive the relay.

0 han gisung over 3 years ago in reply to kai klaas69

Prodigy 10 points

Is the LM2904 fault due to noise on all pins of the LM2904?

0 kai klaas69 over 3 years ago in reply to han gisung

Guru 140200 points

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

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LM2904 output 0.6V