It only says it is an over voltage detection circuit, It was designed by a former colleague but he has gonehi cal,
Where is the circuit used?
Who designed it.?
E
Thanks Eric, your explanation gave me clue to dig more, it is very helpful.hi cal,
Where is the circuit used?
Who designed it.?
E
Thanks , your explanation gave me clue to dig more, it is very helpful.U28, TL431 is the heart of the tooo complicated project. It is used where if "REF" is above 2.5V then pin-1 pulls low. If "REF" is below 2.5V then pin -1 is open.
U29 adds positive feedback. If U29 (& U38) are on, then R12 is shorted out and helps keep U28 on.
48VDC is divided down by to a voltage just under the 2.5V refence.
OV_TEST causes the 24V trip point to be lower.
12V gets subtracted by a Zener and fed into U28.
VR3 is a constant current source.
The "12V" will loose 0.6V of D56 and about 12V from the Zener.You said :" 12V gets subtracted by a Zener and fed into U28. "
Hi Ron,The "12V" will loose 0.6V of D56 and about 12V from the Zener.
At or very neat the trip point, the voltage on the TL431 input will be 2.5V.
At the trip point, the voltage at the top of R12 2k will be 5.9V.
The "12V" will need to be above (0.6 + 12 + 5.8) for the TL431 to turn on and latch.
When the TL431 turns on, U29 shorts out the 2k resistor. For the TL431 to reset the 12V will need to be below (0.6 + 12 + 0.4 +2.5). Remember 0.6 if from D56, 12 is from Zener, 0.4 is the voltage across [U29//R2k], 2.5 is the voltage to turn on/off the TL431.
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