voltage monitor with OK led

Hi,
I have product that have dual redundant power supply. But I would like to have a voltage OK led.

The power supply can take 7 to 50 VDC input voltage and gives out 5VDC. I now only have a led connected to the output of the power converter. But if the input voltage goes below 7VDC the led starts to fading but it still glows at 3VDC in. This is not good. The led should go out immediately if the output voltage goes belove 5VDC.

I have looked around and one way to solve this would be to use a OP amp.

I wounder, is there ready made IC circuits for this? I found an IC with 3.3, 5 and 12 VDC input but only serial connection to read the result. Would be nice with a led driver on the output of these ones.

Do anybody have an idea how to solve this?

//Bertil

1.) Use LM3914

2.) Use opamp.
a.) Connect opamp Vcc to regulator out ( 5V )
b.) Set opamp ' -ve ' input to half Vcc by res.
c.) Connect feedback res. in between opamp ' -ve ' and opamp out.
d.) Connect opamp ' +ve ' to regulator in ( 7V to 50V ) by res potential divider. A decoupling cap. e.g 100nF should be added in between opamp ' +ve ' to gnd to absorb ripple.
e.) Connect opamp output to a npn transistor with series res. in between.
f.) Connect led to transistor collector with bias res.

Setup
Set cutoff voltage at 5V + regulator dropout voltage
Adjust opamp ' +ve ' input voltage ( by variable res. ) to a point just start switching.
Led will switch off once input voltage drops below this cutoff voltage.

Hi,
thanks for the tip, got me searching in a new direction. The op amp solution is not really suitable for my application due to the space. But I found a circuit named MC33161. This is a universal voltage monitor. I am going to use this one. I was also in need of the big temperature range -40 to +105 deg C that this circuit features.

https://www.onsemi.com/PowerSolutions/product.do?id=MC33161PG


//Bertil

Use a zener diode

I think the circuit with a zener driving a transistor has a sharp enough transition between 4.8 and 5v
See attached schematic and simulation graph

Alex

thankssssssssss )))))