High(er) Voltage Power Supply OK Circuit for microcontroller

[ADWSystems]

The MCP111 voltage monitor will sink 10mA to drive an opto-isolator, give a reliable, stable turn-on voltage and draw virtually no current.

Just sayin.....

How about the MC34064 instead? Shipping costs are getting high to make another order from another vendor. Just sayin....

Once you mentioned driving the opto, I can visualize the methodology.

 

[JonSea]

Looks like it should work. The supply current is about 10x what the MCP111 draws but it's still negligible for the voltage divider calculations.

 

[AnalogKid]

How do you maintain isolation with the LM324? I'm not seeing it.

Neither did I in your first two posts.

 

[Nigel Goodwin]

It's been a while since I have worked in-depth with zeners. As I recall, they were essentially high impedance (forgoing the break over region) until they break over then a short circuit passing all voltage through. Are you saying a 22V zener will have a 22V drop and I don't need a resistor at all?

You need a resistor to limit the LED current - but the zener removes 22V of the 24V supply, leaving only 2 volts for the LED and resistor.

The whole point of using a zener is for it's voltage drop.

 

[AnalogKid]

A better choice would be something like 12 V or 15 V. This leaves plenty of headroom for the resistor so small inaccuracies in either part do not significantly alter the LED brightness and optocoupler output current.

If a simple go/no-go signal is all that is required, use a 6.2 V zener for all three circuits, with different resistor values for each so all three LED currents are approx. equal. Commonality of parts usually is better, and three different zener values cost more than three different resistors.

ak

 

[ADWSystems]

I don't see the point of using 6.2V zener when trying to verify the 24V power supply is on. I don't see the 6.2V zener is any better than a straight resistor divider.

 

[Nigel Goodwin]

I don't see the point of using 6.2V zener when trying to verify the 24V power supply is on. I don't see the 6.2V zener is any better than a straight resistor divider.

I agree, an 18V zener would be more suitable.

 

[AnalogKid]

I'm not worried about an exact break over or measuring the voltage, just on-off yes or no.

I wanted to make a sharper threshold for the LED power, than provided by a resistor or resistor divider

 

[ADWSystems]

Those sound like some good user requirements. A sharper break over point than a resistor divider in order provide an on-off signal to indicate if the power supply is online. The exact break over point should be somewhere relative to the supply voltage in order to provide an indication if the power supply is working. A 6.2V zener on a 24V power supply does not do this.