24V interface to micro

[Booleano]

Hi all,

I need to monitor a 24V input through the on board MCU.
This input it from an AC-DC which powers my microcontroller board though an LDO and charge the battery.
When the AC-DC in unplugged MCU has to monitor a Vbat and detect if it falls below 14V.

So basically what i asking for is : could anyone suggest how to interface those 2 inputs to my microcontroller?

many thanks

Booleano

 

[H.E.Tjelum]

Hi

I would use a voltage divider with a capacitor for noise reduction and a zener diode for safety reasons.
I have tried to upload a picture for a simple schematic



The schematic is for a 5V MCU, and if You need to measure higher voltages than 24V, you can replace R1 with a bigger value. The capacitor should be a poly- type to minimize leak current.

Friendly regards

H.E.Tjelum

 

[nigh]

yes
H.E.Tjelum presented a quite decent solution
I would also suggest the same

 

[Booleano]

Thanks for your reply and for the schematic.
I think i will use it to determine if vbat is below 14V.
But what about the 24V unplug? should it be possible to use a digital input to the MCU ?

many thanks regards

 

[nandhu015]

Thanks for your reply and for the schematic.
I think i will use it to determine if vbat is below 14V.
But what about the 24V unplug? should it be possible to use a digital input to the MCU ?

many thanks regards

You can use a optocoupler like pc817 to detect the voltage is present or not.

 

[arbj]

apart from voltage, a current monitor must also be used to monitor battery condition, sometimes a battery may show correct voltage but under load it will fall below normal, this may indicate a faulty battery.

So for a complete check you should monitor the charge and discharge current of the battery. This can be done using a suitable shunt in series with the battery and using the ADC of the micro-controller to read the charge and discharge values, since the shunt voltages will be very less ( a few milli Volts) a suitable amplifier may have to be used.

 

[H.E.Tjelum]

Hi again

To measure whatever there is 24V or not (meaning 0V) it should be fine to use the same circuit and connect it to a digital input. It is just very important that the 24V have the same reference (GND) as the MCU.
If the maximum voltage at the battery is 14V i would replace R1 with a smaller resistor to have a better resolution. I suggest R1=4k7 for this job.
If the 24V is with a different GND i will strongly recomend a optocoupler solution as nandhu015 also mentioned.

Friendly regards
H.E.Tjelum