building a field voltage monitor circuit for input to a plc

[transitmech]

I need help designing a circuit that will monitor the field voltage of a 24 v. alternator so it will set an alarm thru a plc system when a predetermined voltage is seen. This is for a PM program to help determine when the alternator is getting to a stage in its life when it should be replaced before a failure occurs on the road.

 

[BrianG]

Can you supply a few more details like:
1) normal operating voltage range compared to "fail" voltage level.
2) Is the alternator tested when loaded or unloaded, and also with or without the usual control circuit present?

 

[transitmech]

In response to an e-mail, the field voltage under normal loaded conditions (with everything working correctly) varies between 8 and 11 volts to the field terminal, prior experience has shown when the field voltage rises to the 22v.+ region the alternator is on it's last legs and needs to be replaced soon before a failure will occur due to overheating. I'm trying to implement a system to monitor the alternator field voltage while under load so the plc system also used in the vehicle can illuminate a light on the panel to warn of the condition of the charging system. I'm looking for the simplest electronic circuit to build that will be able to be used as an input to the PLC as the field voltage rises above the 22v+ range.

 

[melone]

Can you just use a simple voltage divider with an additional Zener? Therefore, you can easily set the trip point by changing the Zener.