Small query in comparators

[jeffrey samuel]

I have a project currently requiring to compare a voltage to 24V reference and it must trigger a relay output when the input voltage is lower than 24V reference. Both voltages are DC supply. I planned to use LM393 and provide it with the same 24V reference as supply. Can it be done? Does the parameter of input common mode voltage range affect my application? I'm guessing it does, meaning I need to use a voltage divider to drop both input and reference voltage to below 24V? Any help is appreciated.

 

[FvM]

I'm guessing it does, meaning I need to use a voltage divider to drop both input and reference voltage to below 24V?

Yes in case of a comparator that doesn't include positive rail in common mode range. LM393 doesn't, as you know.

 

[Syncopator]

Why don't you simply apply both your reference voltage and your monitored voltage to equal value voltage dividers?

e.g. apply your 24V supply to a 10k plus 10k divider, and the monitored voltage to another 10k plus 10k divider?

You can select the actual values and ratio to suit any other criteria.

 

[BradtheRad]

A zener diode is typically used at an op amp input for this purpose. The op amp can be powered by an unchanging source or by the variable source.

For a relay you might use a SPDT. Hook up to the contacts which are normally-on. When the level reaches 24V, you activate the relay, and the contacts break.

 

[jeffrey samuel]

When I tried using a voltage divider sequence the output is always 0 V

Is there a problem in my circuit or am I supposed to get that when the reference and the actual voltages are same

 

[FvM]

Not sure what your actual circuit is, but LM393 has an open collector output, the output voltage will be always 0 unless you connect a pullup. With exact equal input voltages, the comparator output will reflect the offset voltage. There should be at least a small voltage difference. To control a relay, you'll usually want to add hysteresis to the comparator circuit to avoid relay chatter.