Relay

Is it possible to take a 24vdc power source and reduce the voltage down to 10vdc? I have a 0-10vdc analog input card for a PLC with a potentiometer for a variable 0-10vdc input. I want to avoid buying a 10vdc power supply and use the 24vdc one I have in my panel. I think it is possible with a resistor in series with the POT but I must be using ohms law wrong.

Well, since you are just inputting a signal to a probably relatively high (and relatively constant) impedance ADC input the inefficiency of a resistor voltage divider is not too important. If you put a fixed resistor with 1.4 times the resistance of the "pot" in series with it, it should work pretty well but noise may be an issue, depending on what sort of other loads are being supplied by your 24V supply. You might want to look into a zeneer regulator, as that may work better, but you asked about resistors, and resistors could be used .
For example, assuming the input impedance of your ADC is so high we can ignore it (probably not really true, but maybe close enough) if you have a 10k pot in series with a 14k fixed resistor, the current will be 24V/24000Ohms = 1mA: 1mA * 10ohms = 10V.
You’d want to put the fixed resistor between the +24V and one end of the pot, and take the output signal from the slider of the pot, with the common of the 24V supply and the ADC common connected together.

1) SMPS details Input voltage:230V AC and Output Voltage required:24V DC 6Amps. They have used the DC voltage for switching ON of the solenoids(24VDC).
2) The SMPS is connected with a capacitor (2200mF) across the output Voltage (24V+ and 24V-). What is the use of the capacitor?
3) If the capacitor is uninstalled what would be the output?
4) How is the Capacitor discharged?
5) Should the terminal 24V- at the SMPS be earthened?
Should the negative of the SMPS be ground??

SMPS = Switched Mode Power Supply

2) Capacitor is used to provide initial surge of current need to change the state of the solenoid. It also filters the current waveform coming out of the SMPS’ inductor.

3) If the capacitor is removed, the SMPS may not operate correctly, as it typically uses a feedback sense voltage to control the switching. It is hard to say what you would see, but I would not do this.

4) Capacitor is discharged by the solenoid load, but also by the SMPS, which provides current TO and FROM the capacitor. This is true if the SMPS has a TOP FET and BOT FET which are switched ON at different times.

5) You may connect the -24V on the SMPS to ground, if the solenoid has its return grounded. If the solenoid has two wires, just connect both -24V and +24V wires to the solenoid.