PLC newbie...any advice???

I am looking to buy PLC unit (for the first time) and am curious if there are any out there that allow a user to control the output voltages from the unit itself, without having to go to an external relay. Does this even exist? Any advice would be greatly appreciated.

Thanks,

Nick

Hello Mr. Silverman;
All PLCs I am familiar with have preset digital output signals (24 V DC, mostly, but also 120 V ac, 12 V DC, 48 V DC, 115 V DC...). There are analog output modules that can range ±10 V, 0-5 V... and can be set to output a DC voltage if required, within their range. But I don't know of any that will allow your to control distinct output levels from the modules themselves, outside of analog modules.
Hope this helps,
Daniel Chartier

I'm not quite sure what you mean by 'control the output voltages'.

If you mean controlling the voltage level, that's an analog output module. These are typically 0 - 10 VDC or +/- 10 VDC when you specify a voltage model and 4 - 20 mA or 0 - 20 mA when you specify a current model.

If you mean can they switch voltage on or off to a load without having to go through an external relay, then that depends on the size of the load. Many PLC output modules are rated for 0.5 amps per output point. You can also find modules rated for 2 amps per point. Some contact output modules can handle higher currents than that. You have to read the spec sheets for the modules.

Control output voltage HOW?
ON or OFF...yes
Varying ...yes

Depends on what you want/need. Plcs are digital and the standard outputs offer ON or OFF control BUT there are situations where the output voltage of the PLC does NOT match that of an external device...ie the output may be 24vdc and the starter coil 120vac, in this case you need a relay.

This is one of those IT DEPENDS. Explain what you want/need and more specific information may be given.

Thanks...

First of all, thanks for the responses. Second of all, I apologize for being a bit unclear, I am learning as I go. What I am hoping to do is send three separate voltages to three separate devices. Since we are not sure what voltage level will be required at the output, we would like to be able to manually adjust it until we find something that works. I know there are ways to do this with and external relay, but I am curious if any models of PLCs have this ability on their own. Is this making any sense? Or am I out of my mind...please bare with me.

Thanks again,

Nick

There are isolated contact output modules that could switch the power on and off to the load, assuming that they can handle the current.

I don't know of any PLC analog output modules that go beyond the +/- 10 VDC range. Even if that range is OK for your purposes, a PLC analog output is not rated for very much current, 2 Kohms minimum load resistance being typical, so don't expect to use an analog output module for the armature voltage on a DC motor.

I'm looking at a data sheet for a nominal 12/24 VDC discrete output module, and it says that the load voltage must be between -15% and +20% of nominal, so that would mean it should work with a supply voltage from 10.2 to 28.8 volts, but varying the voltage will have to be done externally to the PLC.

I'm looking at a data sheet for a nominal 120/240 VAC discrete output module, and it says that the load voltage must be between 85 and 265 VAC. Again, varying that voltage will have to be done externally to the PLC.