Potentiometer and analog volt out

Can a voltage analog output be used on a device where a poteniometer normally goes? Ie, I have a DC drive that only accepts a poteniometer and I want to control it using analog outs on my PLC. Thanks in advance.

Matt

Sometimes.

Mostly it depends on the two systems. If the PLC is putting out a true constant DC voltage and that voltage range matches the input requirements of the drive, that's one. If the DC drive can accept an externally produced voltage, that's two. And if the impedence of the two systems matches, that's three.

These are the first few things I can think of.

Steve

In most cases the answer is yes, I have encountered one DC drive that would not accept an isolated analog input. Check the manual for your drive. It should show an example of how to wire an external speed reference.

The drive only has a potentiometer input, no other inputs at all (ie, the only external speed referance input is for a pot). But, the spot to wire the poteniometer has 3 wires, the PLC obviously has 2. Now, the pot wires are a supply volatge, the middle leg on the pot itself, and the common. So I would just wire were the middle leg and the common would go on the drive then (as the PLC is supplying the voltage, I dont need to use the v+ on the drive's pot terminals). Then just send the require 0 - 10 volts to the drive. Is this correct? I need to find out what the impedence is for them still..

Matt

Be very careful about this. You would be hooking the common from your analog output to the "pot" low point. In some drives this low point can be very far above ground level. You may get a huge differential here. If at all possible get the manufacturer's reccomendation on this.

Yes, what you have said is correct. Use the wiper and the common terminals. The supply terminal will be left unused unless you use it to supply the PLC's analog output.

Not knowing what PLC you are using, I know some output cards do allow you to supply the output power externally.

Once again, just for safety, be certain that both systems can be connected to one another. If you can't find enough literature to make you confident, call the respective manufactures.

As others have said on this board before, the advice you receive here is free, and worth every cent!

Steve

Its a SLC NO4V output board btw.

And whats the dc drive type?

Let me explain something if I can.
Potentiometer: A potentiometer (or pot for short) is an electronic component which has a user-variable resistance.
Usually, this is a three-terminal resistor where the center, sliding connection is manipulable. If all three terminals are used, it acts as a variable voltage divider.

NOTE the term voltage divider, it basically splits the resistance seen by the voltage which will vary the voltage output from the center wiper. This is NOT a variable resistor, it is a "changing" resistance.

A rheostat uses 2 wires and is in series with the circuit, therefore it is a variable resistor and controls current.

Many drives have their own DC source voltage to be used for control or analog inputs, not sure that I have seen any that didnt.

Most analog cards used by plc can be referenced by ANY dc source, personally not aware of any that arent.

That said this manual shows wiring for many AB analog modules:
http://literature.rockwellautomation.com/idc/groups/literature/documents/td/1746-td001_-en-p.pdf

On page 9 of that manual it shows a drawing for the NO4V and makes this statement:

The 1746-NO4I and -NO4V analog output modules have connections for user-supplied 24V dc power supplies. When external 24V dc power is used, the module only draws 5V dc current from the SLC backplane.

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Far as I know it dont care where the "user supplied 24vdc" comes from.

Nice references Ron.

I must confess that while I cannot specifically point to any analog output cards that do not accept external voltage, it seems to me that I have run across one or two over the years. But I am not absolutely sure.

As far as drives are concerned, however, I have had an application where you could not simply wire in PLC analog voltage output to use the potentiometer input.

In my case I was using a GE Fanuc 90-30 along with a Seco analog DC Drive, and the drive could not handle having its DC common tied to the PLC's DC common. In order to finally get the combination to work, I had to install Seco's optional isolation board for the drive to keep the two commons separate yet at the same potential.

This same kind of problem could crop up in Matt's system when you take into account another note from page 9 of the manual:

IMPORTANT

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Channels are not isolated from each​

other. All analog commons (ANL COM)​

are connected together internally.​

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Steve

There are always some problems. Take the NO4V and using the drives dc power supply, it may work for the drive but may create problems if you have to output to another device.

It works better all around if "both" devices involved use an external power source and impedance is matched....as you stated.

With my electrical/electronic/maintenance background you would think I would be more comfortable with the use of analog but I am not and I know its not that difficult to understand.

These days with the advent of different types of "bus" and "communication" options I believe they provide more features, options and less wiring that are just as easy to develop and be understood.

Even for some of "us" dumb maintenance guys that just cyber or visit **** sites its not hard to understand that "data" is "packed" (packeted) then sent to or retrieved from a destination and this "data" can be viewed and/or manipulated.

Personal opinion: Trade the analog card for a DNET (or equivalent) scanner then get an option card for the drive.

Dam u Terry, see what you have done.

I do agree that there is really nothing hard to understand about the communications options, but I must say that in most of the industries I have been in, I am hard pressed to recommend using one.

For me, it boils down to this: Build the system to suit the people who are most likely to have to troubleshoot it. For most of my career, that guy is usually an electrican type, usually self-taught, and usually doing well to have a digital voltmeter at his disposal. When this guy (who usually has 5 or 10 "hot" jobs waiting for him) opens the cabinet, he wants to at least see something he can measure. For him, I want to build a system that stays with analog and digital systems and is designed using as much of the KISS principle as I can.

Please keep in mind that I have great respect for these guys and have no intention to minimalize their capabilities. On the contrary. I have learned most of what I know from guys like this.

What I try to do is give the customer the equipment best suited for his environment.

Now it Matt's situation, my instinct is to recommend against the communications option simply because of its additional complexity. Not because I question Matt's competence (which I don't) but because I know nothing about his company, it seems prudent to recommend the more straight-forward and directly measurable analog solution.

I'm guessing Matt has a couple of the old-electrician types working with him.

Steve

I'm guessing Matt has a couple of the old-electrician types working with him.

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This is so funny, thats why I said dam u Terry see what you have done.

I am THE old-electrician type.

Maybe my outlook has changed some. Overall I still prefer Ladder to be used over other languages....I can be adamant on that issue.

I am more open when it comes to devices and how they work with the plc. Lets say it is an analog device and the electrician can use a multimeter to check voltage/current. What does the old electrician type do if its OK and not working? Whats he do when the voltage output isnt what it should be? Finding a problem sometimes can be fairly easy, fixing it can be involved. There are limits to what old-electricians should be expected to do.

In most cases, if not all, a drive display can tell you whether an analog OR communication signal, say a speed reference, is present or not.

Its an "It depends" issue but the comm/bus options offer so many options/features that are easily understood....even by us old-electricians...that I can see no reason they shouldnt be implemented. Old-electricians didnt know about analog until they worked with it, they will not know comm/bus until they work with it.

Whether its an old-electrician or not the ability to install and calibrate an analog device may be on a par with dealing with a comm/bus system if a device needs to be replaced.

Matt,

Get a simple Signal Isolator board (about $75 US). There are many brands widely available. For example, you can get a 10 VDC In, 10 VDC Out Isolator (or 4-20 mA to 0-10 VDC Out) that will guarantee that you have no feedback issues betweeen the Analog Output module and the DC Drive. The drive will never know that it is not being controlled by a pot.

rsdoran said:

Lets say it is an analog device and the electrician can use a multimeter to check voltage/current. What does the old electrician type do if its OK and not working? Whats he do when the voltage output isnt what it should be? Finding a problem sometimes can be fairly easy, fixing it can be involved. There are limits to what old-electricians should be expected to do.

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man... i need a couple of your 'old electricians.' most of the guys on my maintenance crew regard VFD's as black magic. last time we had one go bad, i was on the phone half the first day or my weekend trying to explain how to change the settings for the replacement drive.

the scenario you described above is exactly how I learned about analog outputs. i knew they somehow told the drive to change speeds, but it wasn't until i put my meter on the output and realized 50% on the HMI = 5V from the output = drive running at 30hz (50% of the max freq) in that particular application.