But you can estimate it? If it's greater than the 2ft interval (hopefully not) it complicates distinguishing one level from another reliably.

definitely not 2'. If I were to guess, 6" might be it.

 

Hello. I have been searching the net trying to find a solution and this site has given me the most info out of all of the rest. I am now at a point where I need a tad bit more info, but I cannot seem to find it for my application or design. I am sure if I knew more about things I could adapt a higher voltage circuit, but I am not that knowledgeable.

Without going into too much unneeded detail, I would like to be able to convert 24vdc(or even 12vdc) to somewhere between 6-12vac. The reason is that I have a device I am building that measures water level of a tank. It needs to generate a 4-20mA analog signal, but also has to use AC power for the contact probes so that electrolysis does not happen. If I knew of a way to keep that from happening with DC power then I would just use it.

I have an idea of how to make it happen using 5 wires, 2 for AC power, and 3 for DC power/signal, but if I could convert the DC power in the relay box then I would just have to run 3 wires from the control cabinet where the analog signal is connected to the PLC.

This is the circuit I am referencing.

Except instead of the LED's, I am going to use a photomos relay. (This is my theory anyways) I am not concerned with the 4-20mA signal part as I have already built and have it functioning correctly. I just need to be able to energize/de-energize the relays based on water conductivity, and using AC seems to be about it unless I want to constantly clean/maintain probes using DC power.

Thanks for the help in advance, I hope I got enough detail in there for you guys.

These are not entirely rare as electronics hobby magazine projects.

One approach is to build it with CMOS gates for extremely low current. The ability to run the whole thing off a small 9V battery means you can (electrically) float the circuit. You supply a square wave to the common probe, but as there is no reference to GND - no electrolosys.

 

These are not entirely rare as electronics hobby magazine projects.

One approach is to build it with CMOS gates for extremely low current. The ability to run the whole thing off a small 9V battery means you can (electrically) float the circuit. You supply a square wave to the common probe, but as there is no reference to GND - no electrolosys.

could you explain a little more? sounds interesting, but I have no knowledge of what you speak of.

 

could you explain a little more? sounds interesting, but I have no knowledge of what you speak of.

We can steer you in the right direction - but we're not going to do your project for you.

 

lol. that's fine. I am working on it currently. I just wanted to know a little more of your idea, as in, I really don't understand what your talking about so if you could put it in different terms so I could understand then maybe I could visualize what your talking about.

I have no desire to run it off a 9v battery, but I am not sure if you were just using that as an example. I am just trying to be open minded about the project is all. Not looking for someone to do it for me.

 

I've been simulating a circuit design but it points up problems associated with any immersed probes in a filter system such as yours. In theory, depending on water level, the probes will be either in water or in air, so there should be a distinct inter-probe resistance change. In practice, if the water is fairly pure the resistance change could be small so the detection circuit may need to be 'tuned' carefully to find the sweet spot. Algal build-up on the probes, changes in the mineral content and presence of contaminants would all affect the resistance, throwing the tuning off. With the turbulence and high humidity the probes are unlikely ever to be dry, so there would always be some limited conduction path between them.
I'm coming round to the view that a pressure sensor (as suggested previously) might be more reliable. Either that or a weighing system (based on Archimedes' principle) which senses the apparent weight of a 16 foot pole suspended vertically in the filter tank.

 

lol. that's fine. I am working on it currently. I just wanted to know a little more of your idea,
.

Its not my idea, its been published as projects in various hobby magazines and there's examples on the web for anyone who could be bothered to look.

 

ive already got my plan/project. I was just saying yours sounded interesting. I don't need to look for anything else. I just thought you would expound on what you started to talk about. sorry I asked.

 

I've been simulating a circuit design but it points up problems associated with any immersed probes in a filter system such as yours. In theory, depending on water level, the probes will be either in water or in air, so there should be a distinct inter-probe resistance change. In practice, if the water is fairly pure the resistance change could be small so the detection circuit may need to be 'tuned' carefully to find the sweet spot. Algal build-up on the probes, changes in the mineral content and presence of contaminants would all affect the resistance, throwing the tuning off. With the turbulence and high humidity the probes are unlikely ever to be dry, so there would always be some limited conduction path between them.
I'm coming round to the view that a pressure sensor (as suggested previously) might be more reliable. Either that or a weighing system (based on Archimedes' principle) which senses the apparent weight of a 16 foot pole suspended vertically in the filter tank.

True. When I first started considering the idea, I was going to be using the water as a resistance medium. After just a short bout of testing, it was obvious that the inconsistency would be too great.

That's when I went on with my new idea, just using the water as a conductive path. Sure, it will at times have greater and lesser resistance, but it will still conduct. And all that it needs to do is conduct enough to pull in a small relay coil. Then, the mA signal, which needs to be fine tuned and stay consistent, will be separate from the relay portion of the project.

The weighted pole idea sounds interesting. I'll have to think about that one. The main problem you would encounter with that is your depending on mechanics, which can be effected just the same as a float system.

I have to revert back to the original base fact that there is one system that has yet to fail in the filter, and that is a conductivity sensor. It has never once failed. We currently have one installed as a failsafe for the very bottom point we need to monitor in the filter. The only reason we would not use it for all the other points is the expense and the fact of having to use several input points into the PLC.

However, if we use that same technology, control relays that have different mA currents through them, then bring one mA signal wire back to the panel, then we have a very useful system.

 

That's when I went on with my new idea, just using the water as a conductive path. Sure, it will at times have greater and lesser resistance, but it will still conduct.

That's where the problems begin, as per my previous post.

And all that it needs to do is conduct enough to pull in a small relay coil.

It's not going to manage that, IMO. Assuming you drive the probes with 12VAC and the relay needs only 30mA to pull in, that implies an inter-probe resistance of only 400Ω. Most unlikely unless you have very contaminated water or the probes are only a tiny distance apart (which would be prone to the gap being bridged by algae or crud) . So you'd require a relay-driver circuit. Personally, I wouldn't bother with relays. Use semiconductor switching or summing resistors to vary the 4-20mA loop current directly.
Here's a way you could do that in principle. This assumes you have got a probe assembly which provides a respective 12V positive level at In1, In2, In3 in a 3-level system. You might need to tweak resistor values to get the loop current intervals to your liking.

 

so you do not believe that the second circuit I posted will work? I suppose its possible that someone develops a circuit that may or may not work, just basing it on theory. I guess I assumed that it seemed logical enough to work. I do not believe the circuit is trying to drive the relay coils directly from water conductivity though.

 

so you do not believe that the second circuit I posted will work?

What second circuit? The link in your post #8 is broken.
The arrangement I guess you are referring to (that in Scott-Wang's post #7) relies on the relatively high conductivity (low resistance) of water between probes to provide a current path for capacitor-isolated AC signals. Crud bridging the probes would also provide such a path . I believe you could get that arrangement set up and adjusted to work in the short term, but given the variable water conditions, algae presence, high humidity and turbulence I don't think it would be reliable in the long term. It has the advantage of being cheap and easy to prototype, so by all means give it a go.

 

What second circuit? The link in your post #8 is broken.

I just did the test and the circuit still there, so the link is right.

The arrangement I guess you are referring to (that in Scott-Wang's post #7) relies on the relatively high conductivity (low resistance) of water between probes to provide a current path for capacitor-isolated AC signals. Crud bridging the probes would also provide such a path . .

I just tried to re-link what the TS tried to link in the post #3.

 

What second circuit? The link in your post #8 is broken.
The arrangement I guess you are referring to (that in Scott-Wang's post #7) relies on the relatively high conductivity (low resistance) of water between probes to provide a current path for capacitor-isolated AC signals. Crud bridging the probes would also provide such a path . I believe you could get that arrangement set up and adjusted to work in the short term, but given the variable water conditions, algae presence, high humidity and turbulence I don't think it would be reliable in the long term. It has the advantage of being cheap and easy to prototype, so by all means give it a go.

The only downside is see is what you see also and that is algae bridging the probes. Thankfully, at the low end(where measurement is most critical, the probe will be apart of the "wash cycle" and get scrubbed by air regularly. The high end, less critical, could get buildup, but would be easy to wash off/keep clean. I know the same could be said for a float, but reality is there is quite a bit of difference there(real world speaking, not just engineering thoughts).

I have not thought of/played with the spacing of the probes yet. Humidity, variable water conductivity, and turbulence I cannot see being an issue at all for this. But if someone can reason why they would I would be glad to listen.

I'm building the analog board right now. The CD4093 chips are coming in later this week and I will build the relay board then. Hope to have the prototype in place soon and then it will just be monitored for a while before it actually is put online and used to control things.

I'll keep this updated as I go.

 

Good luck. Keep us posted.

 

So I just saw, this morning surfing the net, that TI made a chip specifically for this purpose. LM1830. Guess it was also discontinued. Aside from buying them from ebay and places, is there another chip similar in nature to this? I would imagine that if there was you all would have already said something about it, but I figured I would ask anyways.

I have not done a lot of looking for it yet. Some, but I am not too concerned about it regardless. More curious than anything. It would make the footprint much smaller to have a single chip like this.

 

So I just saw, this morning surfing the net, that TI made a chip specifically for this purpose. LM1830. Guess it was also discontinued. Aside from buying them from ebay and places, is there another chip similar in nature to this? I would imagine that if there was you all would have already said something about it, but I figured I would ask anyways.

I have not done a lot of looking for it yet. Some, but I am not too concerned about it regardless. More curious than anything. It would make the footprint much smaller to have a single chip like this.

The LM prefix was originally a NS part number - there may be some archived end of line notices and guidance of what new devices should be specified in new designs.