I have been building control panels for 30 years. One of my customers wants to power a pump with flow control from a solar array. He had decided on the supply (solar panel, controller, deep cycle battery, ..) which gives me 12 VDC to work with.
The controls are straight forward. The problem I am having is with a conductivity switch that works on 12 VDC. What we normally do is put a simple high level float on the tank to shut it down when full (plus a second high high level for safety).
He is pumping oil from a spill. He does not want to pump ground water so we put a conductivity switch low in the tank. If he starts to pump water, the oil floats and the conductivity switch is triggered by the water at the bottom of the tank (the switch is not triggered by the oil). I use something like the Omron K8AK-LS (and there are several others)... but all use 24 volts or 120 volts. I searched the internet for 3 hours and could not come up with one that works on 12 VDC!
Can anyone tell me if one exists?
I did find one from Euro but can't find a supplier yet, and the company has not offered a way to purchase it. Actually found sellers in Australia but all said the item is on back order. I thought about a 12vdc to 24vdc converter then the conductivity switch (say $100 for the converter and $150 for the switch ... $250 total).
Just seems to be a "messy" way of doing it, I don't like adding extra parts if I can just find a switch that operates off 12 volts.
Thanks so much ....
I don't think you'll find one for 12 volts, so I'd go with the DC-DC converter, an inverter, or just tell the guy industrial controls work on 24 vdc and he needs a different panel, battery, and charge controller. They make them. For the battery, he can use 2 12's in series. I've done it both days and just having the right voltage is a lot cleaner than trying to shoehorn the components.
Just use an inverter, 12VDC to 120VC and do it right.
You'll need to isolate your instruments from the power wiring somewhat, and 12v inverters are cheap, and available.
The Omron you picked out only needs 1 Watt, it has all the approvals and you are familiar with it. Just get a 2 Watt isolator to power it. 12V to 24V or 12V to 12V isolated.
I see that electronic distributors have DC-DC converters with 24VDC 10W outputs for $50. That is an inexpensive solution even if you install 2 for redundancy with diode auctioneering.
Designing everything for 24VDC might be the best solution next time so that you have a standard supply voltage for any instrumentation.
A 2nd thought.
What are the requirements for the system RE: electrical classification? Is explosion-proof or intrinsically safe equipment required on the tank?
You might be able to find a refraction-based photo level switch that differentiates between the oil and water. Or even maybe a float if the SG of the oil is low enough. The float would be tricky, I think.
I did in fact find a conductivity switch that uses 12 volts ... but the manufacture is in Italy and it is only distributed through one vendor. I try to never use parts that are single source ... if it dies in a few years, I don't want the customer to be stuck.
So, I will go with a 24 volt unit.
This will also allow me to use a standard 24 volt relay.
Just because I had someone try to do something stupid many years ago, I have made 24 vdc the standard for all my control panels (I actually had a dummy try put two metal plates in a tank rather then use a float switch ... then say he was getting a shock off the water ... I was amazed he was not killed .. after that, everything I did was 24 vdc).
I did find a dc to dc converter ... Automation Direct ... $81 for a 60 Watt unit.
I would have gone 24 right from the start but he is using an air powered pump and the compressor he has is 12 vdc.
Just FYI ... he is pumping product from the ground (oil spill). He wants to make sure he is always pumping product and not ground water (you always get some but he doesn't want a 200 gal tank of water). I put a float on the top of the tank (actually two) to stop the pump when it is full. I put the conductivity probes about two inches from the bottom of the tank. If it detects water, it shuts down the system.
Thanks so much again!
Thanks for the feedback.
Many 4-20 milliamp 2 wire instruments will work on 12 VDC provided the load they are driving is not too high. You could count on about 13.5 Volts I would think, the instrument manufacturer will usually give a formula like:
Maximum Loop Resistance = 43.5 * (power Supply Voltage - 10.5)
I looked at your Omron it needed at least 20 Volts unfortunately.
Just FYI ... this gives a good picture of the variety of panels I build. This is a very very simple panel ... probably 10 components. The next one up is the monitoring and control of several large pumps via the internet. This includes daily e-mails of flow totals and any problems with the system. Touch screen, PLC ... control and monitoring via a cellular modem ... should be a fun project!
Here is an example of a simple panel I built.
Thanks again .... Mike