Current monitoring relay?

Community PCB Design
1

I’m a software engineer by trade, but don’t know much about micro electronics, so I apologize in advance for my ignorance. I’m trying to pull together a hobby project and was hoping you guys might be able to point me in the right direction.

The project in question involves a portable UV water purification system. There is a 12 volt, 4 amp battery (charged via solar panel) that is connected through a standard commercial inverter to power a 110 volt 13 watt UV sterilization lamp.

What I’m trying to figure out is how to integrate a component that monitors the power being drawn by the lamp to operate a solenoid valve that shuts off the flow of water until the lamp is fully “warmed up” so that I can prevent water running until sterilization is happening…

In short, I think what I need is some kind of circuit that monitors the current between the inverter and the UV lamp and then trips a relay to energize the solenoid on the valve once the lamp is drawing full power so that water can flow.

There is a PDF describing the project, in general terms, here: http://www.simpleearthstructures.com/re … atment.pdf

I was wondering if you could tell me if there are any pre-made components that I might purchase that would work for this application or if you know of a way that I can pay someone to either draw up plans or manufacture a few of these circuits for me? The PDF made it sound like it was easy to do, but I have no idea.

Thanks so much in advance! Your time and information is greatly appreciated!

2

You can build a circuit to do this, but isn’t the time for the lamp to warm up pretty constant? Just use a timeout. Worst case I can think of is that it may vary by room temperature, so you can measure room temperature and adjust the time you wait for it to warm up.

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Thanks for posting this! I’ve been using UV filters to sterilize our household rainwater system for some time and most suppliers overcharge for their systems (to vastly understate the problem). I hadn’t seen the Sterilight systems and they seem to be a refreshing alternative.

You might email the originators of the concept to ask about the “simple circuit” to monitor lamp current. On the other hand, the documentation for the UV sterilizer says to wait 3-5 minutes after startup, before allowing water to flow, so I agree with lyndon that a simple delay would be fine.

One serious weakness of the bucket design is that the sterilizer won’t work for turbid water. Bacteria and viruses hidden in particles aren’t destroyed by UV. You need to filter water very well for UV sterilizers to be effective in one pass (my rainwater system continuously recirculates the stored water through the filter/sterilizer unit).

4

jremington:
Thanks for posting this! I’ve been using UV filters to sterilize our household rainwater system for some time and most suppliers overcharge for their systems (to vastly understate the problem). I hadn’t seen the Sterilight systems and they seem to be a refreshing alternative.

You might email the originators of the concept to ask about the “simple circuit” to monitor lamp current. On the other hand, the documentation for the UV sterilizer says to wait 3-5 minutes after startup, before allowing water to flow, so I agree with lyndon that a simple delay would be fine.

One serious weakness of the bucket design is that the sterilizer won’t work for turbid water. Bacteria and viruses hidden in particles aren’t destroyed by UV. You need to filter water very well for UV sterilizers to be effective in one pass (my rainwater system continuously recirculates the stored water through the filter/sterilizer unit).

Thanks for the replies, folks!

I’d be okay with a static delay of, say, 5 minutes. Any idea on how to easily do that? I supposed I could just wait after switching it on, but building in a fool-proof way to eliminate operator error makes for better design. There should be no way to operate the device incorrectly.

I did email the author, but haven’t been able to get a reply as of yet.

Also, in terms of turbid water, this particular design uses a 2 micron pre-filter in the top bucket, which isn’t awesome, but does help a bit. For my personal usage, I was just going to use the UV system after running the water through a 0.2 micron Katadyn system, so I’m only using the UV stage to eliminate viruses.

There is a company called “SAFH2O”, which I see make UV lamp systems that run directly on DC current, but their most streamlined system also incorporates a 0.5 micron charcoal prefilter filter, which might make gravity feeding an issue. Also, I want as few components as possible and would like to have as many low tech / high reliability parts as possible. I don’t want a blown ballast, or something, in the middle of a hurricane event and not be able to easily replace it.

The draw of the bucket system is that I’m putting all the components together and would be intimately familiar with the operation and construction of everything so maintenance would be easy.

I just need to figure out how to build in the delay. :think:

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You need to know the characteristics of the solenoid valve before coming up with a circuit. Is it 12 VDC, 24 VAC or 110 VAC? What is the current consumption of the coil? In any case, searching for “schematic relay delay circuit” turns up many possibilities that include a relay for switching.

In order to design a current sensing circuit, you will need to know the current draw versus time characteristics of the lamp circuitry.

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That site doesn’t give any details on the valve used on the project, but I can make some assumptions based on some solenoid valves I have here and go from there. A fixed delay (once you figure out how long it needs to be) is easy to implement. I will put together a demo when I have a few minutes free.

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This is what I did to drive a solenoid valve. I will post a movie of it working when I have a moment, but I used a circuit board I keep on hand for one-off projects. As a test, I set it to fire the solenoid 5 seconds after power up but it’s just as easy to do 5 minutes. The board is based on an AVR microcontroller since it is difficult to get long accurate times from something like a 555 chip. An arduino along with a solenoid driver chip can be used to do the same thing, or I can let you have some of my boards programmed with the timer firmware.

http://www.cedarlakeinstruments.com/Sha … -timer.JPG

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Here’s a crappy cell phone video of it working. Timer set to 5 seconds and LED blinks once per second then stays on when solenoid valve activates. This solenoid draws just over 350mA @ 12V.

http://youtu.be/ZvftTR-WsqY

ps,

Is there a way to get videos to play inline?