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Solenoid Monitoring

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 08 April, 2011 » Solenoid Monitoring « Previous Next »

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muskrat
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Username: muskrat

Post Number: 360
Registered: 06-2009


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Posted on Tuesday, 05 April, 2011 - 05:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I use solenoid valves to feed the plants on the farm. They are of particularly poor design as far as the solenoid coils are concerned. They are encapsulated for water-proofing, with the two connection wires exiting the resin/rubber at the top of the coil. The wires tend to break inside the insulation at the point where they meet the covering.

The coils are 24VAC, inrush current = 340mA, holding current 200mA.

I want to find some method of monitoring the coils' current to recognise failed coils. I have seen reed relay units used in DC circuits, wrapped with a few turns of wire to pull in the reed when current flows, but I'm unsure whether this technique can be used with AC.

I use relays to switch the solenoids.

I would like to achieve this with minimum modification to the existing system.

Any ideas will be greatly appreciated.
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cjaysharp
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Username: cjaysharp

Post Number: 54
Registered: 06-2010

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Posted on Tuesday, 05 April, 2011 - 09:05 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Use a hall effect?

I have a cute little circuit I built many years ago that uses a hall effect for almost exactly this purpose (it was to check for operation of fuel injectors). If you want I'll see if I can dig out the schematic?

You can salvage hall effect devices from brushless fans and DVD/CD-ROM drive spindle motors if cost is an issue.

As an aside, can you not replace the coils with something of better quality?
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alec_t
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Username: alec_t

Post Number: 591
Registered: 03-2009

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Posted on Tuesday, 05 April, 2011 - 09:13 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

When you say 'monitor', do you mean a continuous indication of the coil's current, or a one-off indication that the coil has failed? Is this remote monitoring or local-to-the-solenoid monitoring?

Alec
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muskrat
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Username: muskrat

Post Number: 361
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Posted on Tuesday, 05 April, 2011 - 09:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

@cjaysharp: thanks for the offer. If you can find it, I'd like to see the circuit.

@alec_t: It would monitor when the solenoid is activated and have a signal output to indicate failure. I have a neat little 2-wire comms setup which controls up to 16 solenoids along a pair of wires to the control-room. Some solenoids are about 150 metres away from the controller.

It would be a simple matter to use one of these units to send fault signals back to the control-room.

Each group of solenoids is controlled by the 2-wire setup. 12VDC and 24VAC are reticulated around the farm.
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alec_t
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Post Number: 593
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Posted on Tuesday, 05 April, 2011 - 02:51 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you've got space to wind a pick-up coil around the solenoid coil you can amplify the sensed AC and derive from it a signal, perhaps coded to identify the solenoid, to send back over the comms link.

Alec
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alec_t
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Post Number: 594
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Posted on Tuesday, 05 April, 2011 - 03:01 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

...or a current-sense resistor, say 3R9, would drop sufficient voltage to turn on a transistor when a working solenoid is energised, without affecting the solenoid operation.

Alec
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cjaysharp
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Username: cjaysharp

Post Number: 55
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Posted on Wednesday, 06 April, 2011 - 07:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have the prototype board in front of me but can't find the schematic (think it was from a tech tip in a trade mag). I'll spend a bit of time later and reverse the board.

It's a pretty simple circuit but it works rather well, uses a Honeywell SS495 hall effect, a single rail dual op-amp (OP290 because I had a tube of them) and a dual comparator (LM393) to drive a couple of LEDs (dependent on field polarity).
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cjaysharp
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Username: cjaysharp

Post Number: 59
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Posted on Wednesday, 06 April, 2011 - 11:40 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Schematic attached (I hope)

The software I used didn't have the SS495 in the library and I haven't had time to add it but you just hang it between +5 and 0V then the output should be 2.5v with no magnetic field and it will swing to +5 or 0V depending on magnetic field polarity. Good luck. If you want to prototype it I built mine on stripboard and can send pictures of my layout as a guide?
application/pdfSolenoid coil monitor schematic
tCad1.pdf (14.7 k)

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