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#21324 - 09/06/09 09:37 AM Anyone monitoring a DC motor with a current switch ?
Bruce Sanders Offline
newbie


Registered: 07/10/08
Posts: 17
My SEC16 is now monitoring a spa with a CR9321 current switch. Works great. ( Also monitor 2 doorbells straight into the input side ( all with 1k resistors). Relays for spa, garage doors, etc

Would like to see if my solar attic fan is running and wondered if I can use a CR9321 the same as my spa pump. Of course it's a DC motor so panel getting sun = PV = DC current = DC motor running.
There is only a straight, one lead wire from the panel to the motor.
Any suggestions ?
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#21325 - 09/06/09 01:35 PM Re: Anyone monitoring a DC motor with a current switch ? [Re: Bruce Sanders]
Guy Lavoie Administrator Offline
Beyond All Hope
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Registered: 12/21/02
Posts: 6401
Loc: Montreal, QC, Canada
A device like the CR9321 detects the magnetic field caused by a changing voltage, and cannot detect a DC current through a wire. But monitoring a DC voltage is actually simpler and could be done with nothing more then a series dropping resistor and an optoisolator. Do you know what DC voltage the motor runs on? Divide that voltage by .01 (typical current through an optoisolator) and that will give you the resistance required, in ohms. You also need to multiply the voltage by .01 to get the wattage required for that resistor.
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#21336 - 09/10/09 12:06 PM Re: Anyone monitoring a DC motor with a current switch ? [Re: Guy Lavoie]
Bruce Sanders Offline
newbie


Registered: 07/10/08
Posts: 17
Guy,
According the manufacturer, the fan runs ( variable ) between 1 and 36 volts. The solar panel puts out a max of 20 watts and I have asked what voltage the panel generates.

Maybe I should just pick a voltage ( say 5 volts ) and use that as an on/off point. What do you think ?

I know what an optocoupler does, but have never wired one to anything. On the output side, would I wire it to the SECU16 like a supervised input with a resistor?
On the single DC wire side connecting to the led, don't have a clue.
Thanks
Bruce
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#21337 - 09/10/09 12:23 PM Re: Anyone monitoring a DC motor with a current switch ? [Re: Bruce Sanders]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6401
Loc: Montreal, QC, Canada
You should try measuring the range of voltage being produced by the solar panel to see if it varies a lot between when it gets minimal light to run the fan and when there is full sunshine. This will give a good idea of what voltage you have to contend with for the optoisolator.

For the optoisolator, yes you would connect it across a SECU16 input just like with any supervised input, with the emitter terminal going to common and the collector going to the input terminal. On the input side of the optoisolator, the negative side of your voltage source (solar panel) would go to the cathode, and the positive voltage would go to a series resistor, and then the other side of that resistor would go to the anode of the optoisolator.

On a typical 6 pin optoisolator, the pins are usually:

1 anode
2 cathode
4 emitter
5 collector
_________________________
"If you don't know what you're doing, do it neatly..."

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#21338 - 09/10/09 12:58 PM Re: Anyone monitoring a DC motor with a current switch ? [Re: Guy Lavoie]
Bruce Sanders Offline
newbie


Registered: 07/10/08
Posts: 17
Thanks Guy

Ok, got how to wire the pins. Now to decide the resistors I need.
Just heard back again from the manufacturer - "operating voltage 17.2, Open circuit voltage 21.6"

Given this - would a 2k 1/4w resistor work ? And whatever the right answer is, should I use the same size on the SECU16 side also?

Thanks
Bruce
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#21339 - 09/10/09 01:17 PM Re: Anyone monitoring a DC motor with a current switch ? [Re: Bruce Sanders]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6401
Loc: Montreal, QC, Canada
Yes, 2k and 1/4 w sounds about right for the resistor. On the SECU16 side, you would just put a regular 1k 1/4 watt resistor in parallel across the input along with the optoisolator's output pins.
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"If you don't know what you're doing, do it neatly..."

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