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Author Topic: Reverse Relay?  (Read 816 times)
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I am building an ECM (electro chemical macihning) machine. I have a tool (GND) and a work piece (V+).

I am going to dissolve the work piece using 5V 100A (or perhaps 12V 50A) through a gap and electrolyte.

I want to run 5V ~100mA through the work piece at all times and use a relay off the tool to signal the ARDUINO if there is a short in the system (in which case I will not signal the high current circuit, which is controlled through a more typical ARDUINO/Relay arrangement). I then use this information to run a stepper to control gap size.

I have all of my code and circuits working (with a button in the place of the relay in question) except for this: I am having trouble finding out if a relay exists that will switch at the lower amperage, but survive the high current (on the coil side). Does this exist? How can a high current event be monitored with an ARDUINO?

Or is there a way to measure resistance of the electrolyte gap that wont fry the ARDUINO when I fire the high current pulses?

Thanks,
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My two cents..  You can monitor the current by using Arduino.  You need a shunt resistance in series with the power supply and measure the voltage across the shunt resistor (This is how the ammeters are build).  Of course you need to calibrate that to get the correct values.  You can refer the link http://www.reuk.co.uk/Make-a-Shunt-Resistor.htm if you want to build your own shunt.  Once your arduino starts reading the current flow you can turn on or off the relay at the desired values.
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or you could do the same the easy way, for example

http://www.digikey.com/product-detail/en/ACS756SCA-100B-PFF-T/620-1238-ND/1829841

up to 100 amps, puts out a proportional low voltage analog signal

and they have them up to 2500 amps

http://www.digikey.com/product-search/en/sensors-transducers/current/1966573
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Thank you so much, both responses make perfect sense. I am going to go to radio shack and try both.

I got some sleep, and I have a simple solution. I will run a second relay as a contact sensing circuit which I can turn off (which will completely isolate the ARDUINO) before I fire the high current circuit.

Thanks again. Love this community.
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There are also Current sensors with integrated amplifiers to 1KA mfd by Lem Electronics. I have 15 300 A hall effect devices that would be perfect for your application.
Part #:  Hal 300 S. The thing that makes them "better" than the Allergro devices mentioned is that you don't have to physically connect to the circuit being measured. You simply pass a wire through the hole in the device (20 X 15 mm - .6 X .8"). Both are hall effect transducers but the Allegro requires a special type of PCB to connect the device to the circuit to be measured, one that can handle the full current being measured. I've attached a data sheet for both.

Doc

* hal standard_e.pdf (479.58 KB - downloaded 6 times.)
* ACS756-Datasheet.pdf (351.89 KB - downloaded 7 times.)
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the allegro models are hal effect, but yea you got to hook the intergrated shunts up to something that can handle the current, as its a shunt
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the Lem device is a pass through sensor, you have only to pass the main current carrying wire through the device and it will sense current in both directions, as the Allegro devices will. The one I have is a little better suited to your project as it doesn't require special handling of the current carrying conductor as the Allegro devices do.
Both will work accurately from DC to 50KHz.

Doc
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Another advantage of the LEM device is that you can use an over-ranged unit to measure lower currents.  Say you have a 200A range unit and you only want to measure up to 100A then you simply thread the current carrying wire two times through the core since it is the number of turns within the core that determines the measured range.  I have successfully threaded a thin wire around 40 times through a core to calibrate a 100A circuit using a simply 0-3amp power supply.
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That's why I bought them, had to buy 15 to get 2... $5.00?
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