I'm not an EE but I want to use a microprocessor to display AC current at an electric water heater. I've search here and found some good suggestions on coils and I know I need to end up with a 0-to-5VDC signal, so I guess I need bridge rectifier. What I haven't been able to find is a good description of how to select the diodes and restistors for that part of the circuit. Is there a bridge rectifier calculator somewhere that I haven't stumbled on yet?
This is a mains AC water heater?
You want to measure current through it?
What is the maximum current you expect?
How do you plan to do this?
Could you explain where you expect the bridge rectifier will be used?
I can think of a couple of ways you might be aiming to do this.
Your first task is to determine what method you are going to use to sense the current. Choices are CT (current transformer), very low ohm current sensing resistor (current shunt) and Hall effect sensor pick-up. The first two methods will require a bridge rectifier/filter and the last does not. All then may require a couple of additional resistors to create a voltage divider to keep worst case measurement voltages below the ARduino +5vdc limit. And yes knowing the voltage and maximum current draw for the water heater is needed before one can size any of those three methods.
The Hall effect sensor is the newest method of the three and has the advantage of not having to disturb any existing wiring, and one does not have the danger of dealing directly with the high AC voltage.
Lefty
This is a neat little sensor that clamps over a single conductor:
http://www.crmagnetics.com/products/Assets/ProductPDFs/9500.pdf
I'm using one to measure current through a 115VAC/400Hz circuit and read it through an analog pin. I just map it into the sensors current range X my voltage to get a direct power reading. like this:
power = map(analogRead(powerPin), 0, 1023, 0, 1150);
In my case I have a 0-10A sensor so it's mapped to 115VX10A = 1150W. Since my load is resistive I don't have to worry about power factor. It's within 2% of a Fluke DMM reading at the particular current level I'm using.
It's not particularly cheap at $35US but it beat the alternative that I had cooked up with a sense resistor, diodes, caps, resistors and an instrumentation opamp.
The real benefit is that you don't have to mess with high voltage. The only potential drawback is the response time but if you're sampling over a long time it shouldn't make much difference.
Good luck!
Yes, I'd like to use a current transformer like this one I found while searching other topics here:
http://www.crmagnetics.com/pdf/3110.pdf
Two coils - one at each hot water heater element.
Using it's data sheet it looks like I can select a resistor to go across it that will limit the voltage out. I assume I end up with a 0-5V AC signal which I have to rectify to convert to something close to 0-5V DC.
Mike Forst's solution is a good one. My only concern is that I'd like to find current transformers small enough to fit under the hot water heater covers near each element rather than bring high voltage back outside. Although as I think about it - 220VAC hot water heater may only use one side for each element - I have to check that.
I would not rely on just a resistor load on the transformer limiting the current. Use a series resistor and some catcher diodes to the rails as well.
http://www.thebox.myzen.co.uk/Tutorial/Protection.html