Does anyone have any thoughts about induced voltage being used for something like this?
It is doable. But if your ac wire is at 50/60hz and you need more than a few ma, you may have to use a large pick-up coil.
I saw somewhere a small device that I think was meant to measure load on an AC line like an amprobe ??
That's a current sensor (essentially a current transformer). It can be used to power devices.
Again also, can someone give their opinion of a tiny transformer.
If you don't intend to power something heavy, try those tiny telecom transformers or audio transformers. The telecom transformers are mostly 1:1 to 1:2 and by reversing them you get the right voltage. The issue there is that you have to be careful with their inductance / wiring: most of those transformers are designed for high speed applications and some have a common mode filter. Those will not work. You want to look for something with a primary inductance of at least a few mh.
What is the difference between a signal transformer and a power transformer?
One is meant to supply into a small load and one to deliver lots of current. Signal transformers will work in this case as well, as you are simply powering a mcu.
Here is an alternative for you to think: get a 12v or 24v transformer (you can get them at stores like Radioshack or pretty much anywhere), and put the 2ndary on your 24vac source and on the primary you get 110v or 220v, depending on the transformer used. They put a regular ac/dc adapter on that, and you are done. Less than $10 and minimum amount of work.
How about a Murata OKI-78SR-5/1.5-W36-C? The ebay one linked to looks like a clone of the Murata. The Murata is only $5, max Vin is 36V though so you could drop your 40V with a resistor between your bridge and the power supply. Dropping voltage from an unregulated transformer shouldn't be dramatic, it won't take much resistance (and thus wattage) to get a lot of voltage drop.
tylernt:
How about a Murata OKI-78SR-5/1.5-W36-C? The ebay one linked to looks like a clone of the Murata. The Murata is only $5, max Vin is 36V though so you could drop your 40V with a resistor between your bridge and the power supply. Dropping voltage from an unregulated transformer shouldn't be dramatic, it won't take much resistance (and thus wattage) to get a lot of voltage drop.
Think about that for a moment. At what current draw do you size the series dropping resistor for? If you size it for the maximum rated current of the regulator (so that the applied voltage to the power supply is 36 volts or less), what value of input voltage will the regulator 'see' with little or zero current going to the load?
You can check out the staged regulator designs (pre-regulators) from Walter Jung. If that's too much, get a resistor or zener from Radioshack + another LM317/78xx series regulators and use the resistor / zener to raise the ground pin's potential, and then use the 2nd regulator to step it down further. A typical approach in LM317 datasheet to expand the (input) voltage range.
However, it is rated at 90vac minimum. What happens when it is used at far below its rated minimum?
I guess that several billion wall module power supply designers are right and that a transformer is the best approach.
Does anyone have any recommendations for a tiny transformer to go in front of the bridge?
As i looked at information on transformers, it became obvious that becoming familiar with all the varieties and specifications could take a long time. Does anyone have any quick explanation on how to see the load imposed by the winding on a perpetual basis>
If this post seems redundant, I put one up a few minutes ago that didn't show up and decided to do it again.
You could use something like http://www.digikey.com/product-detail/en/FP30-200/237-1021-ND/242467. Connect the 2 secondaries in series across the 24V supply (I've chosen a 30V transformer because your supply is more than 24V unloaded). Leave the primary open circuit. Then you will get 15V across one of the secondaries, which you can rectify and feed to a 7805.
Or build your own switching regulator, as I suggested in a previous post.
PS: another possibility is to use a 115V to 36V transformer, also available from Digikey. Feed it with 24V and you will get about 7.8V out of it. Rectify that with a bridge rectifier and smoothing capacitor, that should give you about 9.5V to feed directly to the Vin pin of the Arduino.
Thanks everyone for the input. At this point it looks like the best solution is a DC to DC converter and a bridge and cap. There aren't many and they are a little pricey but this one looks like it will do the job: http://www.digikey.com/product-search/en?x=15&y=9&lang=en&site=us&KeyWords=102-1839-ND
It has an 18v to 72v range and is $19.95 q1.
Depends on how much current your circuit draws. If it's small, like tens of milliamps, a zener diode is good enough and won't waste too much power. If it's something like hundreds of milliamps, a switching regulator is better since it's much more efficient. A half-wave rectifier (i.e. single diode) plus big cap is sometimes good enough for AC-DC conversion.
I used a MC34063 switching regulator for my sprinkler controller circuit:
look at the "power supply" section on the top-left corner.
If power consumption is not an issue, there are a variety of simple ways to drop 24V to 12V. Zener diode, 7812 or other linear regulators... But if the circuit is drawing sizable amount current (like mine draws about 180mA current), then (33-12)*0.18 = 3.78 Watt power will be wasted. Note that the 33VDC comes from rectifying 24VAC. In fact, there will be more because the 12V to 5V conversion will waste another (12-5)*0.18 = 1.26 Watt. That's a total of 5 Watt. So in this case switching regulator is more efficient.