Power from 24VAC

A half-wave rectifier will actually give an unloaded output voltage higher than the bridge rectifier, because there is only 1 diode voltage drop instead of 2.

If the unloaded input voltage isn't much above the 40V limit of the device I linked to, then you could use a few diodes in series to drop a few volts.

Thanks for the comments.

Does anyone have any thoughts about induced voltage being used for something like this? I saw somewhere a small device that I think was meant to measure load on an AC line like an amprobe ?? but used to feed a micro for measuring.

Again also, can someone give their opinion of a tiny transformer. What is the difference between a signal transformer and a power transformer?

Thanks.

I'm not sure what you mean by induced voltage in this context - can you explain?

A small 24V to (say) 6V transformer would do the job, but most transformers are designed for 115V or 230V input.

If you are feeling brave, you could make your own switching regulator using this IC http://uk.farnell.com/allegro-microsystems/a8499slj-t/v-reg-buck-1-5a-50v-smd-soic8/dp/1329622 which has 50V max input voltage. The datasheet gives some typical application circuits. Be sure to get an inductor that can handle the output current without saturating.

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.

How much power did you need?
Mouser carries Recom AC/DC power modules like this one at various output currents

Check out something like this:

Used standard in the hobby industry, you can even find them up to 15 amp.
DON'T put AC current through it.

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?

Left

Think about that for a moment.

Ok you're right, I wasn't thinking. So, how about a low-watt, high-ohm resistor between + and - of the bridge output?

tylernt:

Think about that for a moment.

Ok you're right, I wasn't thinking. So, how about a low-watt, high-ohm resistor between + and - of the bridge output?

A non effective solution in any situation that I can see?

Won't it drop a few volts regardless of the current the regulator is or isn't drawing?

between + and - of the bridge output?

Doable, but not as good.

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.

Thanks for the inputs.

The Recom is a neat device http://www.recom-power.com/pdf/Powerline-AC-DC/RAC01_02-SC.pdf

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.

Thanks again.

Search at mouser.com, there is a whole line of Recom Ac/Dc adapters. I had line voltage on the brain when I selected that part.

I looked at Mouser and they start at 80VAC

http://www.mouser.com/Power/Linear-Switching-Power-Supplies/Linear-and-Switching-Power-Supplies/_/N-8s82p/

Same story at Digikey.

Do you have any quick thoughts on a tiny transformer?

Thanks.

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.