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I want to power an Arduino from the available 24VAC by using a bridge and caps and LM7805.  I don't want to use a little power supply because there is no AC outlet available to plug it into.  Splicing in two lines from the 24VAC line is easy.  Low cost and simplicity is important.

My question is:  Is it wise or good practice to use a voltage divider on the AC side of the bridge to knock down the voltage so the 7805 doesn't have to get rid of so much? 

Does the divider represent a constant load that would not be there if I just let everything go across the bridge and let the 7805 handle things?



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My question is:  Is it wise or good practice to use a voltage divider on the AC side of the bridge to knock down the voltage so the 7805 doesn't have to get rid of so much? 
No.   Voltage dividers don't work very well for power, because they waste power and the voltage drop varies with load impedance/current.

However, you could use a 7812 (12V) along with the Arduino's on-board regulator.

With the approximate 1.4 times voltage "boost" you get from the rectified 24VRMS, you'll be coming close to the 35 or 40V maximum rating on the 7805.     So, make sure to check the specs on your particular regulator and measure the rectified DC voltage.
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I just looked and both the 7805 and 7812 say 35v max, and when I hooked up the caps and bridge I got over 40v with no load.  Is there an easy way to knock this down? 
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Maybe a tiny transformer.  What is a signal transformer vs a power transformer?

I guess a transformer will also represent a constant load?
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Given the large amount of voltage you want to drop, the best solution would be a switching regulator, although finding one that goes above 40V may be difficult. I found one that goes up to 40V at http://www.ebay.co.uk/itm/3-Terminal-5V-1A-Switching-Voltage-Regulator-Power-Supp-/390327745637?pt=LH_DefaultDomain_0&hash=item5ae158bc65#ht_3081wt_1163.
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I just looked and both the 7805 and 7812 say 35v max, and when I hooked up the caps and bridge I got over 40v with no load.  Is there an easy way to knock this down? 

It's crude, but how about half wave rectifying the 24V AC? If you're using a switching regulator, I don't suppose it'd be too fussed about the waveform.
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although finding one that goes above 40V may be difficult.

They all use the simpleswitcher, with a maximum rating of 45v.

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I found one that goes up to 40V at

That actually is quite cute. Without all the hassles of a potentialmeter.
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That switching regulator is the neatest thing I have seen for a while, thanks.

I plan to try half wave and see how things look.

Thanks everyone for the input.
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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.
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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.
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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.
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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.

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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.

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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.

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

http://www.mouser.com/ProductDetail/RECOM-Power/RAC01-05SC/?qs=sGAEpiMZZMuEhFEw3BeJbWBvA%252bw%2fvMYdKI7FscCidR4%3d
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