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Author Topic: Power from 24VAC  (Read 3492 times)
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Check out something like this:
http://www.hobbyking.com/hobbyking/store/__6320__TURNIGY_5_7_5A_8_42v_HV_UBEC_for_Lipoly.html
Used standard in the hobby industry, you can even find them up to 15 amp.
DON'T put AC current through it.
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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.


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?

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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?
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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?
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Won't it drop a few volts regardless of the current the regulator is or isn't drawing?
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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:
https://raw.github.com/rayshobby/opensprinkler/master/OpenSprinkler%20Controller/hardware/v1.4/OpenSprinkler_v14.png
look at the "power supply" section on the top-left corner.
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Have I lost the plot here?

The OP wanted to start with 24VAC feed it to a bridge rectifier to get 12 VDC (ish) clean it up with smoothing caps.

All of which is simple and cheap. Then use a LM7805 voltage regulator which gives a nominal 5v (+/- 0.2) as its output and is happy on a 12v input.

The total cost in the UK (inc 20% vat (sales tax)) about 2 pounds.

Mark
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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.
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