Building a 120 VAC power supply for my SX.

edlikestoboogie

Sorry, but I don't know enough about electronics to design a power supply circuit to power my SX 28. I need a very small circuit

to convert 120 VAC to DC 5V, with just enough ma to power up the SX28 and one solid state relay.

http://www.allelectronics.com/cgi-bin/item/SRLY-25/500500/10_AMP_SOLID-STATE_RELAY,_USED_.html


How much power does the SX actually use up, with the relay, I can't seem to figure this out. Would it save space in my design to build a

transformer circuit that exactly fits the amount of power needed? i.e. Smaller transformers?? I need the entire circuit, power

supply, sx28 and relay to fit in a small plastic enclosure, thats why I can't use a regular wall wart.

bennettdan

This is the smallest transformer I could find it has .3 amp output.
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=102517&pa=102517PS
then you can use a bridge rectifer to convert the AC to DC
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=178183
Then use something like a LM2937 to regulate the 5vdc
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=297810
And if you read the datasheet that comes with the LM2937 on page 6 at the bottom shows you a schematic to make a regulated voltage.
http://www.jameco.com/Jameco/Products/ProdDS/297810NSC.pdf
I hope this helps..

Guenther Daubach

At which clock frequency are you planning to run the SX? The SX power consumption is almost a square function of the clock frequency. IOW, you can dramatically reduce it with slow clock rates, like 1 MHz, or even less. You may also consider to reduce the supply voltage from 5 to 3.3 Volt which is fine for the SX, and still high enough to drive a solid state relay.

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Greetings from Germany,

G

LoopyByteloose

You can easily purchase a 5 volt wall wart at 1000ma or less that is rated for digital electronics.

In general, the wall wart will be smaller than anything you can build with discrete components and is likely to be much cheaper - the tranformer and all the circutry is optimized for size. About the only exception would be a 'transformerless design', but not using a transformer creates a very serious safety hazard in the form of electrical shock.

If you must, you can hack off the AC plug ins and solder an electrical cord. Similarly, you can eliminate most of the cord on the DC side.

The solid state relay is likely to use less than 10ma on the SX side as it usually just turns on an optoisolator's LED. You can easily get by for less than 100ma total power supply, but it is really hard to find anything less than 500ma in a wall wart or transformer. I suppose you could wind your own, but would it really be smaller? Where would you get the components to do so? That Jamco 300ma transformer is quite unusual. Don't forget your rectifier bridge and filter capacitors.

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

edlikestoboogie

I am using the SX with the lowest internal clock setting, I think it was 4 khz, should use minimal power that way. bennetdan.. those transformers, are way too big. I use to buy these wall transformers that allelectronics.com sold for 3 dollars a piece. They layed an output of around 5v at around 100-200 ma and they were really small. You know the typical tiny little wall warts that are the size of a zippo lighter, that usually power up and charge your cell phone. I would crack them open and integrate the small circuit into my own. The actual coil transformer was literally, smaller than half my thumb. Anyone know any surplus places that still sell these type of wall warts for < $3. I should have bought a bunch of them while I had the chance.

Gunther mentioned that I can operate the SX at 3.3v.. How about 3v? http://www.allelectronics.com/cgi-bin/item/DCTX-31/815/3VDC_100MA_WALL_TRANSFORMER_.html At $1.25 a piece, not bad!, but only if it would work with the SX. This one is a little bit bigger than the one i mentioned, judging from the reference I am making with the plug it has,, but its dirt cheap..

edlikestoboogie

I found a samsung cellphone charger that is incredibly tiny and really cheap, < $1. But the rating on it, is 0.7 A at 5v. Is there anything I should be concerned about with using this type of transformer circuit? Remember I need it to run the SX and a relay, and who knows, maybe more relays, as my projects grow. What are the fastest oscillations I can run the SX with only 0.7 amps?

Guenther Daubach

Long time ago, I posted diagrams showing the SX supply currents vs. clock frequency. Before searching for that old thread, I attach the diagrams to this message. These diagrams are the results of my own measurements, and I think they are pretty accurate. As you can see, 0.7 A @ 5 V is good enough for many SXes running at 4 MHz internal, as each would only consume around 10 mA.

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Greetings from Germany,

G

CCraig

Almost 3/4 of an amp is a lot of power (for me). Unless you are powering a bunch of LEDs or a light blub, I don't think you'll run out of current.

HTH, Chris

LoopyByteloose

Simply put, everyone would have to just guess.
My guess is that you will use far less than 100ma [noparse][[/noparse]Guenther says 10 ma is a likely right], unless it is a huge relay. Likely 10ma for the SXes general functioning and 25ma maximum for the one relay pin.

The way to accurately find out is to build the device and measure the current draw during actual use.

Of course we all understand that you might want to skip that tedious step. That is normal human nature.

In this case, I truly belive that you won't find anything in a 120VAC supply that provides under 600ma because most if not all these supplies are intended to regard lithium batteries. There is not much reason to build a small supply as it would just slow down charging and the size is about as small as it can physically get.

So why worry?

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan