Can't find right transformer.

Other Hardware General Electronics
1

So I am building up my first stand alone Arduino project which will need a 5VDC power supply. The project controls 120VAC so powering it up off a wall wart is stupid. Going to do it on board. Best estimate i can guess is that i will need about 300ma current so I plan to build in 800ma to 1000ma capacity just in case I am way wrong. Inefficiencies etc etc. After much reading and video watching I bought 161K12 Power Transformer from Hammond Manufacturing. I don't know what i was smoking, this thing is rated for 10VA. That's way more then I should need? Was thinking more like 3-4VA to be on the safe side? Anyhow it's huge and heavy. So I looked on Mouser for a smaller one. I can find some that are about 4VA but only 6.3 Volts Out. I think that is not correct. What I want to know is what the heck are the Chinese doing inside there little tiny light 5vdc 1000ma power supplies. They feel like all air. I would cut one open to see but I need the one I have for a customer and the rest are all in use. Are they Non Regulated? Linear? Switch Mode? I don't even know. I figured they would just be an unregulated piece of crap. All the tiny transformers I find are rated for audio and on 30volts or so. Some one please give me a clue. No, I don't want to just buy a wall wort and steal the parts every time. I want to just be able to order 10 or 20 small transformers and rectifiers and use them in my designs. Hope someone can help.

Update:
I found a 3.3v 300ma wall wart I didn't need very similar in size and weight to the 5vdc one. So I looked inside. It looks like a linear regulated type just with a super small transformer.No writing on the transformer. Where can I get these small transformers rated for 155vac on the primary please? All I can find online are old school iron core clunky ones.

Thank You

2

maybe use 2 or more wall warts in parallel, each powering a part of the electronics?

maybe describe your project more (post some schematic) and we can help you estimate...

3

Get one of these and take it apart, add the circuit PCB to you project.
AC Input: AC 100 - 240V, DC Output: DC 5.0V, 1A.

HERE

4

Switch mode is the only way to go these days, no good reason to build a linear supply unless it is for analog circuits and even then you can still use a switcher in most applications.

Those Chinese Apple knock-off 5 volt USB white cubes are absolute trash and are downright dangerous! No way would I ever use one of those little bombs. Dave Jones over at EEVBlog has a tear-down U-Tube video comparing the knockoffs to the real deal. Frightening, really, the design is that bad.

Buy yourself a decent CUI 5 volt wall wart from DigiKey and just put it inside your project box. Bust it open and just use the board if you want. You can also find decent supplies used for current cell phones. I have about four Motorola Razr chargers that are very nice, name brand 5 volt, 800ma switch mode supplies inside the the little black box.

5

jarrod0987:
I bought 161K12 Power Transformer from Hammond Manufacturing. I don't know what i was smoking, this thing is rated for 10VA. That's way more then I should need?

Which is perfectly reasonable. I hope you do not have any misguided notion that it will supply "too much" power. The advantages to using this transformer will be that it will have much better intrinsic regulation than a smaller one - the voltage you get will be much less dependent on load.

Obviously you would use it in "full wave" mode with the centre tap ground and each end to a rectifier, thence to a capacitor (must be rated at 105°). On no load, it will produce 11 or 12 volts but drop when loaded - to a much lesser extent than a transformer half the size. It will run cool and have a very long lifetime.

jarrod0987:
Was thinking more like 3-4VA to be on the safe side? Anyhow it's huge and heavy. So I looked on Mouser for a smaller one. I can find some that are about 4VA but only 6.3 Volts Out. I think that is not correct.

You would need a bridge rectifier, you would lose another volt, the regulation would be much poorer. Note I say "regulation", I do not mean you would get a constant voltage but it would be suitable (with a capacitor) to fed Vin on the Arduino.

jarrod0987:
What I want to know is what the heck are the Chinese doing inside there little tiny light 5vdc 1000ma power supplies. They feel like all air. I would cut one open to see but I need the one I have for a customer and the rest are all in use. Are they Non Regulated? Linear? Switch Mode? I don't even know. I figured they would just be an unregulated piece of crap.

Switchmode supplies are generally regulated. If they advertise 100 to 250V input range, they obviously must be. As to "crap" (which is more often than not, correct and selling them on to a customer is very unwise :astonished:), just one significant quality problem is whether they include an over-voltage "crowbar" to protect the following equipment in case of a failure which prevents them regulating. Most do not.

jarrod0987:
All the tiny transformers I find are rated for audio and on 30volts or so. Some one please give me a clue. No, I don't want to just buy a wall wart and steal the parts every time. I want to just be able to order 10 or 20 small transformers and rectifiers and use them in my designs. Hope someone can help.

The transformer you have is appropriate. A 5V (fully) regulated switchmode supply from a reputable dealer/ manufacturer would clearly be a better option in terms of efficiency, being ready-built and possibly even with a protective casing. It may be cheaper, or may not. If you are manufacturing things for others (and arguably for your own use), a mains grounded metal (fireproof) housing is the most sensible.

6

Hey Jarrod;

CUI makes a molded AC/DC/converter for board mount (is that what you are after)?
http://www.cui.com/product/resource/pbk-3.pdf
It still requires some external caps, etc. but it gets you a switcher without all the work.

If you're still wanting to go the 60Hz/transformer route, check DigiKey. They stock Signal Transformer. I'm concerned about your choice of 12.6V for the transformer. If you work through all the scenarios of a 115V line, you will find you want about 10V before your regulator. Now if you don't want a 7805 Linear thing there, Recom Power makes some nice replacements. I think an R-785-1 gets you a 5V, 1A DC/DC converter. Hope this helps. --Rob

7

Speaking of CUI, for about the price of that transformer you could have purchased this (or something similar)...

...instead of spending hours researching, designing, and building a custom power supply.

...I need the one I have for a customer...

Are you willing to take on the liability of selling a line-voltage device without a UL stamp? I suspect your insurance company will not assume that liability.

8

I do like these little rubber covered pre made converters. Unfortunately all the ones I found for sale were around 30$. Way overpriced. Maybe there is a more reasonably priced one somewhere?

I am Fascinated by switch mode supplies but they seem very complex. I want to learn all about them. The only book I have is from Radio Shack and is very old. It's only Schematics seem to say that Switch mode comes after the Transformer and show large transformers. The Whole point of the switch mode is to use a smaller transformer and get more efficiency I believe? Can someone point me to a quality Schematic and/or learning material for modern switch mode design ? I would like to really understand this wonderful improvement much more. Google/youtube is not working for me in this case.

Thank You.

9

With the resistor you are assuming the voltage will be pretty close to 2.2V and the reisstor will limit the current to 20mA, or close to it.

Umm ... had you looked at the link I gave above, it's the board-mount version which is UL and CE listed according to the data sheet. He already has one AC feed and wants to "embed" the power supply. It's already designed for you.

10

the transformer you would want if you insist on going that that path would be based on how you want to use it.

if you want to deliver a 5 volt to the arduino, then you need to do some calculations

for a transformer, you get a drop down of voltage. then you add a couple (four) diodes to create a full wave bridge rectifier.

then add some caps to handle the ripple. alas, AC offers RMS voltage so your rectified voltage will be about 1.41 higher than the AC voltage before the DC conversion.

so, your 5volt output would drop by the diode by about 0.7 volts, then multiply that times 1.41 and you get just over 6 volts. not high enough to feed the on-board voltage regulator, but too high to feed the arduino directly.

as many have said, get a wall wart, cut it apart, use the guts to make your power supply and for the little coin it costs, your device will be well engineered.

11

I'm not going to use the whole Arduino, Just the Atmega. I am playing with different supply designs until I find the magic number to give me a stable 5vdc the Atmega whitepapers say that it needs. I am still in the learning process about all the little things that change your voltages up and down when building the supply. Part of why I'm doing this is to learn. Not just to save money. I want all my components on board 1 PBC. Not gluing some tiny board to another board/project wall etc. Why? Just because :slight_smile: Seems nicer to me and I want to be proud of my work. Not look at it and feel like it's a hacked together piece of shit made of simple little boards someone else made that I was to stupid to do myself. I don't care if it costs a few dollars more. However, I do care if it costs 30$ more :slight_smile: That is just bad business.

Thank You.

12

Jarrod;

The link I posted is a 3W, 5VDC switching supply that runs from 85-264VAC. It runs $12.99 at DigiKey. This approach has the advantage that you are buying a device with UL and CE certification which may be important depending on where your market ends up.

Another approach is the transformer/cap/rectifier method you started with. Signal Transformer used to have an app note that took you through all the scenarios of a 115V line's highs and lows. My recollection is that a 10VAC secondary is about right. Anything higher makes you drop and dissipate a LOT to get to 5V.

Transformer: http://www.digikey.com/product-detail/en/ST-5-20/595-1173-ND/953043
Diodes (unimportant); 4x 1N4004 or a 400V, 1A bridge.
Cap: As much as you can fit on the board, 10,000uF, 25V http://www.digikey.com/product-detail/en/ECO-S1EP103BA/P6886-ND/131955 $2.40
Regulator: http://www.digikey.com/product-detail/en/R-785.0-1.0/945-1038-ND/2256218 $7.49

So the "roll your own" method will cost more, but it's your project. You don't have to use that Recom part for your regulator, any 7805 will do but then you will need a heat sink good for 20 watts or so.

13

12$!!! Sold :smiley: