Power supply circuit question - Transformerless good idea? Still pass certification?

Discussion in 'General Electronics Chat' started by Mahonroy, Sep 22, 2015.

  1. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Hey guys,
    I am in the process of deciding on a good rout to take for the power supply for my project. I had a couple of questions about it.

    More info on the device: The device is for home brewing, its a standalone device that sits on the floor, shelf, mounted to a frame, etc. Its actually pretty simple - there is a power on/off, a screen, 3 buttons to interface the microcontroller/screen, a power cable, and that's pretty much it. The microcontroller uses transistors/relays to control a load (internal to the device) that requires 12 volts @ 20 amps.

    I have always just used power adapters/wall worts in the past for all of my projects, so this is a bit new to me. The highest current rated 12 volt power adapter I could find was @ 12 amps and it costs about 30 bucks (ebay). Here is a photo of it as an exampe:
    power_adaptor.JPG

    Something like this would of been perfect if it were offered as a 30 amp, but this doesn't seem to exist.
    So the next option is to use an actual 12 volt power supply, these are rated @ 30 amps and actually cost less (20 bucks). Here is a photo for an example:
    power_supply.JPG

    While this meets the specification, for a consumer product this just doesn't seem like the best idea, even if it were pre-wired (this would still need to be located outside of the device in a similar fashion as a power adapter would be, its a bit of an eye sore, and the electrical terminals are exposed - seemed like someone could get shocked?)

    So this brought me to my 3rd idea... what if I created a simple power supply that resided on the PCB inside the device? I found several plans online on how to make a transformer or transformerless power supply. The device has internal heat ventilation, so the added heat is not a problem. The problem I found with the transformer version was that the transformers are expensive - especially one that can handle enough current for 20+ amps. And to be honest, I actually had a hard time even locating an appropriate transformer for this. So I started looking into transformerless power supply circuits, and this seemed like the way to go.

    So it looks like these transformerless power supplies basically consist of a bridge rectifier circuit, paired up with some resistors, zener diods, and some even had a few IC's like the 78XX linear regulators incorporated, maybe some transistors, all of which could supply 12 volts @ 30 amps. It doesn't matter if the power supply is not very efficient, and that the power is not optimized and is dirty power. Its only going to be used to power a micro controller and the load, so dirty power doesn't matter (I would of course incorporate a few filtering circuits anyways).

    My main concern was mains isolation. With this device will it even matter? Would it be able to still get a certification? People aren't going to be exposed to any of the circuitry so it seemed like this would be ok. Mainly I wanted to get some of your opinions on this, or if there is a better idea that I should go with. Or if I should just go with the eye sore 30 amp power supply I mentioned above and have it sitting outside next to the device? Any help is greatly appreciated, thanks!
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,291
    1,255
    Transformer-less supplies are normally low current. Even if you could make one work at those current levels you could not have anything go outside the box that was not double insulated and have it pass certification.

    What is it? A heater?
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
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    You are going to get many opinions, and most of them will follow the same general lines.

    1. This will not work.
    2. It probably will kill or maine someone.
    3. You have no idea of what you are proposing.
    4. It will not work.

    Then the moderators will shut down the thread.

    A linear offline regulator will have to dissipate over 10 watts of heat for every watt delivered to the load. So you will have to get rid of over 3000 watts of excess heat. A 20 A regulator is far more complex than "maybe some transistors". A non-isolated design of the type you describe will *never* be certified by any agency.

    ak
     
    DickCappels likes this.
  4. peter taylor

    Member

    Apr 1, 2013
    106
    5
    I would go with the 30 A power supply.

    Mains isolation transformers (whether they are the basic 'big, bulky, expensive, inefficient and hard-to-get' ones used in linear designs, or the 'small, efficient, common and cheap' ones used in Switch-Mode power supplies), they are meant to do exactly what the name suggests.

    Electrically remove your circuit from every body else's on your street .

    That is, if there is a catastrophic failure in your circuit (on the secondary side of your transformer), it is unlikely to effect the primary / mains side. Because the primary and secondary winding's never touch, they offer a high degree of isolation (at least, to DC).

    This means that whatever your circuit does, it will not short out all the wiring in your house (if your house fuse is a three inch nail, it happens), but more significantly, it wont send some bazaar high energy pulse down your neighbors power box, setting their house on fire (unlikely).

    I have thought of running stuff directly off the mains. SAMSUNG are not allowed to, even though it would make their VCR's and Wide-Screen TV's a lot cheaper to manufacture.
     
  5. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Thanks for the responses.

    I'll try to answer all of the questions:

    1. If I were to build a transformer-less power supply, I would imagine it would go on the PCB and would reside inside of the devices enclosure along with the rest of the circuitry.

    2. I was trying to research more about non-isolated circuits passing certification, and so far from what I have been reading, there are a lot of misc. items on the market that are not isolated that have certifications. It seems one of the main factors is that the user doesn't need to normally access the circuit? Does that seem right?

    3. Would there be any problems with providing one of those switched mode 12 volt - 30 amp power supplies? Assuming the naive consumer had no problem? Other than it being an eye sore... could this be an acceptable solution?

    4. So here was an example of a high current transformer-less power supply:
    http://circuitdiagram.net/138-volt-20-a-transformerless-power-supply.html
    Couldn't a fuse or other safety precautions to added to this circuit as well?

    5. Lets assume I was to build a power supply with a transformer, here is an example:
    http://www.eleccircuit.com/high-current-12v-30a25a20a15a-ham-radio-power-supply/
    What would be a suitable transfomer for a power supply similar to this? This was the best I could find, am I on the right track?
    http://www.ebay.com/itm/1-20W-EI-Fe...r-/281091471814?ssPageName=ADME:X:AAQ:US:1123

    Thanks again!
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Where did you get the idea that this supply is transformerless? Just because they didn't show it on the schematic doesn't make it so...
    Where do you think they get the 19Vac?
     
  7. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Ah yep... I was too hasty with it... I thought all the way to the left on the schematic was 120VAC input, turns out its some 19.5 volt input... wouldn't work.
     
  8. siedeone

    New Member

    Sep 22, 2015
    3
    0
    That transformer will not work either as it is only good for 20w (1A@20v or 2A@10v) nothing like the 30A you want.
    Even a cheep PC power supply will do better these still use a transformer even though at a much higher frequency allowing it to be much smaller
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The circuit in #4 requires a 25 A transformer for its input. "The power transformer should be capable to deliver at least 25A at 17.5 to 20V."

    The first circuit in #5 is very poorly regulated. The output can be anything from 12 V to 13.8 V and will wander around as the load changes. Note that on the LM31x, 33x, and 35x datasheets there are application circuits for boosting the output current. These have been discussed many times in past threads on this forum.

    ak
     
  10. Brevor

    Active Member

    Apr 9, 2011
    279
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    Maybe there's a different way to approach your problem. I know a little about home brewing, my first question would be what are you powering at 12 Volts that requires 20 Amps? If it's just a heater the usual way to go is use a 120 or 240 Volt heater switched with a zero crossing solid state relay.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    May be wrong here but you don't give much to go on about the 'device'. I assume it is some type of a heating element that make it need 20amps. Why not use the 120V supply for the heaters? Then you only need a 12V low power supply to run the electronics. 120V heat elements are common and the way most products are made. More info on what is using the 20A would probably get you better answers.
     
  12. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Thanks again for the responses!

    Its not a heater, its powering a few TEC peltier chips, and they require a ton of current at 12 volts (anywhere between 10 and 15 volts would work).
     
  13. Brevor

    Active Member

    Apr 9, 2011
    279
    54
    I'm not sure if peltiers need pure DC, maybe a 20 Amp battery charger would work. I'm not an expert on peltiers but I dont believe there very efficiant.
     
  14. Mahonroy

    Thread Starter Member

    Oct 21, 2014
    190
    5
    Peltiers are horribly inefficient... which is unfortunate... but I still need to use it.
     
    Last edited: Sep 22, 2015
  15. siedeone

    New Member

    Sep 22, 2015
    3
    0
    for Peltiers needing 30A I would get a transformer for your 12volts rectify it with a diode bridge (4 terminal package) smooth it with a large capacitor and use it like that as any further smoothing will add losses that are unneeded
     
  16. siedeone

    New Member

    Sep 22, 2015
    3
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    note the output of a transformer is rated in RMS (ie root mean square) the peak output is somewhat higher by a factor of the square root of 2. 12Vrms has a peak voltage of about 17V.
    Also if you find the transformers are rated in watts you need a value of voltage times the current as a minimum and best to have a little higher rated than what you need (eg divide your need by 0.8 which will increase the rating needed). 12V times 30A =360VA then 360va/0.8 gives 450W this is the rating to aim for with the transformer (yes it is a big device, don't expect less than 3KG(or 7pounds(approx Imperial measure))
    After all this I bet that prefabricated power supply is looking like a good option and it probably is, as it is likely approx the same size.
     
  17. peter taylor

    Member

    Apr 1, 2013
    106
    5
    I kind of skipped through the last few posts, but wouldn't it be better to rip a 2000 W heating element out of an old jug, make it heat water, and use the steam to keep your yeast happily between 21 degrees and 27 ?

    A 250 volts mains relay (rated at I = 2000 W / 250 V = 8 A) driving the element would be safe enough.

    Any PIC or MPU would drive the coil as a thermostat, with a temperature probe joined to an analogue input.

    Your circuit is still fundamentally isolated from the mains, as long as you keep the relay switch and element COMPLETELY separated from your control circuit and relay coil. That is, don't even join the ground of your controller to the earth or neutral of the mains.

    And, whatever you do, think before you take advice from me ;)
     
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