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Power supply

Jay KickliterJay Kickliter Posts: 446
edited 2011-03-04 21:58 in General Discussion
Normally, I ask questions here to help me in my design research. However, I'm three weeks away from a deadline and just need to be told what to do.

I have a mostly-working Propeller based true RMS power/energy meter; I'll release details on it when it's fully finished. Now I need to get it off the protoboard make my own board and power supply. I need to use this 9 VAC transformer, rectify it, and feet it into a 3.3 V regulator. The circuit, with the Proto Board's regulators bypassed and hooked up to a bench supply, is drawing 90 mA, with an occasional spike to 200 mA when the XBee is transmitting. However, the bench supply isn't showing the spike. Must be short enough for the protoboard's tantalum cap to handle.

Can anyone please tell me what bridge rectifier, input and output capacitors, and regulator to use?

Comments

  • ercoerco Posts: 20,260
    edited 2011-03-01 09:04
    That XFMR link is dead... no parts found.

    See http://my.integritynet.com.au/purdic/power1.html and http://www.zen22142.zen.co.uk/Design/dcpsu.htm and http://www.electronics-project-design.com/PowerSupplySchematic.html

    for some specific parts recommendations.
  • kwinnkwinn Posts: 8,697
    edited 2011-03-01 11:04
    Jay, for that small a power supply I usually take apart an old AC adapter from my junk box for parts. This has to be one of the older transformer based ones, not the newer switching regulator ones. If you only need 3.3V I would suggest a 5-6V rather than a 9V one. A 470uF capacitor on the input and 10-20uF on the output should be more than adequate.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-03-01 11:17
    kwinn wrote:
    A 470uF capacitor on the input and 10-20uF on the output should be more than adequate.
    Just to clarify, you did mean the input and output of the regulator, not the transformer, right? :)

    I agree that a 9V transformer has a much higher output voltage than is needed to drive a 3.3V regulator. Full-wave bridge-rectified, 9 VAC will turn into more than 11 VDC, including diode drops. Of course, if that transformer is center-tapped, you could do a full-wave rectifier with two diodes and achieve a much more reasonable output level.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2011-03-01 11:49
    Different regulators have different requirements in terms of input and output capacitors. The data sheet will specify suitable values.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-03-01 12:07
    In this case, you can safely ignore the regulator's input requirements, since the cap needed for 60 Hz ripple filtering will be much larger than any minimum value spec'd in the regulator's datasheet. The exception to this is if the regulator's input cap also has a specified maximum ESR, in which case you may have to add a ceramic or tantalum cap in parallel with the electrolytic filter cap.

    This assumes, of course, that the filter cap and regulator are proximal to each other on the same PCB. If not, an additional cap, per the regulator's datasheet, will be needed near the regulator.

    -Phil
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-01 15:48
    Ok, thanks for the pointers. I found a 6 V transformer I can use. Still picking a voltage regulator. Might just copy the Proto Board's 10u tant -> LM1086 -> 10u tant.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-03-01 16:01
    Jay,

    You need a much bigger post-rectifier filter cap than the 10 uF regulator input cap to filter out the 60 Hz ripple. 470 uF should do; 1000 uF would not be excessive. The wall wart power converters that you're accustomed to using have those caps built in.

    -Phil
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-03-01 16:07
    Why not stay with the wall-pack, unless you like head-aches, problems, etc.?
    [They're UL-recognized, you don't have to fuse the AC, and all the rest of it.]
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-03-02 04:27
    Wot PhiPi and PJ Allen said...

    Re the rectifier, 1A bridge rectifiers are bog standard.

    I like wallwarts as you don't have to worry about wiring mains, mechanical strain relief on the mains wires, mounting the transformer etc. You might even be able to find a 3.3V switching regulator wall wart. Certainly there are 5V ones around.
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-02 04:51
    I forgot to mention why I can't use a wall wart. Firt, this circuit is meant to attach directly to a breaker box, even if only as a demonstration for now. Also, we're getting very accurate current measurements (true RMS, sampled at 6 kHz), but the only missing piece is voltage readings. I blew a Vss trace on my Proto Board when I hooked up mains to a voltage divider, even though my board was plugged into a wall wart. They weren't isolated for some reason. Stranger, there's a 50 V differential between Vss and the neutral line at my school. I'd be happy using a wall wart for the time being, if someone could reccomend one that's definitely isolated from mains. But, in the long run, I'm going to have to built my own supply since there typically isn't an outlet near a breaker box.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-03-02 05:06
    Check for an "open circuit" between either output lead and either prong; that's not a test for hi-pot, but it'll indicate a problem.
    [If this stuff was easy then everyone would do it.]

    http://en.wikipedia.org/wiki/Hipot
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-02 05:34
    PJ, not sure what you mean. Are you suggesting I megger my wall wart?
  • LeonLeon Posts: 7,620
    edited 2011-03-02 05:50
    I think he meant it as a test to see if the output is isolated from the mains ground.
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-02 06:58
    Leon wrote: »
    I think he meant it as a test to see if the output is isolated from the mains ground.

    In that case, it's not. Don't know why.

    Edit: actually, I am reading open from the output of the wallwart its neutral and hot prongs. But I'm also reading 50 V from both output polarities to the hot AC line.
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-02 06:59
    You guys got me thinking, though. For now at least, I can continue using a wall wart and use the 115V->6V transformer to step down and isolate mains and measure that. Would probably be easier.
  • kwinnkwinn Posts: 8,697
    edited 2011-03-02 12:38
    Jay, you know you can use the ac voltage from the power supply secondary winding to measure the line voltage. Quite a few of the instruments I have worked on have done that, and it gives a reasonably accurate (1% or better) result.
  • kwinnkwinn Posts: 8,697
    edited 2011-03-02 12:39
    Jay, you know you can use the ac voltage from the power supply secondary winding to measure the line voltage. Quite a few of the instruments I have worked on have done that, and it gives a reasonably accurate (1% or better) result.
  • Jay KickliterJay Kickliter Posts: 446
    edited 2011-03-04 06:15
    kwinn wrote: »
    Jay, you know you can use the ac voltage from the power supply secondary winding to measure the line voltage. Quite a few of the instruments I have worked on have done that, and it gives a reasonably accurate (1% or better) result.

    That's what this guy did. However, I don't understand his logic. He's doing half wave rectification and says to use the minimum capacitance on the rectification filter, so as to not distort the pre-rectifier waveform. Is that right? It seems to be that the reverse would be true. My transformers just came in the mail yesterday so I haven't had a chance to breadboard anything yet.

    However, I hate to admit I might have wasted everyone's time. We got the circuit to work with a wall-wart and direct 120 VAC voltage divided to 3 VAC. BUT, neutral has to be tied to Vss. We immediately blow a fuse when we hook 'hot' up to the Protoboard's Vss. Seems to me that it shouldn't matter. This makes me worry about what would happen if we hook it up to an outlet that was wired in reverse. I only have one shot to have a PCB expedited in time for the trade show (we won a scholarship for this project and get a booth at an upcoming utilities trade show). I'll design a board over the weekend to be re-configureable as possible, with either wall-wart or native power, and either divided 120 V or post transformer voltage sense.

    Thanks for all the help guys, I don't know what I'd do without you.

    Attached is a screen shot of our remote monitoring application. The plots from top left CCW are voltage waveform, current waveform, and power(t). The clipping in the current waveform is because for now we're limited in current sense range, the iron plugged in was drawing about 11 amps, but our circuit pegs at 10.6 amps. Later we can expand the range by switching in different burden resistors for the current transformer, or changing the gain on the op amp buffer.
    Energy Meter.jpg
    1024 x 556 - 40K
  • kwinnkwinn Posts: 8,697
    edited 2011-03-04 21:58
    Jay, the approach to monitoring the AC voltage from the secondary is fairly straight forward and usually based on:

    1 - The load on the power supply transformer is relatively constant.
    2 - The secondary voltage varies in direct proportion to the primary voltage.
    3 - A single diode will charge small capacitor to the peak value of the AC voltage.
    4 - An ADC will measure the voltage and a micro will calculate the line voltage

    The typical calculation is " a + bx ", where a is a constant offset, x is the reading from the ADC, and b is the multiplication factor required to produce the corresponding input voltage.
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