help with a pi filter

Discussion in 'The Projects Forum' started by lokeycmos, Apr 3, 2009.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    im building a high current regulated variable lab supply and looking for some help with filtering. Im using a hand wound microwave oven transformer that puts out 30VAC and TONS of amps. the problem is that the electrical noise and ripple repeatedly burn out the IC and Transistors.(this thing vibrates my whole kitchen table!! :) ) i attached a schematic. I found interesting info @ http://forum.allaboutcircuits.com/showthread.php?t=2687&highlight=ripple+filter

    I found a pi filter calculator too @ http://www.calculatoredge.com/electronics/ch pi low pass.htm

    I have no clue what values to put in, can someone please explain this to me? im assuming a cutoff value would be really low to filter off everything from just above DC and up. or can 1 stage be for 60hz hum and a second stage for ripple at 120hz?

    i dont have a true rms meter or an ocilliscope, but here are the specs: 120v/60hz in 30vac/60hz out of the transformer. 25 amp bridge rectifier, regulator is adjustable from 3-25 volts up to a max 15 amp load( ive added a 3rd power transistor in parallel to the circuit) Thank you so much! Joe
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    A bit more information about the transformer would be helpful.

    When you say "hand wound" does that mean you did this yourself. If it's making lots of noise and vibrating the table, I'm wondering if the design is suspect. Is it vibrating and noisy when not connected to anything on the secondary side - just powered by the 120V mains supply?

    For instance, there might be excessive primary magnetizing current. Are you able to measure the input current on no load? All transformers make some noise but something apparently as significant as your case is of concern.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If your regulator fries with no load, then something has to be very wrong. Either the filtered DC from the rectifier is more than 40 VDC, or there is something wrong with the stuff not shown - the transformer or the rectifier/filter.

    We need to see a schematic and have some information about the transformer rewind - number of turns on the primary, turns on the secondary, and is the secondary center tapped?
     
  4. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    Yes it buzzes at 60htz with or without a load. i attached a pic. i just cut off the secondaries and used about 32turns of 10awg wire no center tap. on the ohm meter the primaries measure .6 ohms. but i know that doesnt apply to an AC input.i also think there may be issues with an initial "surge" when plugging in the xformer. Right now i dont have ANY type of filtering. Just xformer > bridge rectifier > regulator. the rectifier is 50v 25amp. I think the reason it is audiubly loud is because the windings arnt perfect like a factory made xformer. unfortunately i dont have an AC ampmeter. regardless of what transformer i use, im going to need filtering. Not only will this be used to occasionally charge my motorcycle battery, but for digital electronics too, the better filtering i can come up with the better! ive attached the schematic on my first post and here is a pic of the project and transformer. (for testing and ease of use im just using 1 power transistor) I appreciate your help!
     
  5. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    Ive done the math......im going to need about a 2 farad capacitor for best filtering at 15 amps. Any ideas or suggestions where i can salvage capacitors that big(@ 40+ volts)?
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Back of the envelope ... - I think 20,000 to 30,000uF should be sufficient for max 25V @ 15A. You'll need more than 40 V continuous rating for the cap(s) as it's quite possible with 30V ac input you'll have a peak capacitor voltage of > 40 volts DC. If you can't get those cap. values you may have to put several smaller capacitors in parallel - as long as you have sufficient voltage rating. I suggest a minimum 50V - and that's marginal.
     
  7. watergroove

    New Member

    Apr 18, 2010
    2
    0
    Coming in late to this sorry. What needs to happen here is what you are missing on your lethal essentially infinite current input to the transformer. You are killing your system because of startup inrush currents. You also need to hardwire with a switch and a fuse the input to your transformer and protect these. Using these alligator clip cables is a recipe to burn your house down. Anything reactive has an inrush current. If you do not understand that you might want to review some on power supply design and AC theory. What you have is a little beyond what can be done in a simple forum such as this. You also need to do a power analysis to understanding what wire size you need. Looks to me you have built a very powerful 30 amp system here and those kinds of amps you need a separate 30 amp circuit like all built in microwaves need, and the right wiring in the wall to handle this. Do not assume a 20 amp lamp outlet will have the capacity to run your system here. When a circuit is too small in the wall two things happen, the voltage drops and the wire in the wall heats up. Also most if not all homes are built using wire nuts. Over time these can get corroded and high wattage things such as your power supply can cause arcing and arcing causes fires. I would run this thing with an isolation transformer so you isolate this circuit from the rest of the house circuits. That will also manage your inrush current better.
     
  8. watergroove

    New Member

    Apr 18, 2010
    2
    0
    You only need to look at the third harmonic not the second. Noise is odd harmonics only. You are better off looking at 180 hz as you will attenuate too much of your output at 120 hz. And since these elements are so large and with 5 to 20 percent tolerances, it is very difficult if not impossible to find that fc perfectly and you do not want to attenuate too much before the 3rd harmonic, all you end up doing is generating heat. Your output stages are good, you need a PI element post output to reduce ripple. You can put a coil and a cap L network into your input too. You need to bypass your regulator with a 10 uf or larger cap in parallel with a smaller 1 uf cap so your noise is minimized with loads you put on your PS. Remember noise comes from both directions. You need special ESD caps here and that is why your cost so far is low because all your filtering which is missing are the costly elements. Make sure you electrolytics are at least triple your 30 volts. Any inductive elements including your input transformer have kickback voltage that under the right circumstances exceed your 26 volt input. I would also look at a moderate impedance output to keep noise coming back from your load into your regulator.
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Awww, not that late... Whats a year and nine days amongst friends?

    Good addition though.

    While were at it I thought this was funny:
    [​IMG]
     
    Last edited: Apr 18, 2010
  10. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi,

    1. That transformer you have modified is not very efficient, because in a microwave oven application it doesn't need to be and it is fan cooled. It draws approximately 1.5 Amps even with no load. The laminations have been welded together, persumably to keep the noise down, but the extra core loses makes it even more in-efficient.
    2. The windings are side-by-side on the core, which is good for, low current, high voltage isolation, but the magnetic coupling is worse and adds to the in-efficiency for high current applications.
    3. A transformers no-load current is determined by the inductive reactance of the primary winding plus the mutually shared inductance of the secondary winding. Since you cut out most of the inductance, which was in the secondary side and replaced it with very little inductance, you now have a very in-efficient transformer drawing even more no-load current, because the mutual inductance is lower.
    4. When a secondary winding has high currents, and the magnetic forces are also strong and will move the wires around making a noise. They will flex and eventually break. Tie them well with suitable cord and/or epoxy resin to hold them secure.
    5. Fire hazard!: You used an in-appropriate wire with low temperature insulation that will fail when the transformer heats up after a long time in operation. Use proper magnet/transformer wire.
    6. ICs burning out? The 2N2222 cannot drive even 1 2n3055 when the output currents are high and neither can the 723. Replace the 2n2222 with TIP31 or equivilent. Put on heatsink.
    Play Safe,
    Ifixit
     
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