transformer basics

Discussion in 'General Electronics Chat' started by adamclark, Oct 30, 2013.

  1. adamclark

    Thread Starter Member

    Oct 4, 2013
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    hello,, Im currently selecting components to build my new power supply. Im trying to choose the transformer for it and I have a good selection of scavenged transformers. the only problem is that none of them have any markings so I cant find any info on them. my goal is vairiable 3.5v,5v,9v,15v, and 24v with at least 2 amps of current available.. How do I find out what voltages and amps my transformers are? some of them come from old wallwarts and when I plug them in to the wall and measure them most have between 7 and 12v AC.. is that normal? I thought that they had ac input and dc output. Im still pretty new at this and this will be my first major project ive done. Ive found some schematics and most use the lm317 as the voltage regulator and the dats sheet on those says the 317 will only do 1.5a max so whats a better choice? I just need some beginner info asap to base my choices on on and go from there. thanks
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Wall Warts come in all flavours, they can be AC OR DC output, older ones are often the unregulated type and relied on the rated load to bring them down to their rated voltage.
    Modern types now are generally regulated by a switching supply rather than linear type.
    These output the rated voltage exactly.
    The mains transformer in many of the linear type are not that high a va rating, with typically just a a couple of amps maximum.
    Max.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Transformers change AC voltage. A rectifier (bridge diode or pair of diodes) flip the negative-going half of the AC signal (pulsed DC). A capacitor from the diode outputs to ground will filter the pulse to a DC with a bit of a sawtooth on top of the DC voltage. A voltage regulator will remove most of the sawtooth (ripple) and output your desired voltage.
     
  4. adamclark

    Thread Starter Member

    Oct 4, 2013
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    thanks for the info.. my dmm has an oscope in it that also shows the wave when I measure voltage. so this will be interesting when I build my psu. any tips on how to choose a transformer? I have one that has 19.6v output with a full wave on the scope.. will 19.6Vac be enough to get 24vdc after the bridge?? and how would I be able to tell how much ampherage this transformer will make??
     
  5. BobTPH

    Active Member

    Jun 5, 2013
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    Most of your wall wart transformers are not going to produce 2A. Size is a hint at the current capacity. A 2A transformer is going to be about 2 to 3 inches across.

    As far as testing the power capability, you could try increasing loads until it starts getting hot, then back off a bit.

    Bob
     
  6. adamclark

    Thread Starter Member

    Oct 4, 2013
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    not all of my transformers come from wall warts, I have many from old TV's and radios and even microwave ovens... that why im asking so many questions about choosing one.. and im assuming that my current needs are going to rise as the voltage rises, I mean, I can see me needing 2 amps at 12-15 volts more than at 3.5v... also, how would I go about increasing loads on it until it gets warm with my meter attached?? a lot of newbie questions I know, but how else am I going to learn??
     
    Last edited: Oct 31, 2013
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Be careful with the TV and microwave oven transformers. Many are there to creat high voltages for the CRT. Those transformers transform to higher voltages.


    Most lab power supplies are 0.5 amps to 1.0 amp. You don't need anything too big if you are just starting out. Also, remember that voltage regulators like LM7805 and LM317 are linear regulators. Any time they drop the voltage down to a level you need, the POWER must go somewhere. That is, if it takes the voltage down from 24 volts to 5 volts and you try to run 100mA load, that means the load sees only 5 volts x 0.1 amps = 0.5 watts. However, the voltage regulator is seeing (24 volts - 5 volts) x 0.1 amp = 1.4 watts.

    HOMEWORK ASSIGNMENT
    Now, what happens if you want to run 2 amps? Do the calculation and then look at the datasheet to check max power dissipation. Also, look up the size of the heat sink you will need.
     
  8. adamclark

    Thread Starter Member

    Oct 4, 2013
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    so the transformer I think ive chosen for my psu is about 2 inches square and puts out just under 20vac.. what will that convert into in dc voltage? I think it will work nicely for what I want. Ive read a few articles where the guy is using leds off of the filter caps to use extra voltage. does this help with the heat or does it do anything at all except light an led up?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Transformers are rated in Va or Kva depending on size, the Va is largely a factor of core size, the wire size also has to reflect the current required from the winding.
    For example, if turns are add to a secondary winding in order to obtain a higher voltage, the Va remains the same, so the current demand has to be lowered in order to stay within the Va rating.
    20vac results in 20x1.414 =28vdc if a smoothing capacitor is added, allowance should also be allowed for the ~.7v volt drop of the rectifiers used.
    Max.
     
  10. #12

    Expert

    Nov 30, 2010
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    Quickie: 19.6 VAC RMS times 1.414 = peak voltage. That's 27.7 volts. The rectifier will use up about 1.5 volts and you have 26.6 VDC on the filter capacitor. You need about 4000 uf per amp to keep the sawtooth down to 1.5 volts loss between power pulses with a full wave rectifier (that makes 120 Hz.) Your regulator will waste a minimum of 1.5 to 2.5 volts, so, yes, you can get 20 volts DC if you design it right.
     
  11. adamclark

    Thread Starter Member

    Oct 4, 2013
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    that's what I was looking for,,, looks like ive found my transformer.. now what would be an easy way to test the ampherage of it?
     
  12. adamclark

    Thread Starter Member

    Oct 4, 2013
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    I plan on using 2 lm317's paralleled with a big heat sink to increase amps, so do I just build it to be 20vdc and then work on reducing the voltage to each of the other voltages I want? these are going to be selectable by a switch btw... or do I build it with separate filter caps and voltage regulators to make it more efficient?
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is no definitive way, it is dependent on core size and wire dia.
    Two self evident signs of overloading are rapid warm up, hot to the touch, and or voltage collapse.
    Max.
     
  14. adamclark

    Thread Starter Member

    Oct 4, 2013
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    so do I attach a large motor to it and increase power until it gets hot and measure the draw from that point or convert it to dc first then put it under load?
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Add the DC, the size of the capacitor (% ripple) affects the VA rating when running at the required load current.
    Max.
     
  16. #12

    Expert

    Nov 30, 2010
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    This is difficult, even if you're good at it. One method is to weigh the transformer and calculate the VA from a known transformer that is similar in build. The V you need to calculate the VA limit is the peak voltage...capacitor voltage + about 1.5V for the rectifiers. VA/Vpeak = A

    I don't like the heat method. I would rather watch the primary current on an oscilloscope and watch for the distortion of the magnetic field as you overload the transformer with resistors, light bulbs, whatever you have available. When you load the transformer enough to overload it, the magnetic circuit saturates and that causes distortion in the current wave of the primary winding.
     
  17. adamclark

    Thread Starter Member

    Oct 4, 2013
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    makes sense.. my dmm has an oscope built in to it so whenever I measure any kind of voltage it shows me the wave regardless... thanks much
     
  18. BobTPH

    Active Member

    Jun 5, 2013
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    This is a bad idea. Use an LM338 if you want more current.

    Bob
     
  19. adamclark

    Thread Starter Member

    Oct 4, 2013
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    ok so ive chose my transformer, now I need to make the 20vac output of it dc, so I gueass the next step is the rectifier bridge correct? heres one that ive seen in 90% of the schematics ive seen..So I plan on using this one for a starting point and changing it as needed for my application.. I also attached an image of the transformer I have chosen. What size diodes should I use? I have some 1/4, 1/2 or giant 1 watt ones.. does the saying "the bigger the better" apply to electronics? I mean would using the big ones add any benefit or hurt me?
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The transformer appears to be quite small, you will be very limited as to the current capability and keep in mind that regulators consume power also.
    It is a little observed fact that increasing the Capacitor size increases the Va requirements, if you aim for too low a % ripple.
    This is due to the heating effect of the pulse energy to recharge the capacitor, which increases with capacitor size.
    Max.
     
    Last edited: Oct 31, 2013
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