transformer load tester

Discussion in 'General Electronics Chat' started by adamclark, Aug 13, 2014.

  1. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    Hello, How can I accurately measure how much current my transformer can make? I have a transformer I found that im using in a PSU project in another thread and I would like to measure how many amps each coil makes.
     
  2. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    I should add that I have 2 good DMM's and one has an oscilloscope in it. And I have a velleman handheld O-scope that has only 1 lead.. I just got the velleman and im not 100% sure what all the features do as I have never used one before (o-scope I meant) Thanks
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    Post a photograph and give us a reference to the size of the transformer.
    Transformers are rated by VA, volt-amps.
    What is the RMS AC voltage on the secondary winding?
    From there we can figure out the kind of load you can attach to the secondary.
    I would use a power rheostat if you have one handy.
     
    Last edited: Aug 13, 2014
  4. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    heres a couple pics.. I measured it and its 4 inches tall 2.5 inches deep on the frame and 4 inches deep to the very edge.. and its 5 inches wide. it has 3 outputs one has a center tap. the center tapped coil measures 22.5 on each leg from the center. after I put the recitifier on it and 10kuf 50v filter caps it measured 30.04vdc. the second coil measures 8.1vac, I was going to use this one for a 5v regulated supply. the 3rd one measures 38.5vac, im not sure what im using this one for, I was thinking a regulated 12vdc supply but think that the lm317's im using will get to hot. I don't know anything else about it except it came out of a Marantz power amplifier.. I hope I gave you what you asked for.. and I don't have a rheostat.. Ive got a lot of parts and motors (dc and ac). Is there another way to measure it without the rheostat?
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    Do you know the model of the Marantz amplifier?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    You will need some loads. How many 12V automotive headlamps can you get your hands on?
     
  7. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    as many as I need.. have a drawer full of them at the shop.. mostly for newer cars so there just the headlight bulbs instead of the older big square glass ones.. I believe I have some fake hid types that are between 70 watt and 95 watt for the bright side.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    Good.

    For the 8VAC winding, put one 12V bulb across the winding and measure the voltage across the bulb. Add a second bulb in parallel with the first bulb and measure the voltage again.

    If possible, measure the AC current through the winding for both cases.

    NOTE: Cover the lamps with a shield and don't look at the lamps. They will be very bright.

    Don't touch the lamps until they have been given enough time to cool. They will be very hot!!!


    For the 22 + 22 VAC windings, you will need to wire 4 lamps in series for your load.
     
  9. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    good advice.. thank you much. I will grab a handful of them tomorrow and do some measuring and report back.. I am able to measure both voltage and current at the same time with my meters. to measure the ac current where would I place my meter? do I use a rectifier or hook the lamps right up to the windings (ac voltage on a vdc headlamp?)
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I've fixed (and seen inside) hundreds of Marantz units. Based on the transformer dimensions and appearance it's about 100 W (total for all windings). Maybe 120 but 100 is a safer guess.

    That is the main winding, which was used for the power amp rails. It will be about 80% of the total coil area so is about +/- 24v DC at approx 80W.

    Maybe good for 1 amp. 500mA is safer. Was probably used for the HiFi/display microcontroller and 5v logic etc.

    That was probably used for the VFD display which need a low current rail of about 50v. That winding will not be good for any significant current. Maybe 200mA tops(?) but maybe as low as 50-100mA.
    :)
     
    adamclark likes this.
  11. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    For the time being, just testing the amperage of the windings, don't worry about DC voltages. Skip the rectifier and measure AC voltage and current.

    The current meter goes in series from one wire of the winding to the load.

    Use one meter set to measure AC VOLTAGE and use this to measure across the windings.

    Use another meter set to measure AC CURRENT (20A range) and use this in series with the windings.

    When ever you are finished using a meter that was set to read CURRENT, set it back to the highest range AC VOLTAGE.
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    There, RB has provided you with some reasonable numbers.
     
  13. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    Thanks,, that's good reference.. Ill have some actual number from it tomorrow and ill post them up... I have a feeling im about to learn some math on this one..lol..As always, thanks for helping me learn. That's a big part of the reason I chose to build my PSU instead of just buying one to suit me.... this is awesome and I hope My PSU turns out badass...
     
  14. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    a quick ? for you.. should I fuse the transformer before testing it? What amp should I fuse it?
     
  15. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    You're only testing for very short periods and you don't expect catastrophic failure.
    If you must, a 1A fuse should be enough. That would give you up to 120W at the primary.
     
  16. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    Thanks, ill be home from work in a couple hours. Ill be testing then. I just don't want t hurt my transformer.. it took me awhile to find that one. I have 6 other ones, but this one seemed like the best one.. not the biggest though. I have 2 big ones from microwave ovens, but the ac voltage on the secondary is really high..
     
  17. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    ok,, so I started getting ready to test the transformer, I set the load and put my meters on it.. when I turned the switch on, I started to get readings then everything stopped.. I thought I might have hurt the transformer as I couldn't get any voltage from it. bur further testing showed no ac at the switch, then I decided to test behind the switch, I should have seen 120vac from the cord going straight into the wall outlet but there was none.. the outlet tested fine, so I believe the problem is with my poert cord,, so im currently setting another power wire up.. hopefully ill have some results soon.
     
  18. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    OK all,, ive got it working again (molex plug failed between the mains and the primary), so I put 2 lamps on the sec. windings and the ac amps and ac voltage meters are taking measurements. so far im only putting 360 watts of bulb to it.. I have 3 extra lamps for further current if needed.. what exactly am I looking for in measurements? I noticed on the main sec. im using when I put the bulb on either side using the center tap as a common, I got 8.6a ac ,but my voltage dropped to 16vac from 22.5vac unloaded.. I can measure all the sec windings like this, but im not sure what recordings to take? do I keep adding lamps to find the current limitations and the voltage it happens at?
     
  19. MrChips

    Moderator

    Oct 2, 2009
    12,423
    3,357
    You are looking for the voltage drop from unloaded to loaded.
    22V to 16V @ 8.6A you say? That is a reasonable test. That's about 140W.

    How hot does the transformer get after 5 minutes?
    If the transformer is getting uncomfortably warm then you are overloading it.
     
  20. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    it doesn't even get warm at 5 minutes with just 2 180 watt bulbs on it.. should I add more? should I test with all 3 sec. windings with 1-5 bulbs and post results? Is heating up the transformer part of the goal? I cant exactly remember exactly,, but playing around when I first started, the 8.1 coil gave me 3.7a with 4vac... but that's not completely correct...
     
Loading...