Page 1 of 2 12 LastLast
Results 1 to 15 of 20

Thread: Basic electronics question: is it bad to have a transformer thatís too big?

  1. #1
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    1,386
    Blog Entries
    1

    Basic electronics question: is it bad to have a transformer thatís too big?

    My daughter has a bunch of lighting fixtures that each have two 50 watt, 12v PAR bulbs. Inside, there is a 100w toroidal transformer. They have been swapping in 7 watt 12v LED bulbs.

    I want to know if they are actually saving electricity using those low wattage bulbs with a high capacity transformer. My gut tells me Togo find a 15 watt replacement.

    now a related question. They replaced the high wattage bulbs with LEDs and ran it. Now, for some reason, the transformer is bad. There isnít much to these things. They are a doughnut shaped core wrapped with copper. Did running the LEDs lead to the failure?

  2. #2
    Wattage through a transformer is governed by the load it is supporting. An oversized transformer will not appreciably use more power than several small transformers supporting the same total load.
    Lee Schierer
    USNA- '71
    Captain USNR(Ret)

    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  3. #3
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    1,386
    Blog Entries
    1
    Lee,
    so, to be clear, there is no downside to having a 100w transformer powering a14 watt load?
    what about my second question. Was the failure of the 100w transformer related to driving only 14 watts?

    i am looking into 15w transformers and coming up empty.

    input: 120vac
    output: 12vac
    I found some smaller transformers but I realized that the transformer will be completely enclosed. I have to watch out for heat. I don’t want to burn the house down.
    Last edited by Roger Feeley; 03-04-2020 at 2:14 PM.

  4. #4
    Join Date
    Sep 2007
    Location
    Longview WA
    Posts
    21,321
    Blog Entries
    1
    Roger, How did you verified the transformer is the problem?

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  5. #5
    Join Date
    Aug 2010
    Location
    USA
    Posts
    5,582
    Using a higher wattage rating is not an issue. The issue is using one that is underrated.

  6. #6
    I agree with the others.
    In fact, for LED low voltage lighting installations I have done, the recommended practice is to oversize the driver by 25-50% for a cushion.

  7. #7
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    1,386
    Blog Entries
    1
    Quote Originally Posted by Jim Koepke View Post
    Roger, How did you verified the transformer is the problem?

    jtk
    jim,

    at first, I thought it was a bad connection inside the fixture. We have 16 of these fixtures and I’ve seen some arcing leading to a bad connection in the past. I was surprised to see no evidence of arcing or burning. Everything looked clean.

    the next step was to isolate the problem. I removed the transformer. I hooked the primary up to 115vac and measured the voltage on the secondary using a digital volt-ohm meter set to measure AC. Interestingly, I got exactly 0 volts. Did I miss something?

    i can take the same meter and put it across the secondary’s on any other fixture and see 12v.
    Last edited by Roger Feeley; 03-04-2020 at 9:06 PM.

  8. #8
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    1,386
    Blog Entries
    1
    My in-in-law just told me that he wants to replace the transformer and go back to using 50w halogens.

    Hereís the one Iím looking at:
    https://www.newark.com/triad-magneti...0va/dp/11R8063

    The secondary shows 12v @ 8.34A which multiplies to 100watts exactly. No cushion. Is that ok?

  9. #9
    Join Date
    May 2009
    Location
    Northwestern Connecticut
    Posts
    141
    We had flickering lights, and eventually the lights quit working, after replacing higher wattage bulbs with LEDs, and the light switch was the problem. Since replacing the switch with one rated for LEDs, no more problems. It is a dimmer switch.

    YMMV

    Jim

  10. #10
    Quote Originally Posted by Jim Falsetti View Post
    We had flickering lights, and eventually the lights quit working, after replacing higher wattage bulbs with LEDs, and the light switch was the problem. Since replacing the switch with one rated for LEDs, no more problems. It is a dimmer switch.
    It's not commonly appreciated that when you replace lighting with LED's, on a dimmer-based circuit, the dimmer switch also has to be replaced with something rated for that.

    The "bulbs" also have to be rated for it.
    Last edited by Doug Dawson; 03-04-2020 at 10:39 PM.

  11. #11
    Roger,

    As others have said, an oversized transformer isn't a problem.

    The LEDs may have contributed to the failure. Some poorly designed LED replacement lamps only draw current in one direction (half wave rectified DC). Transformers don't like that and will run hotter than they should, even though you are drawing what seems like less power.

    Regarding your new transformer, IIWM, I wouldn't run a 100VA transformer with a 100VA load. It's not good practice. And it will get quite hot. The data sheet says 45 degree C rise and those figures generally assume some cooling air. If you stick this in a little metal box with no air flow, it will get even hotter, and heat is the biggest enemy of electronics. Shoot for 20% margin if you can.
    --Certainty is the refuge of a small mind--

  12. #12
    Join Date
    Sep 2007
    Location
    Longview WA
    Posts
    21,321
    Blog Entries
    1
    I hooked the primary up to 115vac and measured the voltage on the secondary using a digital volt-ohm meter set to measure AC. Interestingly, I got exactly 0 volts. Did I miss something?
    No, it seems you have it covered. Even though much of my work history is with electronics and electric things it is in my fear of what a person who doesn't have a meter might get into. It makes me reluctant to attempt giving advice to those whose electrical understanding is unknown to me.

    jtk
    "A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
    - Sir Winston Churchill (1874-1965)

  13. #13
    Join Date
    Oct 2007
    Location
    Falls Church, VA
    Posts
    1,386
    Blog Entries
    1
    Quote Originally Posted by Jim Koepke View Post
    No, it seems you have it covered. Even though much of my work history is with electronics and electric things it is in my fear of what a person who doesn't have a meter might get into. It makes me reluctant to attempt giving advice to those whose electrical understanding is unknown to me.

    jtk
    Thanks,
    I was a bit puzzled at seeing exactly 0 volts ac. Usually I get some very low random values.

  14. #14
    Join Date
    Nov 2011
    Location
    Tacoma, WA
    Posts
    229
    If you are running a transformer that is made to be dimmed for LED lights and you use a dimmer switch foe LED use that dims the transformer then you can still have a problem with strobing or flickering if you have a a lot of lights connected to it. I run 13 can LED lights and encountered occasional strobing. Like a police car lights. I changed switch and relocated it. Switches are 3-way. I stopped strobing by replacing one LED can light bulb with an incandescent bulb.

  15. #15
    That transformer from Newark is $43....are you sure you can't just buy a new fixture with built-in LED bulb for less? ��

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •