Can I hook up a transformer directly to main (120V)?

Discussion in 'The Projects Forum' started by MattWCarp, Oct 23, 2010.

  1. MattWCarp

    Thread Starter New Member

    Oct 23, 2010
    I'm building a simple 9V DC linear power supply - a 120V to 9V transformer, a full wave bridge rectifier, and a capacitor. I'll insert a fuse and 15V MOV for some protection. This unregulated 9V DC will be sent to another board, where a regulator will be used for a small control circuit.

    The transformer is rated for 120V on the primary and 9V / 0.5A on the secondary coil (so 4.5W or so).

    My question - as I prototype, can I hook up the transformer primary to the mains (120 VAC) without a load on the secondary? Or, will this be a short and then trip my circuit breaker / melt the transformer?

    (I know the dangers here and would be very, very careful)

    I've measured the primary coil resistance as 62 ohms. So, this will create a current of almost 2A, which, may be too much for the transformer. At 2A, that's 240W, right? That's huge for a transformer that's only rated for 4.5W on the secondary.

    I can't use a wall wart design, so let's work under the current approach.

    I have a follow up, but would be interested in any comments so far.

    Thanks, Matt
  2. wayneh


    Sep 9, 2010
    Doesn't that answer the question? The AC impedance could be considerably higher than the DC resistance. Yes, wall warts do suffer from inevitable losses in the primary.
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    What u are measuring is the DC resistance.

    If the transformer is rated for ur mains Voltage. Then u shud be fine.

    Does not matter secondary is loaded or not. U will get the rated output in either case
  4. Wendy


    Mar 24, 2008
    Generally you need a switch, and (more importantly) a simple fuse (in case something goes wrong).
  5. Markd77

    Senior Member

    Sep 7, 2009
    A transformer is fine with no load on the secondary, they draw less current from the mains with low loads than with full load.
    I second putting a switch and fuse on the mains side, and also insulating all mains parts before continuing.
  6. MattWCarp

    Thread Starter New Member

    Oct 23, 2010
    After I went to bed I realized that I wasn't thinking of the AC impedance! The actual impedance will be higher and the current won't be as great.

    Mark, I think your comment makes sense - if there's no secondary circuit, the energy created by the primary isn't being "pulled off" by the secondary, and kind of creates a barrier, which leads to a higher impedance.

    I'm going to have to crack open an old textbook to see the math there, but it seems to be logical now that I've slept on it.

    I think I'm going to take my function generator and see this effect with a 5V 60 Hz sine wave. I should be able to confirm the behavior before I play with the voltages that will kill me!

    After I complete the prototyping, the final supply will definitely have fuse, switch, and a MOV to add some overload protection.

    thanks for the quick comments!
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    whenever I hook up to mains, I consider my personal safety, and to me that means adequate grounding. If the trans isn't mounted on a metal base where I can connect the ground, I'll connect it directly to the trans mounting bracket.

    Fusing and switching come next, but not at the expense of safety. Either device must be completely secure and exposed terminals out of reach.