Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rectified AC through an AC transformer

  1. Apr 21, 2009 #1
    Like the title says, I am wondering if it is possible to rectify an AC signal and run it through an AC transformer. The rise and fall in voltage still exists so it should work, or are AC transformers made specifically for an AC signal and other information needs to be taken into account?
  2. jcsd
  3. Apr 21, 2009 #2
    Yes. As long as the transformer is not driven into saturation and the current limits are not exceeded. AM radio transmitter modulator transformers carry the DC current of the final amplifier, so I'm sure it can be done.
  4. Apr 21, 2009 #3


    User Avatar

    Staff: Mentor

    You can, but for a 50/60Hz waveform, you will get somewhat more losses through the transformer. Quiz Question -- why?

    Why are you wanting to put a rectifier before a transformer?
  5. Apr 21, 2009 #4
    It depends on the transformer.
    There is a large DC bias; the average of the rectified signal.
    AC power transformers are not made for this; the core will saturate.
    A series capacitor between the rectifier and primary will eliminate the bias.
  6. Apr 21, 2009 #5
    Phrak is correct. While transmitter modulation transformers are designed to have a DC bias, power transformers are not. I don't think a small current would cause much trouble, but what you are trying to do might. It all depends on the transformer and how much DC current you will be putting through it.
  7. Apr 21, 2009 #6
    Thanks for all your help. Im wanting to rectify it ahead of time because i want to step up the voltage to about 20000v with a power transformer and since high voltage diodes tend to be more expensive than low voltage ones i was wondering if it was feasible to rectify ahead of time, but it doesn't sound like it will work in this situation since its going to be pretty high current on the primary
  8. Apr 21, 2009 #7
    AC power transformers are designed to be cost-effective to the manufacturer, meaning that they minimize the material cost, iron and copper in particular. The number of turns in the primary is minimum, which pushes the peak core magnetization field close to 1.5 tesla for the maximum peak (not RMS) primary voltage. This peak AC excitation is proportional to the peak input voltage divided by the primary inductance. The only way to reduce this peak excitation is to reduce the input voltage. A DC current will produce a DC offset magnetization in the core and push the peak AC + DC magnetization into a very non-linear magnetization, unless the AC excitation voltage is reduced. In short, for power transformers, there should be no DC current on the input. Power transformers designed for rectification, full wave bridge or full wave center tap, have no average DC current in the secondary, but a half wave rectifiier secondary will, and this could also bias the magnetization, but the manufacturer has probably taken this into account.
  9. Apr 21, 2009 #8
    I hope I'm not misreading your post but it sounds like you have a misunderstanding of basic transformer function. Rectifying the primary will not give you DC on the secondary. You will only get the AC component on the secondary. If I wanted 20kv DC, I would just use a fly-back transformer and rectifier from an old tv set. But please be careful. You can be killed. I have no way of knowing if you have the necessary background to be dealing with such high voltages. From reading your posts it would seem that you do not. If such is the case then I would strongly recommend that you not continue with this project.
    Last edited: Apr 21, 2009
  10. Apr 21, 2009 #9
    Place a voltage multiplier following the transformer. The multiplier requires AC input. You can look it up. It's just a bunch of capacitors and diodes. The current is low, and the impedence is high. But you can get your voltage if it doesn't arc-over. I would select the diodes first and the transformer, dependent on available diode voltage, second.
  11. Apr 21, 2009 #10


    User Avatar
    Science Advisor

    You cannot rectify an AC signal and then pass it through a transformer and expect DC on the other side.
    If you feed pulsing DC into the primary of a transformer, you will get AC on the secondary of the transformer which will still need rectifying to get DC. This is apart from the saturation effects mentioned earlier.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook