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

Need help raising the voltage on a power supply that I am making.

  1. Aug 8, 2011 #1
    I am currently building a DC power supply for a HHO Fuel cell. I am looking for HIGH current at a relatively low voltage (12-15 volts). I already have modified a MOT (Microwave oven Transformer) for high current. I replaced the high voltage secondary with 3 secondaries each consisting of 12 windings of 16 gauge wire. Each of the secondaries are in parallel and then rectified. The 16 gauge wire is rated at 24 amps according to MCMASTER-CARR where it was purchased(Here is the product number of the exact wire that I am using if you don't believe me - 7587K079). So do the math and my 3 coils can withstand 72 amps within the rating of the wire.

    Anyways my problem is that I get about 9.2 volts from the secondary and I am sure my bridge rectifier will drop that below 8 volts which is where it needs to be at to power my HHO cell. I have thought of a solution but was wondering if anyone could point out flaws or offer better solution. So here is my solution:

    I want to modify a 2nd MOT for high current and then put it in series with the other MOT I have. I think that this would double the voltage, however I have not had much experience with AC and especially power sources connected in a series-parallel fashion(Remember that each MOT has 3 secondaries that are in parallel).

    PS: FYI After much research I have found that a Router Speed Controller works wonders for controlling transformers.
  2. jcsd
  3. Aug 8, 2011 #2
    How about the primary, you cannot just increase the current capability of the secondary and think that you can increase the current and maintain the same voltage. As you draw more power from the secondary, it reflect back to the primary. If the wire in the primary is too small, too much voltage drop in form of resistor loss on the primary and you get lower secondary voltage. Even if you parallel up the primary, you still have to worry the core size. If the core is too small and you can run into saturation and the [itex]\mu[/itex] drop and you don't get the power coupling. All you do is generate a lot of heat and burn the transformer.

    This is like getting a 110 to 12 V 1A rating secondary, parallel two more secondary and expecting the transformer magically become `12V 3A capability!!! Don't work this way!!!

    If you have two of the transformer, put them in parallel and you get the increase capability and you don't have to monkey with winding. Just make sure you get the polarity correct or else you can short both of them out by putting them in parallel.
  4. Aug 9, 2011 #3
    I am not trying to squeeze more power out of the primary than it was originally designed for. At first the original primary consisted of hundreds of turns of very thin wire. This was designed to carry very high voltage at low current. I replaced the very thin wire with thick wire that will carry high current at low voltage. Right now with only one MOT I can 9x72=648 watts out of the single MOT, which is much lower than the MOT is rated to handle. I have tested my MOT and it can handle the current that I want to run at without the primary even getting warm. My question is can I connect 2 transformers in series.

    Thanks for the reply
  5. Aug 9, 2011 #4
    OK, you did not mention that you replace the primary also. I re-read your post, Do you mean your secondary winding is only 12 turns of 16 gauge? What frequency are you running? You are expecting 1V per turn, make sure your frequency is high enough. It will never work with 60Hz. I am no transformer expert. I had a switching HV power supply engineer working for me and we talked a lot. You need certain frequency to get certain voltage per turn. So look into this.
  6. Aug 9, 2011 #5
    So the frequency changes the voltage in the coils I will look into that.

  7. Aug 9, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    You need a high enough primary reactance. That's the reason for needing more turns per volt.
  8. Aug 9, 2011 #7
    Not the frequency changing the voltage, you need to have more turns if you run in lower freq. For 60Hz, you need more turns per volt. This is due to the core. I don't know the detail theory but my whole idea at the time was doing high voltage isolation dc to dc converters that can standoff 15KV, we need to have as few turns as possible, I decided to run the transformer at about 1MHz and we get up to 6 or 7 V per turn. My power supply engineer did the design.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook