Simple question -diodes and transformers

Discussion in 'General Electronics Chat' started by Mike Holland, Aug 27, 2014.

  1. Mike Holland

    Thread Starter New Member

    Aug 27, 2014
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    0
    I recently bought an ISO-Tip cordless soldering iron. Unfortunately, the charger is 110 volt and local mains are 240 volts. After some measurements, I worked out that 5000 ohms in the mains lead would drop the voltage into the transformer to 110, but with a heat emission of 3 watts - the resistor gets hot!

    I have just had the brilliant idea of imposing a diode on the input side to the transformer, so that the transformer would see a half-wave of about 120 volts peak to peak. Would this work?

    Thanks,
    Mike
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Is the charger a switched mode supply.?

    Whats the is the wattage rating of the charger,? ie: volts/current.?

    E
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    A diode will not work to reduce the voltage to a transformer. The transformer core will saturate due to the large DC current component from the diode, draw a high current, and likely blow the diode and/or transformer. But you can use a common triac type lamp dimmer to reduce the voltage since it generates no significant DC component. Just make sure that the dimmer can't be turned higher than that required to generate 110VAC (a true RMS type voltmeter can determine that point). An added resistor in series with the pot can be used to limit the voltage.
     
  4. Mike Holland

    Thread Starter New Member

    Aug 27, 2014
    2
    0
    Thanks, Crutshow. I had not thought of the DC component. So much for my brilliant idea!

    A lamp dimmer sounds like a good answer to my problem, and I think I have one lying around in the shed. I do have a 240v-110v dropping transformer, but it is overkill for charging a small soldering iron.

    Mike
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Many off the shelf UK 240Vac transformers are wound with two 120V windings in series on the primary.
    Input 240Vac across the two series windings, 120V output across one winding, ensure that the transformer is well VA rated for the job.
    E
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Since the transformer is being used here as an autotransformer and there's no secondary current, you can use it at or slightly above its VA rating without an overheating problem.
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    hi Carl,
    I have asked the OP for the rating of his charger, perhaps then we could suggest a suitable transformer VA rating.

    E
     
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