Transfomer output

Discussion in 'General Electronics Chat' started by superway, Mar 2, 2011.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009

    Can anyone help me this question?
    I have a transformer 115/208V primary and 2x14V, 20VA secondary.
    I want to apply 208vac to primary on pin1 and pin4 (Pin2 and 3 connected), and I want to connect pin6 and pin7 together on secondary.
    I know the output between pin 5 and 7 is 28vac, but i want to reduce 28vc to about 3 vac to monitor waveform on Scope. Can you give me any advise what kind of resistors need to be added to reduce from 28 vac to about 3 vac?

    Last edited: Mar 2, 2011
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    The transformer output is rated as RMS. And the a volt meter will also measure RMS. But I guess on your scope you want to measure amplitude value. If you are using sine wave on the transformer simply multiply the RMS value with square root of two. Then you can use use ohms law and a voltage divider. remember your scope probe has a 1 Megaohm input resistance. So that should also be taken into consideration.
  4. superway

    Thread Starter Active Member

    Oct 19, 2009
    Hi T06afre

    Thanks for your response. I just want to monitor the waveform on scope thru stepdown transformer from 208Vac input. According to mu caculation:
    As we have V secondary = 28Vac, P =20VA, so I found the resistor is about 40 Ohm and connect series with pin 5 (highside of secondary), and then hook up scope at the end of resistor and pin 4 of lowside secondary.

    Is that correct?
  5. SgtWookie


    Jul 17, 2007
    20VA is ~0.71A @ 28v. That's a load resistance of about 39.4 Ohms.

    The transformer should output right about 28VAC RMS (+/-10%) with that load. It'll be significantly higher under no-load conditions.

    You could use a 10k and an 82k resistor in series across the output, 10k on the grounded side, and measure across the 10k resistor. That way if the output is 28vac, you'd measure 28v / (82k+10k) * 10k = 3.04vac. The 92k divider will only have 0.3mA current, which is practically no load compared to the transformer rating. So, you can test it with no load using just the divider, and add a ~39 Ohm resistor to test it under load.
  6. Teri


    Apr 3, 2009
    From your description of the problem you want to measure only the waveform of the voltage so the VA rating of the transformer is unimportant.
    You will need two resistors in a simple voltage divider circuit. Connect two resistors labeled R1 and R2 in series across the 28v transformer. Connect the Oscilloscope accross R1 only.
    The oscilloscope has a very high input resistance so the total resistance of R1+R2 can be any convenient value; let's say 10000 ohms.
    The ratio of 28volts/3volts must match the ratio of 10000ohms/R1, so R1 = 10000 / (28/3) = 1071 ohms.
    R2 must equal 10000 - R1 = 8929 ohms.
    Using the closest standard values of 9.1k and 1.2k will probably be close enough for your application.

    SgtWookie has given you some other useful values for the resistors and more detail about the transformer load.