Designing a relay trip for a multi-tap transformer

Discussion in 'The Projects Forum' started by Michael Hlavaty, Aug 19, 2015.

  1. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi All,

    I am creating a linear power supply 0-18V, 0-3A and I have a transformer with a 19-0-19 VAC center tap, 25-0-25 VAC center tap, and two 15VAC windings
    (120VAC input).

    The 19-0-19 VAC center-tapped side will be used to create a voltage splitter ---> +/- 5V for the opamps and references on the op-amps and the 25-0-25 center-tapped side will be used for the input side to the pass transistor.

    My question is how to implement a relay switch to move from 50VAC input to 25VAC input(moving from 25-25 to 0-25) when the load current goes very high?

    I have seen some examples such as the solid state version:

    upload_2015-8-18_23-25-40.png
    However I don't want a LM317 in series with my pass transistor, instead I want to use one of the 15VAC ---> rectify for DC ---> into a voltage regulator and use that voltage for a relay with some kind of feedback from the mains output.

    Something similar to this setup
    upload_2015-8-18_23-34-53.png

    However I don't understand how the relay is switched with this schematic. I also don't see a ground reference point for the 12 volt regulator!
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi MH,
    What is the VA rating of the transformer secondary windings.?
    E
     
  3. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi ericgibbs,

    Not quite sure, I received a bunch of free transformers from an electronic surplus store. The E-I core is 3cmX3cm and the size of the secondary windings are about 28AWG.

    Is there a way to test this?
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi M.
    The core suggests about 100VA total, the 28AWG will not be suitable for 3Amp operation.
    What is the wire diameter for the other windings.?
    E
     
  5. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi Ericgibbs,

    Sorry for the confusion the center-tapped windings are 12AWG, and the 2-15VAC windings are 18 AWG. The store owner of the surplus store said he pulled it out of an old power supply unit that went up to 5A output!

    MH
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    That is good news.
    I guess you know that when you full wave rectify and smooth the secondary windings, the Vpeak will (Vrms * 1.414 )- 1.4v [ two diode voltage drops]
    So for the 19v winding that is approx 25V.

    Not quite sure what you mean.??
    Please post a sketch of what you like to do.?
    E
     
  7. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    hi,

    What I mean is I would like a relay connected to the main input. And when the current output goes high the relay trips and moves to the center tap-25VAC which lowers the input voltage. Lowering the input voltage at high current is essential to having a happy pass transistor. Power across the pass transistor = VCE * Iload = (Vout-Vin)*IEmitter.

    Here is an example which uses two relays, but I don't quite understand how relay 2 trips relay 1

    upload_2015-8-19_8-2-43.png
    upload_2015-8-19_8-0-45.png
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi M,
    There are not two relays.
    KIA is the relay coil and KIB are its contact set.
    IMO it should be annotated as KIA for the coil and KIA/1 for the contact set.
    E
     
  9. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi ericgibbs,

    Ohh okay makes sense now. However how does the relay trip? There is a positive +12V on the base of npn transistor and with a cap. The negative side of the cap is connected to the output. My guess is when the load current is high ~ 3-4Amps, the resisitive load is small so the output is close to ~0V, therefore you get a positive VBE across the npn transistor which trips the relay. The reverse diode across the relay is protection for the npn transistor.

    Is this correct?

    Thanks for your help,

    Regards,
    MH
     
  10. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi M,
    The circuit is incomplete, there is no drive to the series power output transistor also the 12V circuit is floating.?

    Do you have a link to the original circuit source.?
    E
     
  11. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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  12. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi MH,
    That original circuit is very difficult to follow, I am assuming that the 12V circuit is at Chassis 0V.
    I would say there are easier ways to make a 0v thru +18v at 3Amps without using the relay method.

    E
     
  13. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi E,

    Yes this schematic has many errors, either to throw people off or because it is Chinese PS(no offense to the Chinese people). I can return the favor now and answers some questions for you,

    [​IMG]


    The 1,2 connector means "connect with a fat wire".
    The earth ground should be an analog ground.
    The analog ground should be an earth ground signal since that is the chassis hoookup.
    Yes I agree with you that the 12V should be referenced to the same earth-ground they use.

    I will solder some wires to my relay today and hopefully have time to test something similar to what they have.

    Do you have any circuit suggestions that is similar to what they have??

    Regards,

    MH
     
  14. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    upload_2015-8-20_7-20-20.png

    Sorry the image is my last reply wasn't working, here it is...
     
  15. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi Michael,
    I do not wish to sound negative, but I would not even consider using that circuit diagram as the design for a 0v to +18v at 3Amp bench power supply.

    I would use either the 19v, 0v, 19v or the 25v, 0, 25v transformer secondary with full wave rectification and a LM150 3Amp linear regulator.
    That would give ~1.25V > ~17V at 2.5A > 3A.

    What you think.?

    E
     
  16. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi E,

    I looked around and saw a reference schematic using a LM10 op-amp, I will try that. I need to be able to go down to milli-volts for testing. I have alot of op-amps, resistors, caps etc to make this, except for LM10 which I can probably get for free through a sample.

    Do you have a reference schematic for 0-18V?

    regards,

    MH
     
  17. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi M,
    For the 0v-18v power supply do you require 3Amps output at the lower voltage settings.? eg: 0v thru 1v.

    What resolution do you require for the Voltage output setting of the power supply.? etc
    The 0v to 18v with a 3A max is not enough information to design a PSU.

    If you could post a more detailed specification of the power supply that you have in mind, I could look it over.

    E
     
  18. Michael Hlavaty

    Thread Starter New Member

    Aug 16, 2015
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    Hi E,

    I have a 0-18, 0-XXX amps power supply fully functional to the specs I want. I just need a way to switch the tapped transformer now,

    If you have a relay switch diagram for switching to center tap on the transformer at high current output that would help me out alot,

    Thanks,

    MH
     
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