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Transformers in parallel

Discussion in 'General Electronics Discussion' started by bigone5500, Nov 11, 2015.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I have two Honeywell 198162P transformers that I want to use in parallel to achieve double the current capacity. The specs for these are as follows:

    Primary: 120vac
    Secondary: 24vac, 40vA

    I have read that they need to be the same ratings to be able to do this. How close do the readings on each of the secondaries have to be in order for them to be used in a parallel configuration? Both have a secondary resistance of 0.8 ohms.
     
  2. Minder

    Minder

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    Apr 24, 2015
    If they are the same part number especially, then it should be no problem.
    Don't forget to ensure the phasing is correct!
    M..
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The secondary voltages must be closely matched or there will be heavy circulating currents between the two secondaries when they are connected in parallel. I learned this the hard way some fifty years ago when I tried connecting several 6.3 VAC filament windings in parallel. There was enough of a mis-match in voltage to cause the transformer to heat up significantly with no load connected! This does not occur with so-called control transformers that are designed to have their secondary windings connected in series OR in parallel. If your two transformers are identical, you should have no problem, as @Minder stated, connecting the secondaries in parallel.
     
  4. GPG

    GPG

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    Sep 18, 2015
    Connect both primaries in parallel. Connect one lead from the secondary of one transformer to one of the other transformer. Power up Measure the AC voltage between the open two leads of the secondary.( If you get ~ 2x the expected voltage swap the leads on either the primary or secondary of one of the transformers). and measure. This should be a low voltage that represents the mismatch. Report.
     
    hevans1944 likes this.
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Connect one of the leads to the other transformer. Connect the other lead to a 1 ohm 0.25 watt resistor and the other end to the other lead of the other transformer. Turn them ON. Feel the resistor.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What you want to do is potentially dangerous:
    Note that a transformer works both ways. You have to ensure that both primary sides are connected to each other (with corect phase, of course). If one primary is open, you will see - and in a worst case scenario feel - mains voltage on the open terminal(s).
    The transformers work like this:

    mains (in) -> 24V -> mains (out)

    If you don't need the 24 V as AC, but need to generate DC anyway, a safer way is by using separate rectifiers for both transformers and connecting the two outputs on the DC side. In this setup you don't have to worry about phase either.
    As long as the voltages are sufficiently well matched, the dynamic resistance of the rectifier diodes may suffice to compensate for small tolerance:
    The higher of both voltages will drive higher output current which in turn will drop more voltage across the rectifier, thus compensating the higher voltage.
     
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  7. bigone5500

    bigone5500

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    Apr 9, 2014
    DC output is exactly what I am after. so I connect the two transformers in parallel. Check the secondaries and should have ~24vac on each. Connect those to two separate rectifiers and then check the voltage.

    Would this be the correct thing to do? My goal is to obtain 24vac 80vA.
     
  8. bigone5500

    bigone5500

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    Apr 9, 2014
    I connected the whites and blacks to each other and connected those to the mains voltage. Checking the secondaries of each transformer I get 26.7vac on each one.

    This is good right?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    After the rectifiers you get DC, not AC

    That's fine. The rating 24V is under load. When you apply a load (40VA per transformer) to the transformer's secondary, the voltage will drop to ~24 V. Note that the transformer as well as mains voltage have tolerances to be taken into account.
     
  10. bigone5500

    bigone5500

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    Apr 9, 2014
    Yes I meant DC. Thanks for the correction.
     
  11. Minder

    Minder

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    Apr 24, 2015
    The DC will be around 32v-33v DC after any smoothing caps added.
    M.
     
  12. bigone5500

    bigone5500

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    Apr 9, 2014
    Ok. So with the transofrmers in that configuration, I should be able to rectify and regulate to 24vdc @ 3A?
     
  13. Minder

    Minder

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    Apr 24, 2015
    If you come up with a voltage regulator for 24vdc output, 40va each is pushing it for 3amp out.
    M.
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Let's assume you need 80 W on the 24 V DC rail. That's 80 W / 24 V = 3.3 A.
    The input to the regulator is ~33 V (post #11), therefore the voltage drop across the regulator is 33 V - 24 V = 9V.
    Using a linear regulator, the power dissipated within the regulator is 3.3 A * 9 V = 30W ! You'd need a power transistor and a very big heatsink. As this is very inefficient, I recommend the use of a switch mode regulator. I do not recommend that you build one yourself as some expertise is required taht (sorry) you don't have. Buy a suitable module.
    Then again, it may be less expensive to buy a 110V/24V/80W switch mode power supply off the shelf instead of the solution you are looking at.
     
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