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So I could not get any stepdown transformer in town larger than 30V. I'm sure there could have been somehwere but just not my regular places. So as I call already I manage to get a control place that said yes be have a 100VA here. It was 40's or 100's in stock. I needed 81 VA's max load on 18 actuators. This place was right next to my buddies office and he gladly went over and grabbed it for me today. So this is where I am.
This is the product model DO 0100HC.

It seems that when I test it out (in the basement) I can get 24V on the secondary side but it is 12v on each terminal (X1 and X4). Am I correct that I need one terminal to be 24V and the other to be neutral, usually labelled R and C on the tekmars and honeywells. Is there an alternative to wire this to achieve the layout of a typical step-down transformer?


  • Steamhead
    Steamhead Member Posts: 14,829
    If none of the 24V wiring is connected to ground, therefore expecting to see one side of the transformer at 24V and no voltage on the other, it should work OK.
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  • hvacfreak2
    hvacfreak2 Member Posts: 500
    edited February 2017
    edit : I was late to the game.

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    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

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  • Hilly
    Hilly Member Posts: 417
    I forgot to add this is the testings from early and why I've confused/worried myself.

    120V L-H1 N-H4

    multimeter readeings
    H1-H4 is 120V
    X1-X4 is 24v
    X1-ground (on 120leg) is 8v
    x4-ground (on 120leg) is 9v

    If this was a typically step-downtransformer would I not want to see x1-ground(or neutral) as 24v and x4-ground(or neutral) as 0v?

    All of this is because I am going to head out of town to help a buddy out on Monday. Sometimes seeing something different just really messes with my brain.
    @hvacfreak2 I'll track down some fuses and test it out your described above method.
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    Ok glad you got to read it . I guess I was composing while Frank was posting .

    Those readings sound like what I see for typical transformers. Once you connect them to the circuit you don't really reference ground in testing so you just don't notice those 8 and 9 volt readings like you have. Some control circuits are ground referenced on the secondary common side to steady the circuit for analog signals ( 0-10 vdc example ) . Either side can be grounded if you decide to do so. Fuse it at 5 amps just to be safe , never hurts.

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

  • ratio
    ratio Member Posts: 2,918
    That's a bog standard multi tap step down xfrmr. The primary side will take either 120 or 240, the secondary side will give you either 12 or 24 volts depending on how it's tapped —same principle as the primary, two coils connected in either parallel or series.

    You're reading strange voltages to ground on the secondary because xfrmr secondaries are electrically isolated from the primaries. If the xfrmr isn't grounded internally, you have to bond one leg to ground so you can reference it—that leg becomes by definition the common/neutral.

    IIRC 100 VA is large enough that the secondaries must be fused. 100 VA ÷ 24 volts is about 4 amps, but if you know your connected load you should size your fuse for that.

    EBEBRATT-Ed Member Posts: 10,213
    When you use a transformer your creating a new power source.
    If the transformer itself and none of the wiring on either side of the transformer is grounded you will get the correct voltages if it's wired correctly but the voltage will "float" you get weird readings to ground because the transformer has no relationship to ground until you ground it.
  • Leonard
    Leonard Member Posts: 903
    edited February 2017
    If your secondary is floating ( not grounded ) then if you try to measure voltage from secondary to ground there is only stray capacitance completing the circuit. And your voltage reading will depend on impedance of your meter, ( meter impedance and stray capacitance make a voltage divider).

    Using an analog meter one way I tell I'm looking at voltage from stray capacitance like this is I change the voltage range on the meter, the meter needle stays at about the same position even though the range scale has changed. I noticed this on my Simpson meter, have not tried it with other brands, but expect they will be same.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,217
    Polarizing/phasing transformers is important when you are dealing with electronics, flame rectification and so forth. If they are not correctly phased, some of the equipment might not work.

    When dealing with a 120v to 24vac transformer, you should have approximately 96volts between L1 and R (hot) on the secondary side. You should have approximately 144volts between L1 and C (common) on the secondary side.

    Here is a document with more info.
  • ratio
    ratio Member Posts: 2,918
    Note that voltage readings between primary/line sides and secondary/load sides are only valid when a) the primary is bonded as required by code (in the US and mostwhere else I believe), and b) the secondary is bonded on one side—this is not required in all situations (and specifically counter-indicated from time to time)! If the secondaries aren't bonded, voltage readings from the secondary to anything other than the other leg of the secondary (or the circuitry between them) are meaningless, as @Leonard mentioned above. If you don't see ±0 volts on one side of the transformer secondaries to ground, you may be missing the bonding. Check the schematic.