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Problems selecting a PCB Transformer

Discussion in 'Datasheets, Manuals or Parts' started by NewcastleSAR, Jan 1, 2007.

  1. NewcastleSAR

    NewcastleSAR New Member

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    Hi Folks..

    I'm finalising all the parts for the Darkroom timer based on two circuits before I decide on which one to build but have run into a difficulty selecting the Transformer I need.

    As I have two circuits to choose from, the transformer the two circuits quote are:

    Mains Transformer 6V 1.5VA (PCB Mounted)

    and

    220VAC 15VAC 1VA PCB Transformer

    I've gone to RS Components to source the Transformers @

    http://rswww.com/cgi-bin/bv/rswww/s...cdgaddjkikjhmhcefeceeldgondhgi.0&cacheID=ukie

    and tried using their filtering system to narrow down the types available but I can't seem to match what they have against what I need.

    Could anyone assist in picking out the code for the transformers I require?

    Based on my searching

    For the 220VAC 15VAC 1VA PCB Transformer, the nearest I can seem to match is the following:

    http://rswww.com/cgi-bin/bv/rswww/s...cdgaddjkikjhmhcefeceeldgondhgi.0&cacheID=ukie

    and for the Mains Transformer 6V 1.5VA (PCB Mounted) I get

    http://rswww.com/cgi-bin/bv/rswww/s...cdgaddjkikjhmhcefeceeldgondhgi.0&cacheID=ukie

    I gather that the VAC is the AC voltage in and the AC voltage out but what is the VA quote and how does it match up with output current.

    Many thanks on this longwinded posting.

    Declan
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    VA is Volt/Amps - essentially the wattage (power) of the transformer. 1VA and 1.5VA are VERY low power transformers.
     
  3. Sceadwian

    Sceadwian Banned

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    Most transformers secondaries are rated for a specific voltage at a specific current, if the current draw goes higher than that the voltage drops as the inductive coupling isn't high enough to transfer more power. I'm going to hazzard a guess that if you try to draw power past the VA rateing of a transformer that core losses in the transformer are going to become very high and the voltage is going to drop significantly.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I don't think there's any need to 'over analyse' it? - try and take too much power and the transformer will overheat and fry!.
     
  6. Hero999

    Hero999 Banned

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    Please post a link to the project and or a schematic so we will be able to tell you if the transformer is suitablity sized.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Really the biggest problem is finding a transformer that fits the PCB, they are usually all a little different.

    But from RS Components there's a 6V 1.5VA under 347-3388, and a 15V 1.5VA under 347-3417 - they don't list a 15V 1VA (at least on the first page).
     
  8. NewcastleSAR

    NewcastleSAR New Member

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    Oops..

    Sorry folks about the RS Components links. I didn't realise that they had an expiry level on them.

    I've attached the two schematic diagrams for kind souls to have a look at. This project is also in reference to another link about an S201S01 Traic although I think I have the triac sorted.

    BTW, RS components is my main supplier as I have an account with them hence the links I was using to RS

    Regards

    Declan
     

    Attached Files:

  9. NewcastleSAR

    NewcastleSAR New Member

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    Hi Nigel..

    Sorry about the links to RS Components. What you have quoted below is what the links I had should have pointed you towards.

    Is the 347-3388 transformer suitable for this circuit?

    In relation to the 220VAC/15VAC 1A transformer, the nearest I could get was the 347-3417. If this okay?

    I've emailed the Author of the 220VAC/15VAC 1A transformer and awaiting a reply.

    Regards

    Declan

     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You've just changed your details! - you previously said 1VA, now you're saying 1A - which is it?. The transformers I referenced were both 1.5VA.

    EDIT: I've just checked the diagrams you posted, the 6V one is 1.5VA, the 15V one is 1A (15VA), so if you wnat to build that circuit you need to look for a 15VA transformer. Bear in mind, most of the RS transformers have two windings, which you should connect in parallel.

    The 6V one lists a Conrad transformer, have you considered ordering that?.
     
    Last edited: Jan 1, 2007
  11. NewcastleSAR

    NewcastleSAR New Member

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    Sorry Nigel..

    There appears to be a typo on the Parts list page of this project.

    I quoted what was on the Parts/component list. It states on this list 220VAC/15VAC 1VA PCB Transformer. Having now looked at the schematic, it states 220VAC/15VAC 1A

    Would that now suggest that the transformer 347-2824 from RS be the one I would be looking for?

    In relation to the 6V one you quoted (347-3388) okay for this circuit.

    The reason for not ordering from Conrad is that they have aminimum order of 50 Euro plus, it has an omber traffic light which I assume means that they don't have them in stock.

    Declan




     
    Last edited: Jan 1, 2007
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That ones 36VA not 15VA - it would work fine, but will be more expensive and larger than needed.
     
  13. Hero999

    Hero999 Banned

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    Well the schematic is wrong, you need a 9V 1VA transformer.

    6V is too low due to the dropout voltage of the LM7805 and 15VA is too big because this circuit only draws a very small current.
     
  14. NewcastleSAR

    NewcastleSAR New Member

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    Hi Nigel..

    Based on the info you now know, which transformers would you recommend in the RS range?

    :confused: Now I have to consider Hero999's comments.

    Hero999.. :eek:

    What would be your recommendations from RS components

    As it is, I'm still waiting on responses from the Guys that designed the circuits.

    Regards

    Declan

     
  15. Hero999

    Hero999 Banned

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    310-1342, 8V and only 1VA but it's still powerful enough for your application or preferably 347-3394, 9V, 1.5VA, it's also cheaper which is suprising.

    Don't just blindly build a circuit, try to understand how it works.
     
  16. NewcastleSAR

    NewcastleSAR New Member

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    Hi Hero999

    Many thanks for that.

    Can you recommend a good book or website location that would help me understand transformers and how to select the correct one.

    I'm still trying to tie in the values of the transformer in the circuit to understand the VAC (in/Out) versus the VA values and the output current it can supply.

    In simplistic terms and at the risk of getting a smacked, with reference to the 220VAC/15VAC 1A transformer, is this essentially a transfomer that steps down 220 Volts AC to 15 Volts AC and can supply an output current up to 1 amp?

    I can't seem to marry the 15VA and the output and 1A for this transformer to one at RS. The nearest I get to it is 347-2694 (16VA) but it only supplies 0.5A. The next one up is the 201-8738. Its supplies 0.73A but the VA has gone up to 22VA.

    :confused:

    In the 6V transformer circuit, is the reason for specifying the 8V (or 9V) transformer due to the 7805 regulator needing a minimum voltage of 7.5V for it to operate?

    Sorry if all the questions sound dumb. :eek:

    Declan




     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    If it's like the other transformers (and I presume it is?), then it has TWO secondary windings (this make it far more versatile), paralleling them gives you the 1A you're looking for!.
     
  18. Hero999

    Hero999 Banned

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    Google is your best bet.

    VA is Volts times Amps, a 15V 15VA transformer can supply a maximum current of 1A.

    Yes
    Don't worry about it 1A is far more than your PIC and optocoupler emmitter will ever use, just use the 1VA or 1.5VA unit I suggested.

    Transformers voltage are always specified as Root Mean Square (RMS), an AC voltage that will do the same work as a DC voltage.



    The voltage across your smoother capacitor is the peak voltage minus the losses in the rectifier (1.4V).



    Now, if you look at the datasheet for you will find that the 7805 needs at least 8V to regulate properly, therefore the output needs to be kept above 8V, so rearanging the above formula:



    So your input needs to be above:


    I'm sorry I don't have either the time of will to explain all of this in great detail (that would take many pages).

    Don't worry about the above formulae, try googling the following terms:

    Voltage regulator.
    Root mean square.
    Dropout voltage.
    Smoothing capacitor.
    Transformer.
     
  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You've not studied the circuit! - the 15V one has four seven segment LED displays, so needs a much higher capacity supply!.
     
  20. NewcastleSAR

    NewcastleSAR New Member

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    Hi Nigel..

    I think things are staring to click together now.

    Having just studied a transformer circuit on the Internet and applying the 15VAC value if I wire the two secondaries together in series, it doubles the voltage but keeps current at same level but wiring in paralled keeps voltage at same level but doubles current.

    Is that correct?

    Declan



     
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Yes it is, it's important though that the windings are in phase, but it's usually obvious how to do that.
     

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