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Does my transformer have sufficient power for an extra item?

Discussion in 'Power Electronics' started by R791945, Jul 2, 2018.

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  1. R791945

    R791945

    74
    0
    Jun 19, 2015
    I bought an ultrasonic humidifier which comes with a transformer. The label for the transformer says it is 24Vac, 1.2A.

    I measured the electricity consumption over 48 minutes:

    0.11A
    22Wats
    245Volts

    I want to split the transformer cable so that it can also run a small computer case fan. Is there spare capacity in the transformer? If I buy a 24V ac fan what is the maximum wattage/ amp I can buy to be able to run it with the humidifier?

    PS
    The ultrasonic humidifier tends to send up spashes of water as well as producing a fog. Do you have a better way to cut out the splashes? I am proposing to put the humidifier in a plastic bottle with a fan to blow the fog out of the bottle but to leave the splashes inside.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    In theory the transformer is good for 28.8 W (24 V * 1.2 A). As the humidifier consumes 22 W, that leaves 6.8 W for other uses. Taking into account some reserve I recommend not to use more than 5 W. A typical computer fan runs on 1 W ... 2 W or so, so this should not be an issue.
    The issue is that computer fans run from 12 V, not 24 V plus they require DC, not AC. You will therefore require a power supply that creates 12 V DC from 24 AC. Or find a fan that runs from 24 V AC.
     
  3. R791945

    R791945

    74
    0
    Jun 19, 2015
    Have I accidentally mislead you? I measured the power consumption of the transformer whilst it is running the humidifier on the basis that measuring the amount of water flowing into a tank will give an accurate measure of the water content. However your reply has made me doubt if I have given you useful figures on actual power consumption of the humidifier?

    I measured the electricity consumption over 48 minutes:

    0.11A
    22Wats
    245Volts




    0.11*245 =27watts. Assuming 22watts is time proportionate then 22/48*60= 27 watts for an hour ie the same as the power label on the transformer. This suggest to me that reading the power consumption of the transformer will not tell me how much the humidifier will draw down.

    Your reply was very useful because it was based on experience of fan power consumption. Do you have any idea how much power the humidifier will draw in theory? It is one of those small round metal pill boxes that cause a fog effect. Here is the model I bought:
    http://www.exo-terra.com/en/products/fogger.php
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Watt is a unit for measuring the instantaneous power. There is no such thing as Watt being "time proportionate".
    When you measure AC voltage and current separately, you cannot calculate the power in Watt from this equation. With AC you'll have to take into account the hase shift between voltage and current (power factor). My calculation in post #2 is based on the 22 W given in your original post. I assumed (possibly erroneously) that the 22 W had been measured with a power meter.

    Measuring on the primary side reveals only part of the truth. The transformer has internal losses which add to the power consumption. If you need to know the power consumption of the humidifier, you'll have to measure on the secondary side.
    Assuming the humidifier in reality uses the full 27 W, there is only 1 W headroom left until the transformer's capacity is reached. It would be possible to add a 1 W fan, but I do not recommend to operate the transformer at its limit.
    You may be better off using a USB powered fan and a small USB wall wart to supply power to the fan - albeit at the 'cost' of some more wiring. This is also imho the safe way to procedd as you will not have to mess with the internals of the humdifier so there is no risk of getting in contact with mains voltage.

    Unfortunately not.
     
    Tha fios agaibh likes this.
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