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24VAC Step-up Transformer

Discussion in 'LEDs and Optoelectronics' started by Suth, Nov 30, 2018.

  1. Suth

    Suth

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    Nov 30, 2018
    I am attempting to add a 24 volt AC LED indicator lamp to an existing circuit that triggers a relay. Due to the length of the wire when I splice the lamp into the circuit the voltage drops from 28 volts to 24 volts which is no longer enough to trigger the relay. I cannot replace the wire with a shorter one or larger guage. Is someone aware of a step up transformer available I can purchase to step the voltage back up to trigger the relay?

    Thank you
     
  2. Alec_t

    Alec_t

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    Welcome to EP!
    What is providing 28V?
    How much current does the lamp draw?
     
  3. Suth

    Suth

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    Nov 30, 2018
    The 28V is coming from a transformer stepped down from 110. The printing on the LED lamp is very difficult to read , but I think it says 20mA. I also have an incadescent lamp available that reacts the same that is .85 watts.
     
  4. Tha fios agaibh

    Tha fios agaibh

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    Can you just change the relay to 24v? Its a standard size.

    Strange that only 20ma knocks out your relay. You may be able to find a relay that draws less current like a ss relay.

    As for the indicator, you could add a resistor (about 1k) to limit the current (and unfortunately light) to the led.

    Or, change the indicator circuit to one that monitors the current through the wire when relay is energized but may require an external power supply.

    They do have boost transformers but it seems to me that changing the relay would be the easiest option.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Placing the led in series with a SSR may be an even better option.
     
  6. duke37

    duke37

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    You can make anAC relay more sensitive by driving with DC obtained with a bridge rectifier. An AC relay driven via a diode will be more sensitive and even more so with a diode across the relay.
     
  7. BobK

    BobK

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    Your description does not make any sense. A 20mA LED should have little effect an the voltage to the relay. And how is the length and size of the wire relevant at all? There should be no change to the wiring to the relay. Show us how you wired it. It sounds to me like you wired the LED in series, when it should be in parallel.

    Bob
     
  8. Kabelsalat

    Kabelsalat

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    Look at the relay markings - it should tell if it's designed for DC or AC. Some old equipment have a dash with 3 dots under that indicate DC only. Or it might use a ~ symbol to indicate AC.
     
  9. Tha fios agaibh

    Tha fios agaibh

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    I'm sure it does say 20ma on the led, but I wonder if this indicator has build in voltage divider resistors designed for line voltage which causes it to draw much more.

    It'd have to be one heck of a long run for voltage drop to be an issue at only 20ma.
     
  10. kellys_eye

    kellys_eye

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    are you wiring the light is SERIES with the existing AC supply?
     
  11. Suth

    Suth

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    Nov 30, 2018
    I do not have an electronics background, but I will try to explain the situation as best as I can. Off of my garage I have a boiler room which houses a boiler for a driveway snowmelt system and garage heat. The system can be triggered by 2 methods. One is a roof mounted snow sensor which is currently out of commission. The other is a set of screw terminals when jumpered together turn the system on for service purposes. A few years back I ran a 2 conductor 16AWG cable from the boiler room to a switch in the house so that I could manually jump the system on. This has been working fine for years. Recently my wife complained that sometimes we forget to turn the system off after it has accomplished its task so I thought it would be a good idea to splice an indicator light into the cable to remind us when it was on. I purchased a 24 VAC LED panel mount lamp and spliced it in after the switch. When I turn the switch on tha lamp lights, but the system no longer turns on. The voltage measures 28V without the lamp. After splicing it into the cable it measures 24.5V. The only reason I brought up the wire guage and distance is I thought that the loss was possibly due to the distance (about 200') or the cable being to small. Since my initial post I have tried bypassing the cabling and simply used the leads off of the LED to jumper the screw terminal in the boiler room. The lamp again lit, but the system did not trigger. I have also tried connecting the LED to the switch in the house in parallel as you mentioned, but that did not work either. Others have mentioned changing out the relay, but that is not an option. Again I have no electronics background, so I hope what I have described is clear enough. Unfortunately at this time i would not be able to pull any additional wire between the house and boiler room, so I am looking for a solution that can use the existing cabling. Thank you for your response.
     
  12. kellys_eye

    kellys_eye

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    THIS is your problem.

    You are wiring the lamp in SERIES with the switch. If you do that then the voltage that is passing through the wire/switch is ALSO passing through the light and being 'used up by the light' before it gets to do anything else.

    It might seem counter intuitive but connect the light ACROSS the switch contacts. This will make the light operate when the equipment is OFF though. You could get used to it.......

    Alternatively, wire a 24V relay coil across the switch contacts and wire the relay contacts themselves to a light (via a secondary power source) and you can the get the light to operate when the switch is 'ON'.
     
  13. BobK

    BobK

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    Okay, we can now see the whole picture. I guessed that you were wiring the LED in series, but now I know why as well (you don't have the connections to wire it in parallel). I would go with @kellys_eye's second suggestion, wire a relay across the switch and use the NC contacts to light a light only when the switch is on.

    The other way to fix it is to run a third wire along with the 2 switch wires so you can connect the LED in parallel with the relay, i.e the third wire would connect to the second terminal for the relay coil and the led would connect between the two wires that connect to the two relay terminals.

    Bob
     
  14. Suth

    Suth

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    Nov 30, 2018
    This is what I suspected, that is why in my initial post I asked about stepping the voltage back up slightly after it leaves the lamp. You are correct when the lamp is spliced in parallel it lights the switch in the off position. Will wiring a 24V relay across the switch cause to much voltage and possibly damage something?
     
  15. Tha fios agaibh

    Tha fios agaibh

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    Instead of relying on seeing the light to remind you to turn it off, I'd replace the switch with a timer.

    Even a small mechanical turn type timer (like used for a bathroom fan) would work.
    That way you could set it for 20min or whatever is required, and you won't have to be vigilant in monitoring the system.
     
    Harald Kapp likes this.
  16. Suth

    Suth

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    Nov 30, 2018
    Okay, I have ordered a 24V power supply to try across the switch, but would anyone mind educating me on why leaving the lamp in series and stepping the voltage back up after the lamp is not a viable solution.

    Thank you
     
  17. duke37

    duke37

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    Jan 9, 2011
    By stepping up the voltage, you will step down the current by an equal factor. In fact, if the available current is not sufficient, the voltage will collapse.
     
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