Can this circuit handle a supply of 12VDC, 15Amperes,180 watts?

Discussion in 'The Projects Forum' started by Pi Chi, Aug 23, 2015.

  1. Pi Chi

    Thread Starter New Member

    Jun 10, 2015

    Good Day!

    This schematic is just a part of the actual circuit that we made.
    The switch there is supposedly a microcontroller. We're using an Arduino Uno R3.
    We used a 27Kohm, 1/4 watt resistor. One out for the Arduino must trigger two transistors that are both connected to their respective relays.
    We have 5 outs in total. 4 have two relays paralleled to it. The fifth one is attached to a single relay for a fan.
    The rest (8 relays) are connected to 12VDC,1A, 20Watts, DC Ballasts. And these ballasts are connected to 8 fluorescent lamps (20 watts).
    We used a DHT11 as a temperature sensor.

    Upon testing:

    We first used a 12VDC adapter. We forgot to consider the ampere rating. It just made the light indicators blink fast and the relays to click as well. But it doesn't even lit up one lamp. So we used one of our regular power supplies. It was able to lit up two lamps. But it was supposed to lit up four. We have noticed in the display that from 12VDC, the reading decreased by2 to 4 volts. And there's also this buzzing sound. I think we had the power supply broken so we can't measure its ampere rating now. We decided that we'll use another power supply so we are about to buy a centralized power supply with 220ACto12VDC output, 15A, 180 watts.

    1.)Will that be enough to make our circuit work properly?
    2.)Should we change the resistors or is it okay to just use 1/4 watt?
    3.)Will the high ampere rating affect it even if it's just meant to trigger the resistors?

    I hope I have described the problem properly :D
    We really need help. We can't afford to break anything anymore.
  2. DickCappels


    Aug 21, 2008
    1. Probably if you use a properly rated power supply and the relays are good for it.
    2. The resistors will dissipate less than 1 milliwatt. You can use any size resistor you can find.
    3. High ampere rating of what?

    Your 27k resistance might be too high for reliable operation. What is the resistance of the relay coil?

    I assume the coil is rated at 5 volts -correct that if incorrect.
    Pi Chi likes this.
  3. ebeowulf17

    Well-Known Member

    Aug 12, 2014
    The part number indicates a relay with a 5V coil, but the schematic shows 12V being applied to the coil. If the transistor limits the current running through the coil, it may actually be protecting the coil a little! Otherwise it appears to be running at more than double its intended coil voltage.

    Not sure if this relates to the apparent power supply problems, but it's worth double checking regardless.
    Pi Chi likes this.
  4. Pi Chi

    Thread Starter New Member

    Jun 10, 2015
    Good day!

    I saw your replies last night, thank you! :)

    We checked our components again. I think we have used a different relay from the schematic. We're not really familiar with relays. A customer from the store told us that what we bought is a relay that triggers at 12V. We tested it and it can be triggered at 7-8V. Still we used 12V.
    We weren't able to use the said power supply because we weren't able to buy one.
    Instead, we used a 300watts ATX. Since it's an ATX we were able to separate the VCC for the trigger and the common pin even when they both use 12V..

    It's working now but I'm not sure how long will it last without overheating or exploding or anything.
    Hopefully it won't do anything crazy :)