Need Review...

Discussion in 'The Projects Forum' started by idleup, Nov 25, 2013.

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  1. idleup

    Thread Starter New Member

    Nov 24, 2013
    I do not know much about electronics and need some help reviewing the below circuit ASAP as it is for a light display I hope to launch on Thanksgiving.

    I designed the attached circuit for some Bi-Color LED strands that I purchased at Costco. The way they work is they run off 120vac but use a full wave bridge rectifier to convert to 120vdc. With the polarity one way the LED's are white, and with the polarity reversed the LED's are color.

    The purpose of the circuit is so I can use a light controller that outputs 5vdc to trigger the reversing polarity output to the light strand to switch between all white, or all color. I originally looked at using an H-Bridge but since I am dealing with 120vdc output it was more practical to use a DPDT relay instead to act as my H-Bridge. BTW, I am using this one from Digikey (

    I have tested the circuit and it worked great for about 25 cycles. Then the Relay started chattering when there was no voltage applied to the coil and the light strand was not lighting (before this the strand lit in white with no voltage applied to the coil, and in color with voltage applied). When I applied voltage to the coil the chattering stopped and the lights came on in color just fine but once removed they turned off and the chattering started again. After taking some voltage measurements I found that I blew one of the fuses on the AC input (The christmas light plugs have a 3A fuse on each leg of the AC input cord) and was getting a low AC reading. I replaced the fuse and the lights came on white with no voltage applied to the coil as they did before, then as soon as I applied voltage they turned color but when I removed voltage on the coil the fuse on the AC input was blown again! I found it odd that it was only blowing the fuse on one leg of the AC input cord and not both fuses as it usually does if you short the leads together. Anyway, after repeating this a couple times I assumed that I had fried the relay and that was what was causing it to keep blowing fuses.

    Anyway, I have soldered 9 of these circuits and started testing the second one as well, it worked as it should for about 10 cycles but then I decided to stop messing with it and have you guys review my schematic before I burned out that relay and it started having the same chattering and blowing fuses as the first one.

    I know with relays you can put diodes across the coil, and resistors and capacitors and what not but have no idea which to put in there, if it is necessary, and what size to use if I do add any of those. I also am not sure if that would have prevented the burning out of the relay and popping fuses on the AC input since adding those components across the coil is primarily meant to protect the coil source power (or circuit) not the other switched power source (in this case the wall AC input).

    Any feedback would be appreciated, thanks in advance!

    P.S. The circuit diagram shows 12vdc as the coil voltage, that is incorrect, I am using 5vdc!


    - Matt
    Last edited: Nov 25, 2013
  2. idleup

    Thread Starter New Member

    Nov 24, 2013
    Nobody has any feedback on how I can protect this circuit? This is a time sensitive project... is it possible that I blew up the relay because one side responded too slow and it shorted the AC? If so, how can I fix that?
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    To power a 5V relay from a 12V u need to calculate the proper resistance.

    Connecting a LED in series with relay is not a good idea since the relay current might be too much for the led.

    the relay data says 28mA so I presume u would need 250Ω. Try the relay alone first to see whether it clicks properly. A 470Ω might be a little too high to properly activate the relay.

    Remove the LED and try with a 470Ω series resistor with the relay @ 12V.
    Measure the coil voltage. If it chatters or if you can't here the relay click instantly then the resistor value is too high. Try a 270Ω 0.5W resistor.

    Connect the LED in parallel with the relay coil with a series resistor of 220Ω before the relay series resistor. This way the led will have no effect on relay current and will light when the 12V activates.

    You should check the load current before choosing a Relay. The contact should be rated more than the total load current.

  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    I think your in breach of the Term of Service , ie. LEDS to MAINS...
    wayneh likes this.
  5. idleup

    Thread Starter New Member

    Nov 24, 2013
    I made a mistake in my drawing, the input voltage from the controller is 5v, not 12v, so it does matches the coil rating.

    As for the rated load of the relay it is below the load of the christmas light strand which I think is 8mA but will check again.

    Any idea how I could have burned up the relay? or how I could protect the relay?

  6. Wendy


    Mar 24, 2008
    We have several rules meant to protect people new to electronics on this site. One of them is to connect LEDs directly to house voltage, which is what you are describing. It is a very dangerous practice to do this.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    This can be found in our Terms of Service (ToS).

    While I regret having to do this, I am closing this thread.
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