DC input voltage cutoff to SS Relay

Discussion in 'The Projects Forum' started by BarryL, Aug 20, 2016.

  1. BarryL

    Thread Starter New Member

    Aug 20, 2016
    I am using 3 solid state switching relays to map RGB LED s strips from a control source
    to another 4 sets of RGB LED strips. The relay ouput side is separately powered at 12VDC, 4A,

    I cannot tap directly to source strip control circuit because it is already near power capacity.

    The SSrelay draws 6ma at 12VDC input. The SSrelay will operate
    at a voltage range between 3 and 30VDC. The output side can handle 60VDC, but in this
    application is 12VDC.

    The RGB LED Strip control source provides fade (likely capacitor driven) between color changes, thus there
    is a voltage reduction where the SSrelay will continue to operate way too long when it would be best
    to be not active. The SSrelay will remain operating at 1 to 1.5 volts when started/operated by a higher voltage.

    I need to cutoff the SSrelay input votage at say anything less than 10 or 11 VDC.
    I can add LED fade on the relay output side with added electrolytic capacitors if I can stop the relay
    from operating during the source fading.

    Is there a simple solution? The control circuit must not be affected by this add-on circuit beyond stealing
    a small bit of power.
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    The outputfrom the controller is zero to some voltage greater than 10, and you want something in series with it that passes through anything greater than 10, but for any input less than 10 the output is zero. AND, hopefully is self-powered from the input signal. Correct? If so, then what is your skill set for assembling a small circuit?

    Comparator or opamp, 8-pin DIP or SO-8 package
    5-ish resistors
    2-ish capacitors
    1 Diode
    1 MOSFET

    One resistor and the diode form a very low voltage reference. The comparator runs on anything above 3 V, drawing around 1 mA. The input is divided down to a point that exceeds the 1-diode reference when the input is above 10 V. At this point the comparator turns on the MOSFET, passing the input through to the output. One more resistor provides hysteresis to prevent output noise bursts.

    How fast can the input voltage change?
    Is it PWM?

    Here is one version that pulls the minus side of the SSR down to the input return. Another way to go is to switch the plus side into the SSR.

    Last edited: Aug 22, 2016
  3. BarryL

    Thread Starter New Member

    Aug 20, 2016
    Thank you for your response. Regarding PWM I have to say I don't know. I am remote from the application
    and have no circuit schematics. The idea is to add-on some more RGB strip lights that map to the original strips.

    I can construct your circuit and it will solve a voltage problem, but now you have me thinking regarding the
    use of PWM control on the original circuit. ( I originally thought about it, and thought the designers were going cheap as possible)

    I have a query into the designer of the original circuit as they have kept the schematics non-public.
    It would help if I had the the application so I could see it in operation and look at it with a scope. All
    All I have is a picture of a plug with 8 wires controlling the two circuits of strips.

    Thank you again.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    The thing about PWM is that the circuit is powered from the signal, and a PWM square wave will not power the opamp correctly. Plus, the opamp needs a decoupling capacitor near its power pins which would appear directly across the signal (C2) and probably distort it.