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Diode I believe?

Discussion in 'General Electronics Discussion' started by Sleepy Dog, Feb 4, 2013.

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  1. Sleepy Dog

    Sleepy Dog

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    Feb 4, 2013
    Hi all, first post.

    I need some help. I designed an automated bottle filling machine for the brewery I own. It's a 24v system that uses four separate control boards for the various procedures needed.

    The fill level is controlled through conductivity. I use an 8 volt separate power source for conductivity. I have the circuit that powers the solenoids (24v) that opens the beer and vent running through a NC relay. When the beer level reaches the two conductivity probes, it closes the circuit and sends 8 volts to a silicon controller rectifier, which sends a clean 24v to the NC relay stopping the beer flow.

    The problem is the SCR. It takes 3.2 volts to open the rectifier. Sometimes for some unknown reason I get some voltage coming back off the probes before they are closed which is strange. Its usually .5-2.0 volts. But occasionally it spikes over 3 volts and trips the SCR and stops the fill early.

    Is there a way I can filter the voltage between the probe and rectifier so that it will not allow voltage through unless it reaches a certain voltage? Like 5 volts?
     
  2. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    3.2V seems to be high to turn on a SCR.

    A 3.3V zener diode in series with the gate would increase the necessary voltage.
    There is normally a resistor gate to cathode, usually below 1k. This could be lowered to reduce sensitivity.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    First let's get the brewery jokes out of the way.

    So... do you have any daughters 18 or older? And would it be OK for me to visit and watch the system in action? It might take me a while to figure out what the problem is. Probably a few months, maybe longer.

    Is it possible that the glitches on the probe signal are caused by drops getting on the probes? Would moving the probes further apart help?

    Is one of the probes connected directly to the SCR? Or is there some processing circuitry in between them?

    You could also add an R-C delay between the probe circuitry and the SCR, so the contacts have to remain closed for a certain length of time before the SCR will fire. You would have to put the probes further down the bottle to compensate for the delay.
     
  4. Sleepy Dog

    Sleepy Dog

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    0
    Feb 4, 2013
    I'll give that a try tomorrow.

    Not really. The fill runs down the sides of the bottle and sometimes it opens before the fill starts.

    Yes, one probe is connected directly the the SCR with nothing in-between.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    I guess the obvious thing to try is a capacitor between the gate and cathode of the SCR.

    If you're using the beer's conductivity to trigger the SCR, the resistance will be fairly high - at least in the kilohm range, maybe higher. So you'll need to use a small value capacitor, otherwise the delay will be too great.

    Try 10 nF and if that's not enough to filter out the spikes in all cases, increase it. If it's reliable but it adds too much delay, decrease it.
     
  6. ninojovannionde

    ninojovannionde

    2
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    Jan 21, 2013
    maybe because the backfire of the relay.. Do I make sense?
     
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