Silicon controlled rectifier

Discussion in 'General Electronics Chat' started by BrewDos, Jun 8, 2004.

  1. BrewDos

    Thread Starter New Member

    Jun 8, 2004
    I've got an oven that uses 4 SCR's that control the heater elements. The problem is that the heat is on all the time. It has a 4 to 20ma tempature control that sends a signal to the SCR control board. The oven is 480 volt 3 phase and 4 SCR's control 2 of the phases while the other phase is conected directly to the heater element through a contactor. The 4 to 20ma control works as I ran it through my meter an looks fine. I wanted to use an oscilliscope to look at what was going on with the SCR's to isolate the problem to either the SCR's or the control board.
    I think the SCR's are controlling the current to the heater elements. What should the signal at the gate of the SCR's look like? Or what would be the best way to isolate the problem.
    Thanks for any comments
  2. beenthere

    Retired Moderator

    Apr 20, 2004

    You might have a problem using an oscope to observe waveforms. I've seen 1/4" of probe tip vaporised by probing where grounds were not common. If it's a portable (battery powered), you may be ok. You can also use a cheater to isolate the ground prong, but that can lead to the oscope case floating to a lethal potential. Be careful.

    Static checking scr's can be fun, too. With a diode function on a meter, the gate will show a small reading to the cathode in both directions. However, it should check open cathode to anode, and a to c. If your meter has enough voltage in the ohms setting, you can put the leads across the scr in the correct polarity, and jump the anode to the gate. The src should go from non conducting to on when you do that.

    The pulse that turns the scr on will float on the AC waveform, and will be about 10 volts above the waveform voltage level. Otherwise, it will be the same potential as applied to the cathode. Duration is usually pretty short, as the scr stays on fro the remainder of the AC half cycle after the gate pulse.

    Try disconnecting the gate signal, and turn on the oven. If it gets hot, the scr must be shorted (assuming it's the only current path). That's the safest method.
  3. BrewDos

    Thread Starter New Member

    Jun 8, 2004
    Well it looks like it is the gate control board. I did disconnect the gate and the oven satyed cool. I finally called the mfg and talk to the technical help personel and he said it was the gate control board. They also had an interesting read on SCR troubleshooting at there website named: Two Leg SCR Unit.PDF
    When the new control board comes in I plan to scope out the SCR's in an On/Off condition.
    Thanks for your input
  4. bipin

    Well-Known Member

    Mar 21, 2004
    Normally(but not necessarily) when SCRs goes diffective it goes short. so you can just check them for dead short. You can probe your scope to the firing ckts and the gate to the cathode by isolating the scope using an isolation transformer. As a better practice isolate the device (oven ) also using a high VA isolation transformer.
    And use only one probe at a time, because when you connect two probes at different places you might end up in shorting the two undesired points where you have connected you groung crocodile pin of the probe.
    best of luck
    bp n