SCR for Halfwave Switch

Discussion in 'The Projects Forum' started by ozarkdan, Jan 11, 2010.

  1. ozarkdan

    Thread Starter New Member

    Jan 11, 2010
    I am trying to use an SCR as a switch in place of a rectifier to control the charging of a cap. The cap needs to be charged to 60V prior to discharging and energizing a relay coil. (see attached ckt.) Normally a 1N4004 would be where Q1 is, and the ckt works great. But, I need to control when the cap C1 gets charged with the microprocessor.Is this possible using an SCR? Anyckt suggestions?

    Thanks Greatly,
  2. SgtWookie


    Jul 17, 2007
    It looks like you are attempting to operate this directly from mains power. This is not safe.

    You must use a transformer to isolate the circuit from the mains.
  3. ozarkdan

    Thread Starter New Member

    Jan 11, 2010
    Good point. Thanks.
  4. k7elp60

    Active Member

    Nov 4, 2008
    To add to what Sgt Wookie has said. As a suggestion, why not eliminate Q2 and use a transitor from the low side of the relay coil to ground. Replace Q1 with a standard diode like a 1N4004.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    We could be more helpful if you would explain what you are really trying to do.

    Senior Member

    Jul 1, 2008
    A SCR is a latching device. Once you've triggered the Gate to Cathode circuit with a positive voltage it will not turn off unless the Anode voltage is reversed or the Anode-Cathode current falls below a minimum holding current as spec'd in the data sheet. This can be accomplished by breaking the Anode - Cathode circuit or by shunting it.

    I guess I'll ask this too. What are you trying to do?
  7. ozarkdan

    Thread Starter New Member

    Jan 11, 2010
    I didn't want to make my post extensive, sorry.
    My ckt normally does have a 1N4005 where Q1 is, and in that configuation, works very well to turn the K1 ON and OFF by the microprocessor. The K1 is remotely located and uses Ground for the negative side, so interrupting that side is not practical. K1 requires a 60V pulse for Pull-in, and a 20mA - 30mA hold current. The described ckt works great and has been tested extensively. However, to meet certification by CSA, a second control device is required since the failure mode of the SCR is a Short and could hold K1 closed continuously.

    My thought was that since an SCR is like a diode, with a gate, I could use it to rectify and control the supply to the cap. However, I don't believe there is enough continuous Hold current while charging C1 to keep the SCR in contuction.

    I really appreciate the feedback here.