To Latch or not SCR

Discussion in 'General Electronics Chat' started by burntwire, Nov 25, 2009.

  1. burntwire

    Thread Starter New Member

    Nov 25, 2009
    5
    0
    (SCR) not latching.
    Sorry to ask what I think is a silly question but I’m stumped. I have a manual switch I’m attempting to replace with a SCR.

    So I purchased my first SCR and figured I start with a simple DC motor latch test. Well I’m having no luck with latching. The data sheet

    SCR calls for
    IH Max. Holding Current 200
    I Max. Latching Current 400
    The test case has two voltages V1 24VDC power source for motor.
    V2 9VDC trigger for the gate

    The test hookup is simple 24VDC into one Motor lead, second Motor lead to Anode with Cathode connectede to ground.

    V2 9VDC connected to normally open push button connected to 1N4001 diode then to the SCR gate
    When I push the button the motor runs pulling .37 amps through SCR
    When I release the button the SCR immediately shuts off any thoughts what I’m missing pertain to having the SCR Latch?

    Here’s the data sheet.
    http://www.mouser.com/catalog/specsheets/9439170t.pdf

    Thanks for your input,
    burntwire
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    You should have a current limiting resistor in series with the gate. 47 ohms should work. It will get hot if you keep the gate drive connected.
    How are you planning to turn the motor off?
     
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    The SCR will unlatch if the current through it is interrupted, even if only a brief interruption. Since the DC motor has brushes, the current is likely not constant and the brush noise is unlatching the SCR.

    You may need to put a capacitor across the motor... I'm going to guess: Electrolytic, 10uF 50VDC rating. Get the polarity the right way around.

    Put a 5Ω 2W resistor in series right after the anode to limit capacitor charge current through the SCR and improve RC filtering charactistics.

    Assuming your circuit now works, you can put a normally closed push button in series with the resistor, which will turn the motor off if pushed.

    Good Luck
     
  4. burntwire

    Thread Starter New Member

    Nov 25, 2009
    5
    0
    I didn't even think of the brushes Doh!. As far as turning it off this was just a test to see the SCR latch. I'll put the 47Ohm between the gate and V2 however V2 will normally only be connected to the gate for a very brief moment to latch the SCR.
    Thanks again and happy Thanksgiving.
    bw

    I'll try and post the results.
     
    Last edited: Nov 26, 2009
  5. burntwire

    Thread Starter New Member

    Nov 25, 2009
    5
    0
    Update:
    Thanks for the advise. I still could not get the SCR to latch so based on your feed back I assumed the circuit was correct. So I removed the DC motor and substitute a EMP coil I had laying around. This increased the current from .37amp to 5 amp. The SCR latched. So I'll need to figure out the minimum latch current for the SCR as the 400mA doesn't seem to do it.

    My next challenge is connecting and triggering 4 SCR in series to fire the Coil mentioned above at full power. The cap bank is 140uF 4Kv

    Thanks again.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    You are going to charge a 140uF cap to 4KV then discharge it through a coil with SCR's? S I'm not too sure that the 4 SCR's in series will do what you want. Beyond that, 4KV stored in those caps is LETHAL!.
     
  7. burntwire

    Thread Starter New Member

    Nov 25, 2009
    5
    0
    Yes,
    Its quite lethal I totally agree.
    I'm totally paranoid about shorting them when they are not in use and with I'm servicing the coil.

    I'm allowing .8Kv tolerance with the 4 SCRs in series as each is rated for 1.2Kv this should hold back 4.8Kv.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348

    That is assuming that the voltage will be equally distributed between the four and that they all fire at exactly the same time.

    PS: I can see how you came up with your Username:)
     
  9. burntwire

    Thread Starter New Member

    Nov 25, 2009
    5
    0
    LOL, Thanks I take that as a huge complement. Some like the smell of scented candles. I truly enjoy vaporized enamel. Normally I'l only smoke my bifilar coils.
    You may have noticed this is my first attempt at using the SCR's.
    I intend to use 4 10MΩ shunt resistors to balance out the voltage across each SCR.
    To date I'v been using a very simple switch for firing the coil. The switch consist of 1 inch bus bar on spring loaded hinge which is held 1/8 inch from second 1 inch bus bar. Triggering is achieved by removing the CPVC separator quickly. Big spark but works. My goal is to replace this very crude switch with something from this century.
    Thanks again,
    bw
     
  10. GreenLite53

    New Member

    Jul 29, 2011
    1
    0
    I would also insure that a well filtered DC power supply is being used in this application. As far as your motor, a NTE5455 would probably work.
     
Loading...