need help troubleshooting NPN circuit

Discussion in 'The Projects Forum' started by aughtago, Dec 15, 2012.

  1. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    I have attached a portion of the circuit containing Q1. The problem is that as soon as the circuit is powered (12VDC) up, the TR relay energizes. When this first occurred it was cycling in and out, but now TR picks up and stays pulled in.

    The CD4060 pin 3 (output ) is still low ).001V and there is 10.8 V across TR(at D1). I first suspected D2 and cut one lead- no change. I replaced Q1, R5, and C4, still no change. only 1.4- 1.9 v across Q1 EC

    after many voltage measurements, calculations and head scratching, today I cut R5, and C4 still no change.

    I have double, and triple checked the polarity of Q1(PN2222ATA) and double checked the board for shorts, I also attached a screen shot of the board layout.

    I am at a loss. hopefully someone can give me a clue.

    If you want more details, see page 3 of my previous thread,http://forum.allaboutcircuits.com/showthread.php?t=68755
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    if you have voltage across TR and D1 then either:
    1. Q1 is shorted or energized or wrong pinout
    2. C4 is shorted
    3. short to COM is elsewhere (PCB issue)

    check continuity across C4 when board is off. there should be no continuity (or at least not dead short, maybe hudred ohms or more). if D1 is not soldered correctly, instead of protecting Q1 it may let TR kill it.
     
    Last edited: Dec 15, 2012
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What are the circles with the crosses in them?
    How much current does your relay coil require?
    What is the value of VDD?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    Unless your relay just need a little current, otherwise R5 should change to under 10K.
    Take D2 away.
    Let Pin 3 alone, add a 0.1uf and test the voltage using AC meter with true rms or if you have scope, just use it to measure the wave.
     
  5. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    1. I replaced Q1 and when this issue started the TR was cycling on and off, so I do not think it could be shorted, I also did the resistance check across Q1 and C4. you may be right on pinout, (I originally had it backwards on my breadboard) but I have triple checked it and it looks right to me, I'm hoping someone can back me up on that.
    2. I replaced c4 and then cut it out of the board- no change.
    3. short is possible, I have gone over that circuit again and again with multimeter and magnifying glass.

    If d1 was backwards I don't think TR would even pick up at all.

    Thanks for the reply
     
  6. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    The circles with crosses are LED's (with built in resistors) to monitor the timing process. I can switch them off by removing the "test" jumper. I have tried it both with them on and off.

    The relay is rated 360 ohms, at the measured VDD of 13v I calculated the following:
    • Ic= .036A
    • Ib= or greater than .00036 (I am shooting for .000559) with a 22k Rb(R5)
    Strange thing is the timing circuit worked fine on the breadboard with only a few tweaks, i then transferred those same components to the finished board with all the rest of the circuit and things began to go wrong.

    Thanks for the questions, hope I answered them all.
     
  7. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    See my answer to Ron on relay current and sizing of R5. when the circuit was experiencing the cycling problem, I let the timer finish its task and when it did the TR pulled in solid, voltage readings I took indicated the circuit perfectly.

    I tried cutting D2 out of the circuit (I did not know its purpose) but it had no effect.

    I don't understand your last suggestion.

    Thanks for your input
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    >Let Pin 3 alone, add a 0.1uf and test the voltage using AC meter with true rms
    >or if you have scope, just use it to measure the wave.
    >I don't understand your last suggestion.

    Because the Pin 3 is a pulse, not a Vdc voltage, so you can't use the normal way to measure it, you have to use scope or true rms AC meter.
    If the frequency of pulse is very low as 2 HZ, and then you can use the normal multimeter to measure it, otherwise you better use the true rms AC meter, some multimeter has that function.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    To guarantee saturation of most BJTs, you need to set Ib≈Ic/10. This is called 'forced beta'. See the attachment, which is excerpted from the Fairchild datasheet.
    Since Ic=36mA, you need ≈3.6mA of base current. R5 needs to be
    R5=((Vcc-Vbe)/Ib)-Ron, where Ron is the resistance of the internal pullup transistor on pin 3. From fig.6 in the CD4060 datasheet, we can calculate that the worst case Ron≈500Ω. We know that Vbe≈0.7V. Therefore, R5=(13-0.7)/.0036-500

    R5≈3kΩ

    This doesn't mean that your circuit won't work with 10k, or even 22k, but it is not guaranteed to work with any PN2222A that you happen to install unless you use 3k.

    I would make this change if I were you, but it still doesn't explain why the relay activates when pin 3 is low.
    Keep in mind that the LED on pin 3 will also load the pullup device inside the chip.
    I believe D2 is intended to stop the clock when pin 3 goes high.
     
    Last edited: Dec 16, 2012
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Maybe this has been addressed,

    Does the LED on pin three work properly, but not the relay?
    If so maybe you've got some bad 2n2222's

    Try disconnecting the Q1 base resistor from the IC and put it to Gnd then V+
    as a foolproof test for relay/Q1 operation.
     
  11. sheldons

    Well-Known Member

    Oct 26, 2011
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    disconnect your power source,with your meter check across the collector/emitter of your transistor for sc....better still remove it and check out of circuit for shorts...with the transistor removed your relay shouldnt operate....ive uploaded a simple relay driver which will give you an idea of what to check.
     
  12. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    I did not Know it was a pulse, I designed it based upon this circuit, the graphic led me to believe it went high and stayed high. If the pin 3 is a pulse then how does Q1 stay conducting? I don't have a true RMS meter at home, but I can barrow one (Fluke 87) from work.
     
  13. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    I will try the 3K resistor, For some reason I thought the gain was 100, looking through the paperwork, I'm not sure where I got that from.

    Ok D2 stopping the clock makes sense, I could not figurethat one out.

    Thanks again for the input
     
  14. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    Q14 LED has been performing perfectly, but I like your test, I will try that when replacing R5.

    Thanks for the suggestion
     
  15. aughtago

    Thread Starter New Member

    Apr 13, 2012
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    Yes I tried that meter test , both with the new one that is in the board, and the original that I removed. reading is Kohms to Mohms in both directions.

    Thanks
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Beta is a minimum of 100 at 150mA and Vce=10V. You are running in saturation (Vce<100mV) at Ic=36mA. Beta is very low in saturation.
     
  17. ScottWang

    Moderator

    Aug 23, 2012
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    Sorry, I apologize for my fault, the circuit is a new application to me, it's not a circuit that I usually do, so I have to do some test.
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    I rechecked the circuit, it's a different from my usually do, I used in pulse mode normally as 32768Hz divid 16384 became 2 Hz, but you used it as timer over 1 sec, here are some points maybe you can change :
    1. If you want to use Pin 12 to be the reset pin, what is your purpose for this circuit?
    2. C4 could take away, where did you get that idea?
    3. I think D2 is used to stop the timer, when the time's up, Pin 3 adding the high to stop to count.
    4. If your relay need the current is 36mA, I think 36mA is for the normal work, a relay has three different current status, 1) active current, 2) normal work current, 3) Maintain current.
    If the 36mA is the normal work current, then the active current will around 70mA, and the maintain current will around 20mA, you have to make sure what is the 36mA belong to?

    Isolating pin 3 is easier to test the pin is ok or not, you also need the Q1 transistor to isolate from other pins, and test it, connecting a 10K from B of Q1, and switch to 12V and GND to test the Q1 and relay.
     
  19. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A relay coil will never draw more than Vcoil/Rcoil. Where did you get this idea?
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    I can't remember that.
    Since you mentioned that, I took a relay 6V/100Ω, and used needle meter and digital meter to measure, I'm adding 5V to the relay, the needle meter shows that around 75mA, but the digital meter shows that around 48mA, I think the digital meter is pretty close to right current, maybe the needle meter was damaged somehow, but I don't know that.

    Btw, Do you have any good method to measure the transient current?
    Does the current probe can do that, but I don't have one.
     
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