Transistor substitution

Discussion in 'The Projects Forum' started by ElectroMagnetic, Sep 22, 2010.

  1. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    First I'd like to thank cfoek for the thread and Bill Marsden for his solution: http://forum.allaboutcircuits.com/showthread.php?t=32944&highlight=newbie

    I was wondering if I could use the following Power Amp Switch NPN

    SK9856 (NTE2351)
    ------------------------
    Vcbo 100V
    Vceo 80V
    Vebo 5V
    Ic 4A
    Ib 400mA
    Pc Ta 1W
    Pc Tc 15W


    as the driver of a small 12v reed relay that has a 550 ohm coil. In Bills circuit the output of pin 3 of the 555 timer powers the base of a transistor to switch the relay coil to ground. I believe the Power transistor he recommends (2N3055) is a little over kill in my case, plus I already have the SK9856 (NTE2351).

    TIA




     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    That's an outrageously large transistor for the job. You only need a little over 20 milliamps! Use a 2N3094 or a 2N2222, or dispense with the transistor completely and just have the 555 drive the relay directly--they have a large output drive capability so you can do just that. But note that if you do, you should connect the relay from the output to Gnd, as the sense will be inverted. Anode of freewheeling diode goes to Gnd too, cathode to the 555 output.
     
  3. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    I'm and extreme novice with this stuff, but I think I know what you mean with respect to the rewiring without the transistor and allowing the 555 to do all the work. I've already blown up a one 555 and damaged the timer setting electrolytic cap. Thank god that was all. ;-)

    So I disconnect the positive rail from one side of the relay coil (current config) attach that relay coil lead to pin 3 of the 555. Attach the other lead of the coil to ground and place the diode parallel to the relay coil with the anode connected to ground and the cathode connected to pin 3 of the 555, correct!

    I'm so anxious and excited about this project I'll probably try it before you respond. So if you hear a loud boom, you'll know I messed up again. ;-) That 555 did pack a wallop though for being so small. I always wear safety glasses (readers) and use a lighted magnifier. It's one of the only times since I've been loosing my eyesight that I was thankful! ;-)

    Thanks Again!
     
  4. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    Sometimes the circuit energizes by itself (without switch being pressed) immediately after I've cycled it, although it appears to be random. Also it seems to always energize when I either turn the power supply on or off (without pressing switch); voltage spike?. Could a filtering cap cure this or is it indicative of a failing component already in circuit?

    For my test rig I'm using a Heavy Duty 30 Amp Continuous Bench 12 volt Power Supply. But while in use the circuit will be powered by another dedicated 12 volt power supply/battery charging board.

    If you need to refer to the circuit, I took it verbatim from Bill Marsden's design, except that I eliminated the 1K resistor and the transistor that it feeds and directly connected the relay to the 555.

    TIA
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    Now you're getting down to real-world behavior!

    One thing occurred to me about driving the relay directly off the 555. It's not good that the output goes high to drive the relay, but then drops to zero. That 0 volt level would fight with the voltage generated by the decaying current in the relay, and I'm not sure what the result would be. I suggest adding another diode from pin3, with the anode to the 555 and the cathode to the junction between the other diode and the relay. That way, current can only flow out of the 555 and not into it.

    Make sure that there are filter capacitors across the power supply of the 555; this is important with switching circuits. 100uF electrolytic in parallel with a 0.1uF ceramic should do it.

    As for the behavior when the power goes on or off, you may be stuck with it. There could well be transients involved as capacitors get their initial charge, or as voltages settle to their normal operating level. It may be possible to deal with this by putting a capacitor to Gnd and a a resistor to Vcc on pin 4, the Reset line. Then the chip could be held in reset for a certain time after power is applied.
     
  6. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    Are these the changes you recommend? Go easy on me, remember I'm a Super Novice. ;)

    Black is current config, Red are additions.

    This is the exact layout on the board.

    TIA

    Delay 1 sec then off in 1 sec.

    P.S. Caps with two straight lines are ceramic discs. Ones with straight and curved are polarized electrolytic.
     
  7. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    Thanks again John P,

    I put the diode in circuit and it seems to have worked, I also placed a cap across the incoming Vcc and Ground rails. but I still have a problem with power on/off. So I was thinking about what you said,
    , but I don't have a clue as to which values to use.

    I'm really sorry for being such a nugget, but do you or anyone else know the formula I'd have to use to calculate the values for the cap and resistor mentioned above.

    TIA
     
  8. Member02

    New Member

    Mar 23, 2010
    28
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    I believe the point he is making is to have a high impedance resistor trickle charging a decent cap, so as to delay the startup of the chip while the circuit powers up
     
  9. ElectroMagnetic

    Thread Starter New Member

    Sep 15, 2010
    13
    0
    but I still don't know how to calculate the values.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Use a 100K resistor and a 0.1 uF cap to start with.
     
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