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Controlling Darlington with SX... — Parallax Forums

Controlling Darlington with SX...

SteelSteel Posts: 313
edited 2006-09-21 04:11 in General Discussion
I am seeing issues controlling my darlington transistor with an SX.

I want to be able to provide source current to pin1 of the connector.

The Transistor is a ZTX603...My base resistor is 370 ohms, and my Collector resistor is 1k.· Should I swap both of these resistors with 10k?

Thanks.
Shaun

Post Edited (Steel) : 9/20/2006 5:49:10 PM GMT
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Comments

  • SteelSteel Posts: 313
    edited 2006-09-20 17:45
    edited to add circuit
  • BeanBean Posts: 8,129
    edited 2006-09-20 18:17
    Steel said...

    ·I want to be able to provide source current to pin1 of the connector.
    Shaun,
    · The only thing in your circuit connected to Vcc is the 1K resistor. So that is the only path to "source" current to the connector.

    · What exactly does the connector connection require to·operate ?

    Bean.


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    ·
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-09-20 18:39
    Steel, if your goal is to provide current, the path must go through the controller, this means you have to provide two pins on the controller, the high side which is connected from Vcc to the controller, and the low side which is connected to the collector. Whether you need a resistor as well (the 1k as shown in the schematic) depends on the resistance of the path through the controller and your desired current flowing through the controller. As you have it now, a voltage is being supplied to the controller, this is fine if the impedance looking into pin 1 of the connector is high, but if it's low, the controller will be always be on.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • SteelSteel Posts: 313
    edited 2006-09-20 21:53
    The object is to provide enough power for a solenoid.

    I know that the SX pin will not provide enough current for the solenoid, so I am putting in a darlington transistor in circuit to provide current from the source voltage.

    My goal was:

    When the SX Pin is high, no current is flowing to the connector at the collector.
    When the SX Pin is Low, the source voltage (5V) is flowing to the collector at the connector.

    When I measure the voltage on the SX pin, it is correct 0 or 5V.
    When I measure the voltage on pin1 of the resistor, it is correct (based on what the SX pin is saying)
    When I measure the voltage on Pin2 of the resistor, or anywhere beyond the Transistor, I am reading about 2V.

    I think the Transistor may be blown, and I want to replace it, but I want to make sure I have the correct resistor values/theories behind it to prevent it from blowing again.

    Shaun
  • Dave HeinDave Hein Posts: 6,347
    edited 2006-09-20 22:57
    Shaun,

    According to the spec sheet, the ZTX603 has a current gain of 2000 at it's maximum collector current of 1 amp. A collector current of 1 amp would require only 500 micro-amps of base current at a gain of 2000. The base voltage is 1.8 volts, and if the SX is running at 5 volts the base resistor would be 6.4 K-ohms. I would use a lower resistor value in the range of 1K to 3.3K to ensure that the Darlington is fully on.

    Even though the Darlington has a high gain, I believe that you can't drive the emitter to ground. This is because the collector of the first transtor has to ride at the voltage of the base of the second transistor, which must be around 0.9 volts. If the emitter drops below 0.9 volts the first transitor will turn off.

    You should connect one end of the solenoid to 5 volts and the other end to the emitter. You may need a limiting resistor in series to ensure that you don't drive too much current through the solenoid coil. What's the resistance of the solenoid coil, and how much current does it need to operate?

    Dave
  • Dave HeinDave Hein Posts: 6,347
    edited 2006-09-20 23:13
    There was an error in my previous post. I used the word emitter when I should have said collector, and collector when it should have been emitter. The emitter should be tied to ground as you have it, and the collector should be connected to one end of the solenoid coil with a series resistor if needed. The other end of the solenoid coil should be connected to +5.

    Dave
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-09-20 23:20
    Shaun,

    What are the specs on the solenoid you are trying to drive? What is the driving voltage (voltage required to operate), the driving current and its resistance (which can be measured by using an ohmmeter across it's terminals)?

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • Beau SchwabeBeau Schwabe Posts: 6,563
    edited 2006-09-21 04:11
    Shaun,
    Here is a FAQ that might help... Don't get HOT choosing the wrong transistor
    Even though it references a motor in the schematic, a Relay, Light bulb, Solenoid, etc. could just as easily be substituted.
    Note: Because your transistor is a Darlington and the B-E saturation voltage is 1.8· ... use·1.8V in place of the 0.6V in the formulas above.
    ·
    ·




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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
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