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Pass Transistors and Current Limiting

Discussion in 'General Electronics Discussion' started by Raven Luni, Jun 19, 2012.

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  1. Raven Luni

    Raven Luni

    Oct 15, 2011

    I've now got all the parts I need for my car power adapter (power transistors - TIP41a, TIP42a and a selection of resistors rated from 2W to 7W) and I just wanted to be clear on a few details. It will have a number of voltage settings, but the 12V setting wont use the regulator. If I wanted to draw about [email protected] with a resistor alone, I would choose a 4.7 ohm resistor (which would give me about 2.55A), but that equates to about 30.64 watts (and a toasted resistor). Am I correct in assuming that when using a transistor, the above resistor is replaced by the base resistor and its value multiplied by the transistor's current gain (for example 47 ohms if the gain was 10)? And does this provide safe current limiting?
  2. ctrlfreak


    Jun 20, 2012
    You could always use a fuse to protect your circuit.
  3. Raven Luni

    Raven Luni

    Oct 15, 2011
    I want to build a robust power supply. I will of course include a fuse as part of the design but that doesnt really answer my question.
  4. john monks

    john monks

    Mar 9, 2012
    You can do something like that but you don't know just what the current gain of the transistor is. Normally you have some kind of current limiting such as a resistor in the emitter or the collector to prevent thermal run-away and to provide current stability. Maybe you should consider a power mosfet such as a MTP10N05 or IRF510 with a 1 ohm resistor from source to ground (chassis ground) with a voltmeter across the 1 ohm resistor.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Can you start by posting a schematic and full specification?
  6. Raven Luni

    Raven Luni

    Oct 15, 2011
    No because I havent designed it yet. I'm just making sure I've got all the principles right before I do anything.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    For protection, some sort of current limit may be desirable. Note that a current limit may only protect your fuse :)

    Depending on your load, you may want to consider foldback current limiting which will protect your load and the power supply by winding back the current to a very low level if the limit is exceeded.

    The downside of foldback current limiting is that it can be triggered by a brief high current demand (such as capacitors charging when a device is turned on) and then limit the current to a value too low for the device to function.

    If you expect that the power supply could be shorted, and remain shorted, then foldback current limiting is a good idea. If you expect that you might have brief overloads that are transient, then simple current limiting may be a better option.

    Depending on your power supply, there are other options too. Some switchmode power supplies can go into a hic-up mode where they periodically try to start and if the current demanded is too high, they shut sown again for a while.
  8. Raven Luni

    Raven Luni

    Oct 15, 2011
    Interesting, hadnt heard of foldback before. I wonder if thats why those inverters were always such a pain to start up.

    I've had decent simulation results with a standard 2 transistor limiter - if the load is on the collector side the current levels off nicely and theres very little power in the limiter circuit. I was thinking of using a latch to pull down the main pass transistor (and light a warning LED) which could be set to trigger after a short delay - that would allow any capacitors to charge and turn the whole thing off in the event of an overload or short, while never passing any more than the designed maximum current.
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