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Thread: power off circut

  1. #1
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    Post power off circut

    I made a circuit that will power down my AT motherboard after about 15 min. The base of the transistor (npn generic) sees up to 15V will this damage the transistor? I am used to working with them up to 5 volts on the base. BTW if anyone wants the schematic I can draw one up, it is only a relay, capactitor and transistor.

  2. #2
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    Originally posted by alexchannell:
    <STRONG>I made a circuit that will power down my AT motherboard after about 15 min. The base of the transistor (npn generic) sees up to 15V will this damage the transistor? I am used to working with them up to 5 volts on the base. BTW if anyone wants the schematic I can draw one up, it is only a relay, capactitor and transistor.</STRONG>
    ??? 5 volts at the base of the transistor? Is the emmiter at ground or what? base to emitter is suppose to be .7 volts or so. going higher than that will burn the device up. Use resistors to limit the voltage and current.
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  3. #3
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    The base is tied to the capacitors which are charged to the battery voltage (about 13) and the collector has the battery voltage directly to it. The emitter connects to one side of a relay coil. The other side of the coil is to ground. When the capacitor voltage drops to low (as it is slowly discharging through the base) the voltage out the emmiter lowers until the relay opens and shuts off the computer. It works right now, but will it eventually blow up?

  4. #4
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    Originally posted by alexchannell:
    <STRONG>The base is tied to the capacitors which are charged to the battery voltage (about 13) and the collector has the battery voltage directly to it. The emitter connects to one side of a relay coil. The other side of the coil is to ground. When the capacitor voltage drops to low (as it is slowly discharging through the base) the voltage out the emmiter lowers until the relay opens and shuts off the computer. It works right now, but will it eventually blow up?</STRONG>
    Hmm, sounds like an emitter follower. This should be fine. The voltages you want to look out for are the collector to base, emitter to base (reverse biased), and of course collector to emitter. What is the part number of the transistor you are using?

    Presslab

  5. #5
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    This should work better




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  6. #6
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    Actually this circuit kind sucks now that I think about it. Put the resistor where B is and put a diode pointing to the right where the resistor was.
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  7. #7
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    Okay, thanks, I'll give it a try.

  8. #8
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    You should put a diode across the relay's coil to protect the transistor from spikes.
    If it ain't broken, open it to see what makes it "tick".

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