Transistor Switch

Discussion in 'General Electronics Chat' started by pntrbl, Apr 30, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I need to design and build a self contained gas powered battery charger in support of a commercial venture my Son-in-Law's are collaborating on.

    The attached circuit is my 1st attempt at a starting circuit. Some googling has revealed a deep cycle battery at 50% depth of discharge will be down to roughly 12V from a fully charged 12.6V. With the emitter essentially tied to gnd thru the relay I'm planning on using the R1/R2voltage divider to get the base just over the .7v (Vbe?) threshold. As the battery discharges it'll drop below, switch the transistor off, de-energize the relay, and with a normally closed relay that "was" being held open, start the motor.

    On the plus side this circuit will also automatically shut itself off. When the motor starts and the supply voltage comes up to charging levels the Vbe potential will go back up and turn the transistor back on.

    That's also the negative side in that it means the transistor and relay will be energized/on in a fully charged state. The idea here is to charge the batteries as necessary. Not create an additional drain .... Seems like it begs a PNP instead of the NPN I drew, but I can't get my head around what that kind of circuit might look like.

    I hope I've explained my thoughts well enough but any help, advice, opinions, and especially any corrections on what I think I've learned, are greatly appreciated.

    SP
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    A transistor by itself makes a lousy comparator. Its conduction is dependent on its temperature.
    Your transistor is an emitter-follower that does not switch anyway.
    A transistor switch has the load in series with its collector.
     
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Thank You for your opinions Audioguru. You are apparently more knowledgeable than I am ... so would you have any suggestions? :D

    SP
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Something else - you don't have a constant voltage reference such as a Zener diode. Without a voltage reference, there is no practical way to determine what the actual voltage of the battery is. Besides, if you had a relay coil energized the entire time the battery was in a "charged" state, you would be consuming a fair amount of power just in the relay coil itself.

    Charging of lead-acid batteries usually requires supplying them with current at a voltage in excess of 13.5v. If the voltage charging the battery is held constant, the current will decrease as the battery charges. If the charging current is held constant, the voltage will increase as the battery charges. It would shorten the life of the battery to continue charging it at high current when it was nearly fully charged.

    Discharging even deep-cycle batteries more than 30% will shorten their lifespan. If the generator is cycling on and off too frequently, you'd need to add more batteries in parallel.

    You need a bit of logic in there to control the start-up and shutdown of the generator. For example, you wouldn't want to just turn on the starter when the battery voltage dropped - you might want to have the circuit check the fuel and oil levels, and sound an alert if they're low. You might want to allow a maximum time for the starter to run before disabling it and sounding an alert, when the generator won't start for some reason. You may want to add a "service reminder" alert, because after the generator has run for x number of hours, it'll need things like an oil and filter change and a new spark plug.

    These kinds of things would be reasonably easy to do with a microcontroller.
     
  5. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    OK and Thank You SgtWookie. Looks like it's back to the drawing board!

    BTW, current plans are for no more than a 50% depth of discharge and I'm not happy about keeping the relay energized either. That was part of the reason for the post.

    I had wondered about a zener and I'll study up ......

    Totally agree on fault logic too. A belt might fall off and the starter would run until it burnt up or murdered the battery. All kinds of possibilities, but I kinda need to start at the beginning by getting it started and shut off. One step at a time.:)

    I don't know nuthin' from nuthin' on a microcontroller either. The last time I studied electronics was back in the slide rule days of the early 70's. Not likely I'm still smart enough for a micro anything so I'll just have to do it the old fashioned way.

    Thanx again. I'll keep digging.

    SP
     
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Well here's the 3rd attempt at Circuit Number 2. Something tells me we're gonna need about 42 more before I get this right ...:confused:

    Incorporated a zener on the base for a constant voltage reference as SgtWookie suggested.

    Moved the load to the collector based on what I learned from Audioguru.

    Seems like it'd work to me. As the supply voltage drops from the battery discharging the emitter will become less positive than the constant zener voltage on the base. Transistor then becomes conductive? Forward bias is forward bias right? Energizes a normally open relay. Starts the starter motor.

    Should shut itself off too. Right now I'm looking at a 14.6V charging level from a GM alternator and that should bring the emitter back to being more positive than the zener controlled base and it'll shut off the transistor?

    As always, any opinions, analysis, and general slaps about the head are welcome.
     
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