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BS2 high power transistor control problems — Parallax Forums

BS2 high power transistor control problems

WarumaWaruma Posts: 21
edited 2010-04-12 18:45 in BASIC Stamp
Hello, I need some help on this because I am stumped...

I am using the configuration in figure 6-1 of volume 1, colum 6 of "Nuts and Volts": http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv6.pdf
I've included the figure as an attachment below.

I am trying to use a HIGH pin command hooked up to a Darlington high-gain transistor to pull a large current through a small resistance load.

The problem is when I turn the pin HIGH, my Board of Education seems to reset (almost like it's a brown out or I'm short-circuiting something). I'm fairly certain that I have the high current portion of the circuit isolated from the BS2 via the transistor. I'm using a 9V battery to power the circuit, but having the NPN power source (supply) pull from the Vdd terminals on the breadboard. Now, when I disconnect the wire connected to the transistor base, and manually plug it into the Vdd bar on the BOE, the circuit works fine. I want to control the circuit by sending one of the BS2 pins HIGH to turn on the transistor - but it won't work.

The transistor I'm using is a 39K1529 Darlington NPN Transistor, and the RB as shown in my picture is around 1800 ohms. Assuming a 1.4V V_be potential for the transistor, I should only be sourcing about 2mA of current - so it's much less than the maximum 20mA sourcing limit. (5V-1.4V)/1800 ohms = 2mA. A BS2 pin has a +5V output, so that's the same potential as the Vdd bar on the BOE. Does anyone know why my circuit won't function when I try using the HIGH command from the BS2 pin?

Thanks
Darlington Transistor Problem.jpg
395 x 256 - 9K

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-08 03:40
    1) The configuration you've shown should work. Since it doesn't work, that means that what you actually have is not what you've shown.

    2) Make sure that your power source can handle the load. It's not clear from your description whether you're trying to power the Stamp and the load from the same power source. The voltage regulator on the board can only handle so much current (if you're powering the full load from Vdd). It will shut itself off (resetting the Stamp) if the load is too great.

    A schematic drawing of your actual circuit would help.
  • WarumaWaruma Posts: 21
    edited 2010-04-08 03:47
    This is pretty much the entire circuit. I am powering both the stamp and the circuit with the 9V battery, but I'm trying to only use the 5V Vdd supply to run the circuit off of. I'm trying to pull upwards of 4A to burn through the .7 ohm load. Once it's burned, I can use the LOW command on my BS2 pin. This circuit is only on for maybe 5 seconds max - so even though I have such high current requirements, it's not constantly pulling this current.
    Darlington Transistor Problem.jpg
    395 x 256 - 10K
  • FranklinFranklin Posts: 4,747
    edited 2010-04-08 04:30
    You can't use the stamp to provide power for the part of the circuit controlled by the transistor! The stamp can't provide that mich power and you can easily damage the stamp beyond repair.

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    - Stephen
  • ercoerco Posts: 20,256
    edited 2010-04-08 04:35
    A 9V battery can only put out maybe 100 mA for a minute or two, it will never put out 4A. Voltage will drop down below 5V long before then.

    Also, your 1800 ohm base bias resistor seems very high. What transistor are you using?

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    ·"If you build it, they will come."
  • WarumaWaruma Posts: 21
    edited 2010-04-08 05:04
    I didn't think I was using the stamp to provide power to the rest of the circuit. I'm only using 2mA to activate the BJT, and it's pulling power straight from the battery power supply. And I've gotten the 9V battery to put out a lot more than 4A. I'm basically short circuiting the battery since an almost 5V drop across a .7 ohm load = 7.14A. I'm not using this continuously - only for a couple of seconds every so often. And the transistor I'm using is one I ordered from newark.com and the newark part number is 39K1529.
  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-08 05:26
    Vdd is the output of the 5V regulator on the BOE which is rated at 1.5A if I remember correctly. As I mentioned, the regulator will shut down if more than 1.5A is drawn from it (or if it overheats). Vin is the unregulated BOE supply voltage.
  • WarumaWaruma Posts: 21
    edited 2010-04-08 05:41
    But that doesn't make any sense - no offense - but whether it' coming +5V through the stamp I/O pin, or +5V through the Vdd header on top of the breadboard area, it would still be regulated either way. It didn't shut down when I manually turn the transistor on, but it does shut down when I try and turn it on through the stamp. Is it possible that although my 9V battery says it's still got a 7.4 ish potential in it, that my 4A current draws have depleted the actual current level?
  • JonathanJonathan Posts: 1,023
    edited 2010-04-08 15:42
    I'll bet it's your battery. You just can't pull that much juice from a 9V battery. 7.4V is a toasted battery. Look for a wall wart.

    Jonathan

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  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-08 15:55
    You still haven't given a schematic of your actual circuit. You wouldn't believe how many times people have found that their wiring doesn't match what they've posted as their "pretty much the entire circuit" and the difference was what kept it from working.
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-08 16:05
    You'll probably have to order a 5V 4A wall supply. Four AA NiCd or NiMh batteries would be 4.8V and should have no problem supplying 4A at short duration with no regulator. I wouldn't exceed about a 5-10% duty cycle though.

    I'd go visit a hobby store with model airplane stuff. 4 AA packs are commonly used to power the receiver. I bought a 1200mA/hr NiMh pack for about $15. I would probably NOT get the common 600mA - 800mA packs... They'd supply 4A, but you're asking a lot. Or course you'd need to charge them as well, but there's many inexpensive ways to do that, or buy a 120 - 500mA wall-wart charger from Radio Shack or whatever.

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  • ercoerco Posts: 20,256
    edited 2010-04-08 21:41
    Suggest you post a large, clear photo of your exact setup showing board, battery and every bit of wiring.

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    ·"If you build it, they will come."
  • WarumaWaruma Posts: 21
    edited 2010-04-09 04:58
    Thank you everyone for the help. My circuit is working now and the culprit to my problem was my battery. Once I swapped out my battery with a new one, circuit works fine! Thanks again. [noparse]:)[/noparse]
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-09 15:39
    a 9V battery delivering 4A????? NAAAAAAAAAAAaaaaaaah. no way! Really? I would say that's not possible, but.....

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    "puff"...... Smile, there went another one.
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-09 15:57
    I pulled a data sheet off Energizer's website for a 9V alkaline.... If I'm reading the charts right this 9V battery can supply ~500mA for an hour till voltage drops to 4.8V. There's no data on more than 500ma or less than about 25ohm.

    http://data.energizer.com/PDFs/522.pdf


    Weird. I dunno.

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    "puff"...... Smile, there went another one.
  • ercoerco Posts: 20,256
    edited 2010-04-09 17:12
    That's a good link, but it looks to me like the charts indicate ~500 mAH life at 100mA intermittent drain. That's a total of ~5 hours, drawing 100 mA only occasionally and then letting the battery recover for a while, before it gets down to the 5V range.

    If you use the battery continuously at 100 mA, it will not last anywhere near 5 hours.

    A 9V battery won't last nearly that long powering 5V electronics through a typical voltage regulator, a lot of power is wasted as heat. And the regulator needs almost 2 volts of overhead to function reliably. From my experience, once the battery no-load voltage gets down below 8 volts, the Stamp may operate with random glitches, especially if you're switching a few relays.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    ·"If you build it, they will come."
  • ercoerco Posts: 20,256
    edited 2010-04-09 20:02
    Waruma: You must be using something other than a small rectangular 9V alkaline battery, correct? If you're using some type of 9V lithium or rechargeable ibattery, that's a very different situation.

    I tested a new 9V Alkaline Energizer by shorting it directly with a 10A meter. I got 3 amps for ~3 seconds, then it dropped steadily down to 2 amps after 10-15 seconds. I'm sure the voltage dropped way down under that drain, although I didn't measure it.

    But if you have an alkaline 9V battery that's delivering over 4A through a 0.7 ohm load for 5 seconds, and is able to do that multiple times, please advise type & brand.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    ·"If you build it, they will come."
  • WarumaWaruma Posts: 21
    edited 2010-04-09 20:12
    I don't know the specific mAh rating on my regular Duracel 9V battery, but looking it up online (in general) a 9V battery has a 500mAh life.· Drawing an occasional 4A from it by basically short-circuiting the battery would mean that my battery wouldn't be able to last more than 12.5 minutes.· But it's probably way less than that since I'm assuming my useable voltage has to be greater than 5V for the circuit.· I don't expect this battery to last long and I may end up switching to a wall adapter for more frequent usage.

    If I did switch to a rechargable 9V battery, what is the mAh rating on those?
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-09 20:22
    The Energizer is 175mah
    http://data.energizer.com/PDFs/nh22-175.pdf

    I'd hazard to say the Duracell is very close to that and also that if you pull 4A through a 175mah rechargable ( 23C or 23 times it's rated capacity) you'll toast the battery.

    Get the wall wart, of the AA NiCd or NiMh pack I mentioned above. You'll be fine then [noparse]:o[/noparse])

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    "puff"...... Smile, there went another one.
  • sumdawgysumdawgy Posts: 167
    edited 2010-04-12 18:45
    Spiral_72 said...
    The Energizer is 175mah
    http://data.energizer.com/PDFs/nh22-175.pdf

    I'd hazard to say the Duracell is very close to that and also that if you pull 4A through a 175mah rechargable ( 23C or 23 times it's rated capacity) you'll toast the battery.

    Get the wall wart, of the AA NiCd or NiMh pack I mentioned above. You'll be fine then [noparse]:o[/noparse])
    Hopefully, that's not medical equipment you're trusting to a 4A drain from a 9vdc battery. (It's basically a stack of N batteries wired in series.)

    I'm an old school electric remote control racer....7.2 is my weapon of choice... the 9.6vdc packs are availiable & a·little bit smaller.· Either way, even intermitantly, the foil connectors between the cells of the 9vdc battery isn't rated for·over 500mA·& I wouldn't trust them.

    Rechargeable 7.2 V SubC NiMH 3000mAh Battery Pack·(a stack of series wired C batteries.)
    Rechargeable Racing Pack containing 6 x 3000 mAh or... spend a few bucks & go with the 3900mAh unit.

    Either way packs not too excesively big and, You can reliably power stamp & circuit off it.
    typical Dimensions are 130mm H x 47mm W x 25mm D. Weight is 403g.
    Whatever you do.... hope it all works for you.

    -dawgy
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