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2N3055 amp with collector on the negative rail?

Discussion in 'Electronic Basics' started by Ingen Ingensteds, Feb 16, 2004.

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  1. I'd like to use a 2N3055 in a watertight project box. My project is a
    556 timer driving a TIP, which drives a 2N3055 to oscillate a motorcycle
    headlamp. The diagram is at

    The need for a watertight enclosure limits my options for heat-sink, as
    there would be no airflow to the 3055. I read a note at
    ....that it may be possible to have the collectors of the 2N3055 on the
    negative rail, so that they might be affixed to an grounded enclosure.
    That would be great for me. However, I'm a bit confused by this. I
    thought that the polarity of the base and collector had to be the same,
    with respect to the emitter. But the link above seems to imply that the
    base can be positive, while the collector and emitter are negative?

    I tried this, and got some odd results: usually, my primary 555
    oscillator (within the 556) would be stuck open with voltage constantly
    on the Output. Twice, the circuit seemed to be working for several
    seconds, then quit when I jiggled the wires (going to the
    non-oscillating mode). It appeared that this problem might be dependent
    on the order in which the positive and negative rails were connected to
    the circuit (diode->cap->556) versus the emitter and collector of the
    3055. However, I couldn't nail it down from trial and error.

    When I reconnect such that the collector is positive, I have no
    problems. Any suggestions for using a negative-rail (grounded)
    collector on the 3055 would be appreciated! I may try a BUZ71 instead
    of the TIP+2N3055, but believe that I will have the same problem (heat).

    -Norman Newby
  2. John Larkin

    John Larkin Guest

    With a Darlington (two-transistor) configuration, Q2 doesn't properly
    saturate so it will get pretty hot. And no, you can't ground the

    If you used a big N-channel power mosfet instead of the Darlington,
    the 'on' voltage would be very low and you wouldn't near near as much
    heat sinking.

    You could use a PNP Darlington as a follower, and ground the
    collector. It would still dissipate about the same power, but might be
    easier to heat sink.

  3. Jamie

    Jamie Guest

    if you look closer to that print (power supply) you will see that
    the Bridge rectifier is actually connecting the + out to the ground.
    thus you can then connect the 2N3055 cans directly to the case.
    normally this is only done to get better heat sinking and usually the
    inners are not exposed.
    what is some what common is to have the - side of the Rectifier
    connected to the emmiters of the 2N3055 while the Collects are thus
    connected to the gound side. this makes it nice if you have a few
    2N3055 hanging off the back and worry less about them hitting gound
    points around you. the + side of the rectifier is on the binding post as
    the isolated output..
    but using that for a motorcycle on the bike it self using its own
    power source you may have problems.:))
  4. John Larkin wrote:

    Yep, I'm leaning that way. I also noticed that federal regulations
    limit the voltage drop through a modulator at 0.45v, which the
    Darlington will probably not be able to achieve.
    The main advantage of the NPN Darlington was it's availabitily at the
    strip mall for a quick Sunday afternoon project. But if I have to
    mail-order, then I might as well try a power MOSFET. It sounds like I
    could get a power FET with an isolated flange for easier mounting, or
    else a mica insulator and a normal NPN FET; or else use an PNP FET,
    which would have the virtue of failing closed, so that the headlight
    would still work, and could have the flange mounted to a ground
    plane(?). But I'm not sure how to drive a PNP FET (which I think needs
    negative voltage on the base?) using a 556 timer (which supplies
    positive voltage from its output).

    Still pretty new to this fun stuff!
  5. Dan Dunphy

    Dan Dunphy Guest

    Here is a link (Kisan) which shows the legal ramifications. This
    circuit, in it's present form is not legal becasue it does not have
    the light sensing device to turn the modulator off in dim light.
    Kisan builds a DOT legal version, called pathfinder. Their customer
    service is reputed to be excellent.

    This circuit has one other flaw, that of using a darlington. One
    should seperate the collectors and put a current limit resistor in the
    collector of the 2N3055's driver. It should be large enough so as to
    not exceed the base current spec on the 2N3055. This will allow the
    2N3055 to saturate, and give a very low voltage drop, when on.
    Chances are you will not need a heat sink at all, if you do this. In
    fact I suggested this to Jim Buchannon years ago, but he ignored me.
    The way he designed this, as you approach saturation, the 2N3055s
    driver collector is starved for current, and it will not drive the
    output device into complete saturation, Let's assume it goes to .7
    volts, x 5 amps = 3.5 watts dissapated in the output device alone.
    If you can drive it to .15 volts, at 5 amps, you get .75 watts, a big
    difference. In a T3, the 3066 is a 60 watt device, at room temp
    (case) if I remember correctly, and at .75 watts it probably won't
    hardly get warm. 3.5 watts will burn your fingers.

    No you can't tie the collector to the negative rail, NPN's don't work
    that way.


    Colorado Springs, CO
    My advice may be worth what you paid for it.
  6. Louis Bybee

    Louis Bybee Guest

    I didn't see any link?

    Remove the two fish in address to respond
  7. Dan Dunphy

    Dan Dunphy Guest

    I just looked at the 2N3055 Data sheet. Vsat at 4 A is 1 volt. This
    maked the FET in Steinwinkles modified design look like a much better

    Colorado Springs, CO
    My advice may be worth what you paid for it.

  8. Definitely go with the MOSFET solution. Although the author of the webpage
    you linked to deserves kudos for making such information available, he is
    not very well informed about modern MOSFET technology. His general
    impression of MOSFET technology is more applicable to say the early 1980's
    timeframe. They have come a very long way since then and have long since
    vastly surpassed bipolar junction transistors in this type of application.

    However, do not use the BUZ71 MOSFET for this application. That is a really
    wimpy and pitiful device and indeed may end up dissipating as much heat as
    the 2N3055 in this application. Don't be fooled by the seemingly impressive
    14A rating. Just because it is rated for that does not mean it is really
    well suited for or even meant to handle 14A.

    Use some much more robust device such as the IRFZ48V for instance.

    Datasheet at:

    This device is rated at 60V with a Rds(on) resistance of 12 milliOhms. When
    the device is on the device simply behaves as a resistor of around 12
    milliohms (although about twice this at 140 deg. C junction temperature).
    When off it behaves as a near infinite value resistance.

    If you want to pump say five amps through the device, we can estimate the
    continuous DC power loss by using the simple old formula P=I^2 * R. So for
    instance at 5A continuous DC current the power loss would be something like
    5^2 * 0.012 = 0.3W. Without heatsinking the junction temperature will rise
    somewhat and the steady state dissipation will be slightly higher than this,
    but in any case this is a very manageable figure. No heatsink will be
    necessary at all. It may get a little bit warm to the touch, but still well
    within its capabilities. This appears to run the lamp at less than full
    duty cycle so the actual dissipation would likely be even less than this.

    Or if that bothers you use an even bigger MOSFET still. Some device like
    the IRF2804 would certainly be massive overkill for this application (but I
    include it in this post to give you an idea of what exists out there in
    terms of power MOSFETs):

    This device is rated at about 2.3 milliohms with a breakdown voltage of 40V.
    Steady state DC power dissipation at full duty cycle on with 5A would be
    something like 5^2 * 0.0023 = 58 mW. This is negligible and will not
    produce any detectable temperature rise.

    Both devices and many more are available from Digikey among other
    distributors at very reasonable prices.

    The IRFZ48V can be had for US $1.05 in single unit quantities or for $0.84
    in ten unit quantities.

    The IRF2804 is more expensive (and massive overkill for this application) at
    $3.33 in single unit or $2.22 in ten unit quantities.

    Oh by the way. There is no such thing as a NPN MOSFET or a PNP MOSFET.
    There are however N-channel MOSFETs and P-channel MOSFETs. The N-channel
    and P-channel devices are however quite analogous to NPN and PNP bipolar
    transistors respectively. Most semiconductor devices (both N-channel and
    P-channel MOSFETs included) will fail short circuit if you abuse them
    excessly. If you massively abuse them beyond simply failing short circuit
    sometimes they will act like a fuse and eventually become open circuits, but
    this isn't very common when playing with low voltage. The primary advantage
    the P-channel MOSFET offers is its ability to switch a load from the high
    side. In this case the original schematic was designed for low side
    switching using an NPN device, so an N-channel device will make for a fine

    Once you get some experience using power MOSFETs you will probably end up
    wanting to spit at 2N3055 transistors for they are such pitiful devices. I
    know I want to.
  9. A 1-page printable version of FMVSS-108, which you can carry under your
    seat with your insurance info, is at

    Kisan is well-reputed. Their device modulates at 50:50 mark:space from
    full brightness down to 40% brightness at 4hz. The Comagination
    VisiPath is cheaper, and modulates down to 17% brightness, the legal
    limit for "most obnoxious". These are effective, but can be annoying to
    other motorists. I wanted something more subtle. My timer is running
    at 4hz, 70:30 mark:space (the legal limit for "least obnoxious"), and
    only modulating down to 75% intensity (that is, 75:25 mark:space at
    ~200hz). I'm pleased with the visual results on my test lamp. I think
    a 50% duty cycle modulating down to (say) 90% intensity would also work

    Reading , it appears
    that if my voltage is 11.5-14.5v, and lamp load is about 5A, and the my
    HFE min-max are 20-70, then I need a base resistor of between
    (14.5/(5/20))=46 and ((14.5/(5/70))=203 ohms. 100 ohms sounds
    reasonable. So I'm guessing that a 10W 100-ohm power resistor between
    the TIP31 emitter and the 2N3055 collector would be sufficient to keep
    the 2N3055 base from starving? (or should it be placed between the 12v
    rail and the collector of the TIP31?). Then the R2 resistor in should probably be changed from
    100 to 1k.

    This would give the TIP31 "Q1" a load of [email protected] Using the same
    calculation (and HFE 10-50), Q1 would need a base resistor "R1" of
    1k-6k, such as 2.2k, rather than the 680ohm resistor I have now. But
    perhaps this is an incorrect conclusion.
  10. Dan Dunphy

    Dan Dunphy Guest

    Here's teh text from the Kisan Web page.

    Modulators are legal in all fifty states. Below you will find the
    federal Mandate which makes modulators legal. No state can override
    the federal law, and therefore any motorcycle can modulate their
    headlights, provided they follow the federal guidelines. Kisan
    modulators meets or exceed the federal standards.

    Federal Motor Vehicle Standards

    Department of Transportation
    National Highway Traffic Safety Administration
    Federal Motor Vehicle Safety Standards
    49 CFR Parts 571
    [Docket No. 97-57; Notice 1]
    Executive Order 12866

    Motorcycle Headlamp Modulation System

    s7.9.1 A headlamp on a motorcycle may be wired to either the upper or
    the lower beam from its maximum intensity to a lesser intensity
    provided that:
    (a) The rate of modulation shall be 240 +/- 40 cycles per minute.
    (b) The headlamp shall be operated at maximum power for 50 to 70
    percent of each cycle
    (c) The lowest intensity at any test point shall be not less than 17%
    of the maximum intensity measured at the same point.
    (d) The modulator switch shall be wired in the power feed of the beam
    filament being modulated and not in the ground side of the circuit.
    (e) Means shall be provided so that both the lower beam and the upper
    beam remain operable in the event of a modulator failure.
    (f) The system shall include a sensor mounted with the axis of its
    sensing element perpendicular to the horizontal plane. Headlamp
    modulation shall cease whenever the level of light . . . . less then
    270 lux.

    Partial reprint of:
    Federal Registar: Feb. 21 1996 (Volume 61 No. 35)

    Colorado Springs, CO
    My advice may be worth what you paid for it.
  11. Dan Dunphy

    Dan Dunphy Guest

    Jim Buchannon's design has been around a long time, close to 9 years,
    I think. I don't think Rick Steinwinkle's response is much newer.
    12 milliohms, now that's more like it.

    Colorado Springs, CO
    My advice may be worth what you paid for it.
  12. The high voltage drop probably also makes it fall outside FMVSS108,
    which states a max. of 0.45 volts dropped.

    Also, using the timer configuration listed would yield more than 280hz,
    also not within FMVSS108.

    Regarding the light sensitivity requirement, I could use an LDR,
    transistor and 3 resistors to hot-wire the TIP31 or MOSFET on during the
    night-time; or I could use the LDR and a resistor as a pull-down on the
    reset pin of the 555 (but I can't remember if lowering the reset pin
    would hold the 555 output high or low, or undetermined).
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