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PWM Control of a 24Volt (20W) Light Bulb — Parallax Forums

PWM Control of a 24Volt (20W) Light Bulb

ArchiverArchiver Posts: 46,084
edited 2004-02-16 23:47 in General Discussion
I would like to control the brightness of a 24 volt (0.8amp, 20Watt)
halogen light bulb using a BS2P and the PULSOUT command. Looking on page
128 of the Earth Measurements V1.2 shows how to control a low current
5vdc pump using a transistor. Will this approach work using the
appropriately sized transistor (FET, SCR, etc.)? If anyone can point me
towards example circuits I'd appreciate it. I have a solid 5VDC supply
and between 22 and 28VDC for the lamp depending on battery levels
available.

Thanks
Chris

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Comments

  • ArchiverArchiver Posts: 46,084
    edited 2004-02-14 22:51
    Chris,

    Yup, it will work the same, and the circuit is the same. The only
    diffenrence will be the transistor and the resistor
    values. You could use a 2N3055, easily found at Radio Crack. It's a little
    over kill, but will last forever.

    Jonathan

    www.madlabs.info


    Original Message
    From: "Christopher Dundorf" <cdundorf@j...>
    To: <basicstamps@yahoogroups.com>
    Sent: Saturday, February 14, 2004 11:31 AM
    Subject: [noparse][[/noparse]basicstamps] PWM Control of a 24Volt (20W) Light Bulb


    > I would like to control the brightness of a 24 volt (0.8amp, 20Watt)
    > halogen light bulb using a BS2P and the PULSOUT command. Looking on page
    > 128 of the Earth Measurements V1.2 shows how to control a low current
    > 5vdc pump using a transistor. Will this approach work using the
    > appropriately sized transistor (FET, SCR, etc.)? If anyone can point me
    > towards example circuits I'd appreciate it. I have a solid 5VDC supply
    > and between 22 and 28VDC for the lamp depending on battery levels
    > available.
    >
    > Thanks
    > Chris
    >
    > ________________________________________________________________
    > The best thing to hit the Internet in years - Juno SpeedBand!
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    > from the same email address that you subscribed. Text in the Subject and
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  • ArchiverArchiver Posts: 46,084
    edited 2004-02-14 23:58
    >I would like to control the brightness of a 24 volt (0.8amp, 20Watt)
    >halogen light bulb using a BS2P and the PULSOUT command. Looking on page
    >128 of the Earth Measurements V1.2 shows how to control a low current
    >5vdc pump using a transistor. Will this approach work using the
    >appropriately sized transistor (FET, SCR, etc.)? If anyone can point me
    >towards example circuits I'd appreciate it. I have a solid 5VDC supply
    >and between 22 and 28VDC for the lamp depending on battery levels
    >available.
    >
    >Thanks
    >Chris


    No, that circuit is designed to control a 3 volt pump, and you will
    need a different circuit to control your 24 volt lamp.

    The circuit you referred to uses the transistor in a "common emitter"
    configuration to turn the pump ON and OFF. It would also work with
    the PWM command to control the speed or power of the pump.

    ,
    Vdd
    | /
    |'
    P3 --/\/\---| ZTX1049 (superbeta e-line)
    100 |>
    | \ 10ohm
    `--/\/\--3vPUMP---Vss



    There are a number of circuits you could use to control the 24 volt
    lamp. One is a power n-channel mosfet with the lamp in the drain
    circuit to 24 volts:

    d
    ||
    LAMP
    24V
    100 g|
    P3 --/\/\--o--||--; IRL520
    PWM | ||--|
    | |s
    | 22k |
    `-/\/\-|
    |
    Vss

    The 22k resistor pulls the gate of the transistor to ground when P3
    happens to be an input. That turns the lamp off. The maximum power
    dissipation in the IRL520 would be 12 watts if by some chance the
    gate floats, and that would be HOT! With PWM control, the transistor
    should not get too hot, because in simple terms it is switching in
    PWM between full ON and full OFF, both of which are zero power
    dissipation in the mosfet. I reality, the switching takes time and
    the transistor warms up.

    The Stamp can only do one thing at a time. It can vary the brightness
    with a PWM command, but it has to leave the lamp either full on or
    full off while it executes other pbasic commands. If you want to set
    a medium brightness and then have the Stamp go off and do other
    stuff, you will have to look into an external PWM generator.

    -- regards,
    Tracy
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-15 15:39
    Less than 100% duty cycle significantly reduces Halogen
    service life. You might look into how they work - Quite
    different from incandescents.

    regards, Jack
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-16 13:48
    check out this web site for control circuits to tie a 24v bulb to the stamp.
    http://www.geocities.com/jimforkin2003/
    jim

    Original Message
    From: Christopher Dundorf [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=YTjoSMBtIJ1bAaGbMpZIG3NWntJOUWtC_gi83l5ZSxBYH6Opb4kKeXw1RLo2VztvaUnAW7-eUs43NA]cdundorf@j...[/url
    Sent: Saturday, February 14, 2004 2:31 PM
    To: basicstamps@yahoogroups.com
    Subject: [noparse][[/noparse]basicstamps] PWM Control of a 24Volt (20W) Light Bulb


    I would like to control the brightness of a 24 volt (0.8amp, 20Watt)
    halogen light bulb using a BS2P and the PULSOUT command. Looking on page
    128 of the Earth Measurements V1.2 shows how to control a low current
    5vdc pump using a transistor. Will this approach work using the
    appropriately sized transistor (FET, SCR, etc.)? If anyone can point me
    towards example circuits I'd appreciate it. I have a solid 5VDC supply
    and between 22 and 28VDC for the lamp depending on battery levels
    available.

    Thanks
    Chris

    ________________________________________________________________
    The best thing to hit the Internet in years - Juno SpeedBand!
    Surf the Web up to FIVE TIMES FASTER!
    Only $14.95/ month - visit www.juno.com to sign up today!


    To UNSUBSCRIBE, just send mail to:
    basicstamps-unsubscribe@yahoogroups.com
    from the same email address that you subscribed. Text in the Subject and
    Body of the message will be ignored.

    Yahoo! Groups Links
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-16 19:08
    Jack,

    Do you have any PWM Duty Cycle vs Hours of Life curves or know where I
    could find them. Depending on the the degree of reduction it might not
    be an insurmountable problem.

    Thanks for the thought, I never considered this until now.
    Chris

    <<<>>>

    Date: Sun, 15 Feb 2004 07:39:00 -0800
    From: goflo@p...
    Subject: Re: PWM Control of a 24Volt (20W) Light Bulb

    Less than 100% duty cycle significantly reduces Halogen
    service life. You might look into how they work - Quite
    different from incandescents.

    regards, Jack

    ________________________________________________________________
    The best thing to hit the Internet in years - Juno SpeedBand!
    Surf the Web up to FIVE TIMES FASTER!
    Only $14.95/ month - visit www.juno.com to sign up today!
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-16 20:43
    Tracy,

    I had a little trouble visualizing the character based diagram likely due
    to my email shifting spacing around but I think I follow your approach.
    Is your design the same as what is shown on Al Williams web link with the
    exception of a 100 ohm resistor directly coming off of P3 and before the
    junction with the 22K resistor? Is the 100 Ohm for overcurrent
    protection to the Stamp in the event that the FET failed?

    http://www.al-williams.com/fetrly.htm

    As for the "cycle time" of the Stamp I was unaware it worked that way.
    What you're saying though makes total sense. On this particular project
    I have a single Stamp performing many tasks with about 24+ I/Os in use.
    Yikes! Therefor I'm going to take your advice on the external PWM
    generator. I've seen several of these units described in N&V magazine.

    Thanks for your help,
    Chris

    <<<>>>

    Message: 9
    Date: Sat, 14 Feb 2004 15:58:45 -0800
    From: Tracy Allen <tracy@e...>
    Subject: Re: PWM Control of a 24Volt (20W) Light Bulb

    >I would like to control the brightness of a 24 volt (0.8amp, 20Watt)
    >halogen light bulb using a BS2P and the PULSOUT command. Looking on
    page
    >128 of the Earth Measurements V1.2 shows how to control a low current
    >5vdc pump using a transistor. Will this approach work using the
    >appropriately sized transistor (FET, SCR, etc.)? If anyone can point me
    >towards example circuits I'd appreciate it. I have a solid 5VDC supply
    >and between 22 and 28VDC for the lamp depending on battery levels
    >available.
    >
    >Thanks
    >Chris


    No, that circuit is designed to control a 3 volt pump, and you will
    need a different circuit to control your 24 volt lamp.

    The circuit you referred to uses the transistor in a "common emitter"
    configuration to turn the pump ON and OFF. It would also work with
    the PWM command to control the speed or power of the pump.

    ,
    Vdd
    | /
    |'
    P3 --/\/\---| ZTX1049 (superbeta e-line)
    100 |>
    | \ 10ohm
    `--/\/\--3vPUMP---Vss



    There are a number of circuits you could use to control the 24 volt
    lamp. One is a power n-channel mosfet with the lamp in the drain
    circuit to 24 volts:

    d
    ||
    LAMP
    24V
    100 g|
    P3 --/\/\--o--||--; IRL520
    PWM | ||--|
    | |s
    | 22k |
    `-/\/\-|
    |
    Vss

    The 22k resistor pulls the gate of the transistor to ground when P3
    happens to be an input. That turns the lamp off. The maximum power
    dissipation in the IRL520 would be 12 watts if by some chance the
    gate floats, and that would be HOT! With PWM control, the transistor
    should not get too hot, because in simple terms it is switching in
    PWM between full ON and full OFF, both of which are zero power
    dissipation in the mosfet. I reality, the switching takes time and
    the transistor warms up.

    The Stamp can only do one thing at a time. It can vary the brightness
    with a PWM command, but it has to leave the lamp either full on or
    full off while it executes other pbasic commands. If you want to set
    a medium brightness and then have the Stamp go off and do other
    stuff, you will have to look into an external PWM generator.

    -- regards,
    Tracy


    ________________________________________________________________
    The best thing to hit the Internet in years - Juno SpeedBand!
    Surf the Web up to FIVE TIMES FASTER!
    Only $14.95/ month - visit www.juno.com to sign up today!
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-16 21:39
    At 11:08 AM 2/16/2004, you wrote:
    >Jack,
    >
    >Do you have any PWM Duty Cycle vs Hours of Life curves or know where I
    >could find them. Depending on the the degree of reduction it might not
    >be an insurmountable problem.
    >
    >Thanks for the thought, I never considered this until now.



    Don Klipstein' lighting page is a *great* resource for lighting info!
    Here's a page that talks about (among other things) failure modes of
    incandescent and halogen bulbs (and why halogens don't like to
    be dimmed)

    http://members.misty.com/don/bulb1.html

    Steve


    [noparse][[/noparse]Non-text portions of this message have been removed]
  • ArchiverArchiver Posts: 46,084
    edited 2004-02-16 23:47
    > Do you have any PWM Duty Cycle vs Hours of Life curves or know where I
    > could find them.

    No, sorry.

    I've seen OEM minimum on-time recommendations for rated life
    expectancy - IOW, how long the lamp should stay on once turned
    on. Don't remember the numbers, but it was minutes, not milli-
    seconds.

    regards, Jack
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