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Propeller Proto Board Input Voltage Limit really 9V? — Parallax Forums

Propeller Proto Board Input Voltage Limit really 9V?

R PankauR Pankau Posts: 127
edited 2012-08-10 22:23 in Propeller 1
Quick question.
On the Propeller Proto Board it appears that the only thing the Input voltage really touches is the 5.0 volt regulator (LM1086), which according the the National website can tolerate 30 volts on the input.

Sooo I wanted to put this board to use in a car and I'm thinking that it would be really simple to just use the 12 V (nominal) available to power this thing rather than try to get it below 9 volts first.

Can anyone think of a good reason that this would be a bad idea? Besides transients, I may put a snubber circuit on the input to protect against inductive spikes from the starter etc...


thanks.

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Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2010-03-08 03:00
    It's usually the capacitors that have a lower rating but otherwise 12V is usually fine, it just depends on what else you want to run from those regulators. If the board draws 100ma from 5V then that means the regulator needs to dissipate 12-5 *100ma = 0.7W. So it doesn't matter even if the input could tolerate 100V it comes back down to the fact that the greater the input voltage the greater the power that has to be dissipated by the regulator and there is a very finite limit. Check the datasheets plus this also depends upon heatsinking. For this reason switch-mode regulators are used for higher voltages and currents etc as they are far more efficient in converting the power.

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    *Peter*
  • Mike GreenMike Green Posts: 23,101
    edited 2010-03-08 03:01
    In principle, what you want to do should indeed work. The "bad idea" part is that there's really a lot of bad stuff riding on that 12V "nominal" power source. You need more than just a simple snubber. You want lots of filtering, at least a hefty pi filter to cut those transients down to size. You want to avoid the worst of the transients from causing false resets. You want the thing in a shielded case with any signal inputs bypassed and shielded. If the inputs are any distance from the device and run alongside other wiring, they'll inductively pick up noise as well. Use common sense.
  • Brian RileyBrian Riley Posts: 626
    edited 2010-03-08 04:02
    Peter Jakacki said...
    It's usually the capacitors that have a lower rating but otherwise 12V is usually fine, it just depends on what else you want to run from those regulators. If the board draws 100ma from 5V then that means the regulator needs to dissipate 12-5 *100ma = 0.7W. So it doesn't matter even if the input could tolerate 100V it comes back down to the fact that the greater the input voltage the greater the power that has to be dissipated by the regulator and there is a very finite limit. Check the datasheets plus this also depends upon heatsinking. For this reason switch-mode regulators are used for higher voltages and currents etc as they are far more efficient in converting the power.

    Once common solution would be to put 2-4 1N4000 series of diodes in series in the 12 V line. Each one will drop the line 0.7v.

    cheers ... BBR

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    cheers ... brian riley, n1bq, underhill center, vermont
    The Shoppe at Wulfden
    www.wulfden.org/TheShoppe/
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2010-03-08 04:16
    Brian, that is a common solution indeed plus I have even used a series zener up to 5V @100ma as a quick fix. Either way though there is a heat problem and it really depends upon the other loads as well and as Mike pointed out, all those nasties!

    Cheers

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    *Peter*
  • mctriviamctrivia Posts: 3,772
    edited 2010-03-08 05:58
    if running off a car 12V(13.6V when engine running) you need to at absolute minimum have a diode with a small series resister followed by a large storage cap. This simple input filter will eliminate most of the voltage problems caused by the starter and alternator(assuming the cap, resister, and diode are suficient size and current rating). I usually pick a resister of size and power rating to charge the cap at half or less of its max current capability and a capacitor large enough to withstand a 2 second black out.

    The problem is the higher the current the more expensive the cap gets
    20mA = 5,000uF
    100mA = 25,000uF
    *Assuming 5.5V cut off voltage


    Usually the better way is to drop the voltage first then use this circuit only on the CPU with a brown out detection circuit to handle telling the processor when the car was started.

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    24 bit LCD Breakout Board now in. $24.99 has backlight driver and touch sensitive decoder.

    If you have not already. Add yourself to the prophead map
  • max72max72 Posts: 1,155
    edited 2010-03-08 08:55
    I used the protoboard with a 12V lead gel battery, even is it is "discouraged".

    The main problem is heat dissipation, so in my experience it is ok as long as you don't exceed with current needs. For instance I ran 2 serial LCD, GPS and a little bit more, with no problems, as long as the backlight is disabled. Otherwise the regulator gets too hot.

    Massimo
  • R PankauR Pankau Posts: 127
    edited 2010-03-08 18:54
    Good answers thanks.
    I was planning on using a AA pack of batteries as well(5 or 6 cells), I need this thing to operate while the car is off, but I would not expect my pack to last all day.
    Looks like I should Drop the Voltage with series diodes, use a filter of some kind to limit spikes. And I'll need to employ a charging circuit & logic for my battery pack.

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  • max72max72 Posts: 1,155
    edited 2010-03-08 21:16
    A 6 pack AA NiMh rechargeable batteries is a nice alternative.
    I have 2900mAh batteries and they last a lot. It depends on your requirements, but with the linear regulators in the proto board al the voltage drop is wasted, so 7.2 V nominal (6 AA NiMh) are a good option, and depending on the peripherals you'll have a lot of hour of autonomy.
    Moreover if you have a current controlled charger you can expect a uniform battery behavior.

    Needless to say 12V out of yor car battery are available at no cost...

    have an idea of you power requiremets?

    Massimo
  • R PankauR Pankau Posts: 127
    edited 2010-03-09 02:57
    Not sure of total power yet. It will be the board, SD card, GPS receiver and some diagnostic leds. Not too bad. I have a good design for a constant current source that I use to charge some NiMh button cells that I may re-use with the proper current tuned in. then all I need is to sense battery voltage for charging. I figured I'd use a simple voltage divider to measure voltage, maybe through a transistor so I don't have to waste battery life all the time. Could try Sigma DeltaV for the A-D portion to keep parts count low.

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  • jrjrjrjr Posts: 22
    edited 2010-03-09 04:59
    If it rhymes with vehicle you should read this >>

    http://en.wikipedia.org/wiki/Load_dump

    Enough is never enough, unless the circuitry is active (transient/filtering & overvoltage disconnect)
    as well as the mentioned passive protection(s).

    or just plan on losing one occasionally, if the vehicle misbehaves.

    I like the local battery 'float' idea, but the I/O MUST be addressed as well.

    Mike's not overstating the problem at all, it's all there.

    jr
  • max72max72 Posts: 1,155
    edited 2010-03-09 12:48
    In my regatta computer prototype I have a protoboard, a 5V GPS, 2 serial LCDs from [url=http:///www.robot-electronics.co.uk]http://forums.parallaxinc.com/www.robot-electronics.co.uk[/url], with keypad, no backlight.

    Is is powered from a 6 NiMh 2900 mAh pack, and it works 8-10 hours.

    GPS are usually not power hungry, and some of them have very reasonable current needs. I played with SD cards but I have not idea. Nonetheless I would say a SD card is not an issue. In both cases you have not alternatives..
    Lexar says the current drain of their SD cards is about 35 mA www.lexar.com/digfilm/sd_specs.html, so it is equivalent to a couple of leds on.

    To save power in case you need the leds, blink them in place of having them on all the time, and reduce the current across them.

    Massimo
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2010-03-09 18:01
    You might use this in place of the 5v regulator on the board.
    rg2575.jpg
    www.ereshop.com/index.php?main_page=product_info&cPath=75&products_id=188

    They are low-cost switching regulators that are drop in replacements
    for linear regulators.
    the site said...
    $3.78
    RG2575-5 use switching mode for control output voltage. It can source current up to 1 Amp without heat-sink. Pin diagram is compatible to 7805 standard linear regulator. The RG2575-5 supports input voltage up to 40 Vdc. And output voltage is 5VDC.
  • R PankauR Pankau Posts: 127
    edited 2010-03-10 05:54
    I did some simulating with PSpice and I ended up with 6 diodes in series, a pi filter (.1u cap in front, 100mH , and 10u cap in back)
    With a 200 volt spike lasting 1uSec the output only sees a roughly 1volt spike. I should be able to get the big inductor from the local university's ECE store.

    Anyone know how long the inductive spike actually lasts for a typical starter motor? Or what the voltage could go to? I'm guessing that the insulation is usually 300v, so it must not go above that.

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  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2010-03-10 06:37
    I've never bother with an inductor as I usually use the old diode + resistor + big capacitor trick with automotive supplies. It's mainly the big drop you get when turning over the starter motor that I worry about. Include an 18V transzorb across the filtered output to clamp the voltage. With a 22 ohm resistor, a 470uf (more is better) electrolytic and the series diode the voltage only rises from 12 to 14.5 with a 200V spike for 100us (1us is way too short). With the transzorb the voltage would be clamped and the 22R resistor should be of a type that won't fuse at the first sight of a surge. Polyswitches are very useful too if you are handling several amps in automotive circuits as copper tracks tend to vaporize very quickly otherwise.

    Load dump though can spike the supply up to 120V for 400ms which the transzorb and polyswitch will handle to protect the electronics as well as other sustained overvoltage conditions.

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    *Peter*
  • R PankauR Pankau Posts: 127
    edited 2010-03-10 20:53
    I guess I never really looked closely at the voltage coming from a cigarette lighter. So would it be safe to assume that all of the electronics in a car are shielded from spikes as mentioned above? Is there an industry standard that most devices follow? M.O.V. or something of that nature to keep costs low. Do these things exist in cell phone chargers for example?

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  • Mike GreenMike Green Posts: 23,101
    edited 2012-08-10 22:23
    I don't know for certain about C3, but I'd suspect that it's rated at 15V since that's the next highest rating above 9-10V (the upper limit of the Protoboard's voltage input rating). That would not be adequate for running the Protoboard from a car's power supply since that commonly goes up to 13.8V and often will peak substantially higher than that. I'd use a minimum of a 25V capacitor there or find some way to drop the input voltage (like a Zener with an appropriate current rating or another voltage regulator set for 8 or 9V.
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