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Power Consumption? — Parallax Forums

Power Consumption?

Armored CarsArmored Cars Posts: 172
edited 2005-04-14 17:42 in General Discussion
I have a stand alone SX that will not work with five AAs.· I can only get it to work with a wall adaptor, is there any way to cut power consumption?

Comments

  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-04-10 20:50
    Hey,

    five fresh AAs mean 1.5 * 5 = 7.5 Volts or even more. The absolute maximum ratings in the SX28 data sheet specify "Voltage on Vdd with respect to Vss at 0 to +7.0 Volts". I hope, you did not "fry" your SX which might lead to excessive power consumption.

    With four AAs, you (and the SX) should be in good shape, and the SX should work down to 2.7 Volts, i.e. with quite exhausted batteries, bovided that the rest of your setup can tolerate such low voltages.

    Besides this, SX power consumption is a function of the clock frequency. As a rule of thumb, the higher the clock frequency, the more "juice" the SX will need. The data sheet specifies typical values for a 5 Volt supply of

    77 mA @ 50 MHz
    17 mA @ 20 MHz
    7.5 mA @ 4 MHz

    BTW: When using the SX-Key to program/debug the SX, make sure that both are powered from a stabilized 5 Volts source, as the SX-Key will not work correctly with voltages off too far from 5 Volts.

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    Greetings from Germany,

    G
  • ForrestForrest Posts: 1,341
    edited 2005-04-10 21:22
    FYI Parallax is currently selling this device which contains an SX28 (running at 4 MHz) and (3) 5 x 7 LED's and it runs for 30 hours on (3) AAA batteries http://www.parallax.com/detail.asp?product_id=28099.
  • Armored CarsArmored Cars Posts: 172
    edited 2005-04-13 00:35
    It does have a volt reg. and will not program without power from a wall outlet. Even without the SX it will still not run with 4 new AA's. I have two pins tied up with 10k and 22k resistors but I've done that on a Stamp without problems. It also has two relays, should these have some kind of resistor on them or do they create enough resistance on their own? I'll try reprogramming it for 4 Mhz and see if that will work also.
  • Armored CarsArmored Cars Posts: 172
    edited 2005-04-13 00:38
    Wait a sec... I'm using four AAs. Most RC guys count AAs as 1.2 volts, giving me only 4.8 volts. That could be part of my problem also.
  • ForrestForrest Posts: 1,341
    edited 2005-04-13 02:25
    What voltage regulator are you using in your setup - it could make a difference. As an example, an LM7805 requires 7 volts in to get 5 volts out. On the other hand a LM2940-5 typically requires 5.75 volts in to get 5 volts out. If you don't need more than 150 mA maximum - look into the LM2950-5 or a MIC2950-5 - these are very effiecient and come in a TO-92 package.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-13 02:38
    Armored Cars said...
    It does have a volt reg. and will not program without power from a wall outlet. Even without the SX it will still not run with 4 new AA's. I have two pins tied up with 10k and 22k resistors but I've done that on a Stamp without problems. It also has two relays, should these have some kind of resistor on them or do they create enough resistance on their own? I'll try reprogramming it for 4 Mhz and see if that will work also.
    Hello,

    ·· Are you saying these Relays are connected directly to I/O pins?· Mechanical Relays?


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Armored CarsArmored Cars Posts: 172
    edited 2005-04-13 21:55
    Yes, they make clicking noises when they go on and off.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-13 22:11
    ·· Okay, you're not supposed to do this for more than one reason.· But instead of going through that again, I would recommend·reading any of several threads from the last 2 days on driving loads with a Stamp I/O pin.· I'm surprised you haven't burnt the pin already due to voltage spikes produced by the voltage collapse on the coil when the relay is de-energized.· Ah well...You live, you learn!· shocked.gif

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-04-13 22:30
    In many of my SX-based applications, I'm driving relays directly from the SX output pins.

    I'm using reed relays designed to be driven with 5 Volts, having a coil resistance of 500 Ohms, and - most important - having integrated recovery diodes to "kill" the reverse voltage generated by the coil when turned off. You can find such relays in the Digi-Key catalogue (HE207-ND).

    You may use relays without such integrated diodes, but only with an external diode (like a 1N4148) connected in parallel to the coil (cathode connected to the positive side of the coil).

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    Greetings from Germany,

    G
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-14 16:32
    Guenther,

    · Reed Relays are kind of an exception in a way...However, even when I use them in applications I still use a transistor to drive it.· When I've mentioned driving a Reed Relay with an I/O pin in the past, several users got the misconception that was long as they had the diode, they could use any relay.· So if they had used the transistor driver from the beginning, it would've worked either way.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-04-14 17:15
    Chris,

    I'm using the SX together with reed relays in some commercial applications where every component and PCB space counts, Therefore, I like these reed relays which only require 10 mA drive current @ 5 Volts, so they can be directly driven by SX outputs.

    On the other hand, I agree with you, using a driver transistor would be less dangerous for the SX or BS outputs, and allows for more flexibility in the choice of a relay. Again, there should be a recovery diode in parallel to the coil to protect the transistor in this case.

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    Greetings from Germany,

    G
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-14 17:42
    Guenther,

    ·· Thanks.· I appreciate your point of view.· Over the years I have adjusted my responses somewhat to account for inexperienced users who might not quite understand fully the need for transistors.· In this case it would be to prevent the under-experienced (Who might not know the difference between a reed relay and a standard mechanical relay) from thinking they can just connect these devices to I/O pins without protection.

    ·· Reed Relays are certainly acceptable for connection to the pins if they're within current limits and have the protection diode.· I just wanted you to understand why I always recommend the transistor or other driver on the outputs.· It keeps inexperienced users relatively safe.· Although I'm sure occasionally we're going to see some blue smoke stories!

    ·· My own personal experience with that dates back to 10 years old with a Radio Shack 150-in-1 kit where I connected the 9V battery to a TTL logic chip and blew the plastic top off the chip.· Since then I have been very careful about what connects to what, how much power, current consumption, etc.· Of course, 5 years of engineering saw it's share of disasters too, but nothing blown up, luckily.

    smhair.gif

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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