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How to use AA size battery to drive Solenoid — Parallax Forums

How to use AA size battery to drive Solenoid

trilobytetrilobyte Posts: 3
edited 2008-04-06 02:55 in Robotics
I use 4 AA size battery to drive a solenoid of 4 Ohm. When batteries are new, everything is OK. But when voltage goes down to 5V, there is a suddent dip in
battery voltage to below 2V. I use a MCU to turn on a switch for 500ms, which drain current of 850mA. How can I drive the solenoid in this condition without causing the voltage to drop to 2V?
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Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-03-28 17:39
    trilobyte said...
    How can I drive the solenoid in this condition without causing the voltage to drop to 2V?
    Um, replace the batteries with fresh ones?

    When batteries discharge, their internal resistance increases. That's why you see the voltage plummet under load. Once they reach this point, there is little you can do to extract more useful energy from them.

    -Phil
  • Mike GreenMike Green Posts: 23,101
    edited 2008-03-28 17:48
    Please delete your duplicate post. Use the "X" box in the upper right corner of the message display.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2008-03-28 17:57
    trilobyte

    I use 4 AA size battery to drive a solenoid of 4 Ohm.

    Is there any reason that you have to run·this drive a solenoid of 4 Ohm
    Because it may better to run it off of a tranformer....>>>this is just an idea.gif





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    Sam
  • ercoerco Posts: 20,256
    edited 2008-03-28 18:31
    Solenoids are extremely inefficient, and current hogs to boot. Their pulling power increases dramatically with a shorter stroke. Another way to look at that is you reduce the voltage (thereby current) going to the solenoid coil after it pulls in fully. I've been able to reduce the holding current to as little as one-fourth of pull-in current, based on the stroke distance. That can extend battery life.

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  • allanlane5allanlane5 Posts: 3,815
    edited 2008-03-28 18:33
    Perhaps replace it with a 'latching' relay?
  • ercoerco Posts: 20,256
    edited 2008-03-28 19:04
    Great idea, change to a toggle mechanism with 2 solenoids that you energize one briefly for on and the other for off.

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    ·"If you build it, they will come."
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-04-01 16:59
    Duplicate post removed.

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    Parallax Tech Support
  • trilobytetrilobyte Posts: 3
    edited 2008-04-06 02:12
    Actually I switch to use DC motor, but I see the same problem while the batteies are weak. The voltage can drop from 5.4V to below 2V when driving the motor.
  • trilobytetrilobyte Posts: 3
    edited 2008-04-06 02:17
    By the way, how do you display your picture here?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-04-06 02:55
    Seriously, trilobyte, at some point batteries just run down and you have to replace them with fresh ones. The 5.4V you measure without a load means nothing, really. It's like taking fresh batteries and measuring their voltage through a series resistor. Without a load, you'd still see 6V. But you wouldn't be able to power anything with it, due to the voltage drop through the resistor when a load is applied. With dead batteries, this resistor manifests itself internally to the batteries themselves. So the only voltage reading that matters is the 2V you see under load. The remaining 3.4V gets dropped through the dead batteries' internal resistance, and there's nothing you can do about it.

    -Phil
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