New discussion on voltage drop switch

Discussion in 'The Projects Forum' started by j_w_Bruce, Oct 26, 2011.

  1. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    I was trying to follow the discussion on the voltage drop switch. I should be able to do this stuff once I get going a little, but for now... I'm having a hard time following it.

    I would like to make a switch that will turn on above 13.5 volts and off below 13.5 volts. My circuit has a breaker in it for 15 amps.

    Also, I saw that automotive discussions are not allowed. Although this could have automotive applications (obviously), this particular project is non automotive. My wife just doesn't appreciate my use of lighted toggle switches everywhere to keep batteries from running down...

    Any help would be apprecitated
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You have to include a margin called hysteresis otherwise your switch, electronic or otherwise, will chatter. An example of hysteresis is the switch should turn on above 13.6 volts and turn off below 13.4 volts. This gives you a 0.2 volt margin (sometimes called backlash - similar to how your house thermostat works).
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What is your actual load? How many amps?

    Another question: If this is a lighting switch, is the power supply available (+ and -) at the switch or just one side of the power supply and a wire to the load?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It also depends on how your source (battery, I suppose) is being charged, and what other loads you might have.

    Mains-powered chargers will usually charge until the battery is ~14v to 14.5v, and then reduce to a trickle, perhaps 13.6v.

    With an alternator, if you have transient loads (like heavy-duty motors starting up) or "load dumps" (loads being turned off) you can get very high momentary voltage spikes.

    What is actually providing the power for this project?

    [eta]

    Just to make it clear; this type of idea must not be used on lighting, where the lighting is related to safety (head/tail lamps, navigation lighting, etc.), particularly on something that is moving (motor vehicle, boat, aircraft, etc.)

    The reason is that if the engine stopped/stalled for any reason, or the alternator fails, the safety lighting would turn off, and would not turn back on until the engine is running again, and the alternator producing enough power to bring system voltage above the threshold.
     
    Last edited: Oct 26, 2011
  5. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    Thanks, I understand the safety issue, it is not a lighting application. One place I might make use of this is in with my aircraft radio to avoid having to have an avionics master switch. Another place I can see using this switch would be in a (gasp) automotive (gasp) application, where they no longer include an accessory wire to power an aftermarket radio (because apparently the new factory radios are smart enough to not need an individual memory and power wire anymore).

    so it's for a alternator, 12 volt battery, and voltage regulator setup. only one wire is available (not + and - )

    and, I'm not BS'ing about the plane, Private pilot certified (485 flight hours) since 1999...

    Thanks!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    All right, now you're introducing yet a new problem; zero power consumption when the system voltage is below the threshold, AND an automotive rating.

    We're not going to get to zero power consumption. It might be quite low, but it will not be zero; there has to be some current flow in order to be able to actively monitor it.

    That's the nice thing about a master switch - when it's off, that's all she wrote.

    I suppose you're in the process of building your own experimental aircraft? Which one is it?
     
  7. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    Not Experimental, Production Beechcraft 4 seater referred to as the Musketeer

    The more advanced the radio installation is, the more likely they are to have a second (Audio) master switch. This switch come after the main master switch and keeps you from having to turn several different on and off knobs. So in this case, the power consumption doesn't have to be zero. If you are away from the plane, most likely you've turned off the main master. If you didn't, your battery will still last much longer at that small consumption over running off and leaving all the audio equipment on accidently. In my case, I'm just looking at a radio and possibly an intercom. Alot of installation are 2 or 3 radios, and audio panel/intercom, and a transponder all powered from the 'avionics master'.

    Most likley my radio upgrade will involve getting a 'field approval', so any switch I build for the application would ultimatley have to be approved.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    What it sounds like you need is something that starts beeping intermittently in case the system voltage falls below ~13v, and then after so many beeps (or a certain amount of time), the power is turned off. That way it will not just turn off immediately without warning. After all, if the prop stops turning and attempts to relight fail, you might forget that the radio's off when you try to declare an emergency.

    An audio master switch could be used to bypass the automatic shutoff circuit; enabling the power again. Without some sort of bypass, you would not be able to turn the radio back on.
     
  9. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    Yes, a bypass would be a nice addition to the switch. I think if I can get what I need for the voltage drop, adding a nice bypass wouldn't be hard. Maybe even something that would reset after the master was shut off. That way if you have a reason to kick up the radios while your batteries are discharging you don't have to remember to turn that off later (micro switch or something).
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    What I'm thinking of is a 3-position bat-handle switch, where you have:
    1) UP for ON, bypassing the automatic power control,
    2) Center for AUTO; power is automatically connected when the bus > ~13.4v, off when below ~13v after a warning period (perhaps buzzer and visual warning like a flashing LED next to the switch)
    3) Down for OFF; power is disconnected from the automatic switch and therefore the rest of the circuit.

    That way you would have a means of completely bypassing the automatic circuit in the event of failure.

    Here is a switch that can enable that functionality:
    http://www.newark.com/carling-technologies/2gg51-73/switch-toggle-dpdt-15a-250v/dp/54H0425
    It's a double-pole, double-throw switch that has three positions:
    ON-ON-OFF
    where:
    Position 1: Both circuits on (On position)
    Position 2: One circuit on (Auto position)
    Position 3: Both circuits off (Off position)
     
  11. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    Yes, that would work. Thanks

    However, I am still unsure about what I need to complete the Voltage Drop switch. The other post about the voltage drop switch was a bit over my head. Some of it was OK, and when they said that they were including a circuit diagram, I didn't find those in the post...

    JB
     
  12. LoganFife

    New Member

    Feb 7, 2010
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    for the actual switching what you need is a n/o relay which can handle the current, and with a coil voltage below the voltage you want to sense.

    there are a few options to turn the relay on and off, which differ in complexity and predictability. the simplest way is a few resistors, better is an analogue circuit with a comparator, and a filter, better still (maybe) a microcontroller.

    what is your experience with these ideas? where is the other discussion you mentioned?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    What I'm thinking of is using a battery voltage monitoring circuit, which would have a very minimal draw when the circuit was off. There would need to be a voltage divider to compare the battery voltage to a fixed reference, but that would be do-able. As a switch, I'm thinking of a P-channel power MOSFET, just to keep things reasonably simple and relatively low-power. After all, you don't want to add more load on the electrical system than absolutely necessary; every bit of power you save translates to better fuel economy.

    I'm not on my normal computer now (helping a friend recover from a lousy virus) so am pretty limited as to what I can post. However, I'm thinking along the lines of a MAX834, MAX835 or MC34161; these are purpose-made for battery monitoring.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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  15. j_w_Bruce

    Thread Starter New Member

    Oct 26, 2011
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    I think that the other post I was looking at was called "voltage drop switch". My experience level is low at this point.

    And thanks for the link, I'll check it out.
     
  16. LoganFife

    New Member

    Feb 7, 2010
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    one of the reasons i suggested the relay is because it has built in hysteresis. but the voltage monitor looks nice.

    http://datasheets.maxim-ic.com/en/ds/MAX834-MAX835.pdf

    see figure 5 for a simple diagram of close to what you want, though it will need a little adjustment as it is auto off only. a logic high on the clear input would switch it on. R1 and R2 form a voltage devider, so you just need to work them out for the voltage you need. also, vcc on that particular part is around 11 volts, so thats a problem. but its also a starting point.


    how are you planning construction?
     
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