When to use 24V/12V/5V?

Discussion in 'General Electronics Chat' started by skysurf, Jun 5, 2006.

  1. skysurf

    Thread Starter Member

    Oct 20, 2005
    14
    0
    Hi,

    newbie to electronics so please pardon me for my tardedness.... Quick question though, I was desigining a home security system with some reed switches and was wondering whether there is a guideline as to which voltage I should use (24V, 12V, or 5V.) I know that using a lower voltage such as 5V is probably not a good idea because the wire resistance for longer length of wires might cause problems.

    Is there any guideline for which voltage levels should be used for which type of application (I know industrial applications mainly use 24V, how about for home??) Or is there a guideline for what voltage you should use depending on the length of the wiring and amount of electromagnetic interference?

    Would appreciate any help!!


    Thanks!!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    12 volts is probably the best to use. Many devices use 12 volts, and that level is generally accepted as being safe to use.
     
  3. skysurf

    Thread Starter Member

    Oct 20, 2005
    14
    0


    Thanks!! I figured 12 Volts was probably going to be the most logical. However, is there an actual standard or specification (ISO??) that defines what level is to be used for what applications??

    Thanks again!!
     
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    yep, definately 12 volts. Nearly all non-passive sensors such as PIR, Glass-break, microwave, ultra-sonic are all powered via 12 volts in the cable run.
    Also, most panels (and external sirens) use a 12 volt Gel cell battery for Back up power.. so it just makes sense to power it all at 12 volts with the Main PSU being more a Glorified 13.8 volt standby charger.
     
  5. skysurf

    Thread Starter Member

    Oct 20, 2005
    14
    0

    OK, 12 Volts it is.... I have a Wall transformer that supplies 12 Volds DC that I can use for testing (I don't remember it's rating at the moment.) However, would it be a good idea to run all 12 volts circuitry and components internal and external to a board directly from the wall transformer?? Or is it better to have slightly greater input voltage (13.8 V??) and use a voltage regulator to step this input voltage down to 12 Volts to power the circuits (would this provide more protection??)

    Thanks for your help!!
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The bigger question with the plug-in supply is: will it source enough current to power everything? How big are the horns/bells/sirens you will use? The initiating devices, the control board, and the dialer might not add up to much, but those notification appliances can pull a lot of current.
     
  7. skysurf

    Thread Starter Member

    Oct 20, 2005
    14
    0
    Hi, thanks for the good point. I will be using only one siren that draws about 500mA. Other then that I was going to have a few reed switches, some sensors, and some processors and control circuitry. I did find one 15V power supply that is adjustable from 13.5V to 16.2 V and can provide a max current of 2A. Would this current specification be enough or should I go with a power supply with current rating higher than 2 Amps??

    Also, the siren has an operating voltage of 6-14V. Can I simply adjust the power supplies voltage down to 13.5 Volts and use it directly to drive the siren and reed switches, or should I step it down using a voltage regulator to 12 Volts first?? Is there any advantage/disadvantage with either method??

    Thanks for all your help!!!!


     
  8. billbehen

    Active Member

    May 10, 2006
    39
    1
    I don't know much specifically, but many alarm systems use normally closed switches to alarm if the wires are cut or come apart over time. In this case, you're really using a current source, which ought to make the thing less sensitive to absolute voltage at the source, so long as it's enough to push the requisite current.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The sensors I've worked with all are normally open switches, but have specified resistances across the contacts. The panel (or control board) will see, for example, 47K Ω in normal operation, open if the wires are damaged, and short in alarm condition.
     
  10. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Most of the systems I have worked with use a 4k7 resistor in series at the sensor if the sensor uses a NC circuit, or across the contact if using a NO contacts.
    If the panel see's 4k7, then all is normal, if it see's an O/C or S/C or a different resistance then it alarms.
    There is normally another set of contacts that detect tamper.
    Most dont draw a huge amount of current unless alarming... which is normally limited to a 10 min period, although the Dialler can continue for a while. In alarm operation they can call on the Battery reserves for that alarming period.. and infact most Sirens have an internal 12 volt gel cell fitted, not just to supply the current that the alarm cabling may have problems with, but also to keep the alarm operation in case of cable tampering.
    Most Alarm PSU's are around 1.5 to 2 amps, 13.8 volt and working mostly as standby chargers..keeping the standby batterys topped up. The panels and sensors would be lucky to draw 500mA in most cases.
     
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