Question about 'wall warts' and DC power.

Discussion in 'Survival Communications' started by TnAndy, Feb 5, 2017.

  1. TnAndy

    TnAndy Senior Member Founding Member

    OK...I have a Dakota alert for driveway, and it takes 8 AA batteries.....12v DC, right ? I want to use a AC/DC transformer (plug in wall wart) instead of replacing AA batteries regularly. I have AC power at the location of use. I cut the ends off one, (says 12v, 300milliamp on it) put a voltmeter on it.....19v DC output. Set it aside, afraid that might be to much voltage. Got another that says 12v, 1amp....cut the end off it, reads 12v.

    Should be the same, right ? Well, the unit works fine on the AA battery pack, but won't run on the wall wart.....yes, I checked polarity, and it is attached correctly.

    What wall warts put out some kind of funky DC that won't work ?
    Kinda like 'modified square wave AC' that comes off cheap inverters when going the other way ?

    Come on Bruce..or somebody....clue me in....
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  2. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

  3. Flight-ER-Doc

    Flight-ER-Doc Monkey+

    DC is DC.

    Are you feeding it at the external power connection?
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  4. TnAndy

    TnAndy Senior Member Founding Member

    Not sure what you has a two connector block on the board marked +/- where the battery pack leads connect....that where I'm connecting anything I've tried.

    Just tried a 12v lead acid battery spare I works fine (voltage on it reading just over 13, came off trickle charger I keep it on) batteries work, but wall warts don't.....near as I can tell.
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  5. ghrit

    ghrit Bad company Administrator Founding Member

    Things get complicated with wall warts. Some (I think all, actually) have transformers in them and the question becomes what the transformed voltage is. That transformed voltage is still AC and then gets rectified to DC. That DC can be variable, from a smooth ripple wave to all manner of odd forms.
    Yup. You'll need to know what range of voltages and form of DC the Dakota will tolerate, that may be a factory question, dunno. 12 V may actually not be enough, they might want a bit more, like the nominal 12.8V that lead acid batteries develop open circuit.
    Last edited: Feb 5, 2017
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  6. TnAndy

    TnAndy Senior Member Founding Member

    Yeah, and their base receiver unit runs off a supplied wall wart (but has terminals for 12v batter also)....but can't get the transmitter unit to do so....

  7. TnAndy

    TnAndy Senior Member Founding Member

    Yeah, I don't have a scope to really see the output....just a VOM. Like I said, one WW said 12v on it, but put out 19v. One switchable one I bought just for this (1.5v-12v) put out one lousy volt no matter what the switch setting...POS.....

    It comes set up for 8 AA's....measured, I get 12.14 out of that battery pack with new AA's. Problem I've found is the batteries don't last long.....3-4 months, meaning it's a battery eater.

    By the way, this is their MURS unit with the magnetic probe, not the MURS w/motion detector...those units use 6-AA's, and I seem to get about a year out of them.
    Last edited by a moderator: Feb 5, 2017
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  8. ghrit

    ghrit Bad company Administrator Founding Member

    Hm. If your multimeter is not a pretty high quality instrument, it will read low. At least that's been my experience with the el cheapos. YMMV.
    sec_monkey likes this.
  9. BTPost

    BTPost Stumpy Old Fart Snow Monkey Moderator

    I would try the second Wall Wart, only with a 1000uF Capacitor across the DC Leads.... sometimes the the Ripple out of a cheap Wallwart, will mess around with the internals on flaky designed Electronics.... Most of these small Electronic devices use a 3Pin Regulator as the first active device in the Power Supply System, and usually they can tolerate the 19Vdc on the Unloaded First Wall Wart... and when it gets a load on it it will drop to the nominal 12Vdc it is spec'd to supply...
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  10. Tevin

    Tevin Monkey+++

    Wall warts are known for having very dirty, unstable DC output. Normally this is not a big deal but for some reason maybe your Dakota just didn't like it.

    And by the way not all DC is the same as @Flight-ER-Doc alludes. Although he's not wrong, he does oversimplify things a bit.

    Many wall warts, especially the cheap ones (which is most of them) simply turn full AC into pulsating positive voltage. It's not true DC, it's just AC that never goes to the negative side of the cycle. On an oscilloscope the waveform will look like camel humps. Usually this is enough to bullshit your device into thinking it is a getting actual DC power.


    I'm surprised Your Dakota does not work with the wall wart (I did something similar with a motion sensor in my garage and it worked fine on the garbage output of the wall wart). If it doesn't then something is wired wrong or maybe it is so picky that nothing but the purest, cleanest DC will do.

    Hope this is helpful.
    Last edited: Feb 5, 2017
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  11. ghrit

    ghrit Bad company Administrator Founding Member

    Yep, that would be so if there were batteries in the circuit as dampers. Not so sure that will hold if there aren't, since demand by the device may not be constant.
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  12. ghrit

    ghrit Bad company Administrator Founding Member

    If it is as the graph shows, then if the peak is at 19V, then the effective voltage will be 12, IIRC.

    (Sounds familiar from a VERY long time ago now that post 7 above also connects --)
    Last edited: Feb 5, 2017
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  13. Tevin

    Tevin Monkey+++

    You got it 100% right. That's why it's (usually) no big deal to connect a "19 volt" wall wart to a 12 volt device. Even if the voltage is too high, it's constantly changing and does not hang around the 19 volt peak long enough to matter anyway.

    While I'm thinking about it, some wall warts elevate the cheapness even further and are designed as half wave rectifiers.

    Where a full wave will take the negative cycle and flip it positive, a half wave simply chops the negative side off and forgets about it. You have the juice on literally only half the time! There is no way to know if you have one of these low end junkers without an oscilloscope, but this could be the reason why the Dakota will not work.

    sec_monkey likes this.
  14. ghrit

    ghrit Bad company Administrator Founding Member

    I actually thot of that, drafted it into a response above, then doubted my memory. @TnAndy, you obviously need an o'scope. :lol:
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  15. stg58

    stg58 Monkey+++ Founding Member

    In this case you want it running all the time but keep in mind power packs suck power all the time so called "energy vampires"
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  16. tacmotusn

    tacmotusn RIP 1/13/21

    Just 2 cents . Why not do like a lot of folks with game cameras and feeders that operate off 12vdc or 6 vdc. Investigate and find one that runs off 12vdc. Buy both one of their 12vdc sealed batteries, plus a 12vdc solar charger. This seems to give constant power that seldom needs any attention. Or you could use and auto float trickle charger that is meant to keep a 12vdc battery topped off and also plugs into 115/120 vac wall socket. jus sayin'
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  17. TnAndy

    TnAndy Senior Member Founding Member

    Yeah....that's my plan. I've got some 25amp/hr batteries given to me by a guy that changes out emergency exit signs/light batteries....most of them are still fine, they just change them on a fixed schedule.....they are sealed 12v, I've kept them on a GOOD trickle charger/desulphanator for a few years.

    Where I'll put it, I can plug in a maintainer.
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  18. duane

    duane Monkey+++

    Several different wall warts-battery chargers-dc supplies types. Cheapest has a small transformer and a diode and puts more than enough voltage at the specified current draw and depends on the using the supplied device to regulate the voltage and filter the unfiltered pulsating dc. Works well with a rechargeable battery as it is used more as a huge filter capacitor than a battery , but is noisy and a lot of things will not run on it alone. Next best has a transformer, a voltage regulator, and some sort of filtering, but puts out a fixed voltage, and is usually used for electronics and computers and will replace most battery supplies. I have had problems with computers, electronics ,etc failing as the voltage may drop with a larger current draw. Real problem with some Raspberry Pi and to small of a current supply and voltage dropping below minimums.. The third type is the one used to charge most power tool batteries. It uses a capacitor circuit and switching to reduce the 120 to some low voltage and not a transformer to supply dc voltage, also used in computers etc, and limits current flow and senses the desired voltage in the load and will work with several different voltage batteries, either by sensing the battery or a regulated voltage as in a computer or other electronic devices. Once you wish to replace a battery supply with a wall wart, it can be a challenge if you don't use the one designed for your unit. The engineers have hopefully did their home work and it will work first time out. If you don't wish to use their unit, it really helps if you can check the output voltage, its regulation and the current rating of one of the correct units for your device.
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  19. Tempstar

    Tempstar Old and crochety Site Supporter+

    Try reading the voltage with the xformer plugged in and connected. I think that will tell the tale,
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  20. sec_monkey

    sec_monkey SM Security Administrator

    Try a good computer PSU. They provide regulated 12VDC.

    Get the 2 pin to 24pin adapter that keeps them powered on.

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