Posted: 11/7/2013 3:21:50 PM EDT
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Couldn't think of a better place to ask it ... lot of knowledgeable people here.
Say you have a small DC electric device like a network switch, cell phone charger, whatever. It plugs into AC power via a wall wart transformer that has "12 V, 0.6 A" written on it. Is there any reason you can't just take a 12V car battery, or two 6 V batteries wired in series, or 8 AA batteries in series, or some other battery arrangement that puts out 12V, cut the cord off the wart, and power the device straight off the battery or batteries? Do you need a voltage regulator to ensure it's 12V and not 13.9? Seems dumb take a 12 V battery, connect it to a DC to AC inverter to get a 120V AC outlet to plug the wart into just to get 12V DC again. Thanks. |
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It really depends on the device. Some gadgets have an internal voltage regulator that will convert the 12V into 5V or something else,
and powering with a battery is fine. Other devices are expected 12.00 V exactly, with the regulation coming from the power supply; these devices may not work correctly with direct battery power. The "normal" way to power isn't with a DC to AC inverter, but a DC to DC converter. There are several on the market that take 12V battery power in (which is generally 11-14V in) and put out 12.0V or whatever you need out. |
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Quoted:
Couldn't think of a better place to ask it ... lot of knowledgeable people here. Say you have a small DC electric device like a network switch, cell phone charger, whatever. It plugs into AC power via a wall wart transformer that has "12 V, 0.6 A" written on it. Is there any reason you can't just take a 12V car battery, or two 6 V batteries wired in series, or 8 AA batteries in series, or some other battery arrangement that puts out 12V, cut the cord off the wart, and power the device straight off the battery or batteries? Do you need a voltage regulator to ensure it's 12V and not 13.9? Seems dumb take a 12 V battery, connect it to a DC to AC inverter to get a 120V AC outlet to plug the wart into just to get 12V DC again. Thanks. G-D, we do it all the time, in fact several sws, network bridge radios, this message is being sent via, IP cams, etc, are being powered directly via a plug and cable cut off from a 12 to 18 vdc wall wart or wired into the device directly. A couple sws that I like to run off 12vdc are the Linksys 5 port sw SD205, other Linksys/Cisco sws, some Netgear units are very nice. I've spent some time in the past 2 years taking them apart and testing on 12 volts nominal [could swing from <11 to 16] and have good results. Most of them are internally regulated to 5 or so VDC. These are pennies on the dollar on ebay. Any questions for specific needs, just ask. |
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I just ordered one of these 12V netbook chargers---has lots of adapters for different netbooks/laptops. Also has a USB port which I'm hoping will work with my smartphone and other small devices. I plan on using it with a 12V deep cycle battery bank.
These things are cheap enough that I don't think it's worth taking the risk modifying an AC charger and hooking it up to a battery. |
| If the device is powered by 12v it should be fine using a battery, even if it's the 13v+ you get from a car battery. Things listed as 12v are (usually) made to run off of either a wall-wart or a DC cord plugged into your cigarette lighter in your car, so they should be good in that 10-14v range. The caveat is something like a notebook computer would be unable to charge it's batteries if the voltage falls below a critical amount, even if it could continue to function. |
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Thank you all. Should've put more info in the OP I guess ...
For now, just need to run an IP camera & 802.11 network bridge off it. It'll sit about 1200-1700 yards downrange and serve as a target cam. Just parts I have laying around - network bridge = 24V 0.6 A camera #1 = 12 V 0.6 A camera #2 = 12 V 1.25 A Camera #2 has better resolution so I think I'll probably use that. Any specific battery recommendations? Thanks. |
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Quoted:
Thank you all. Should've put more info in the OP I guess ... For now, just need to run an IP camera & 802.11 network bridge off it. It'll sit about 1200-1700 yards downrange and serve as a target cam. Just parts I have laying around - network bridge = 24V 0.6 A camera #1 = 12 V 0.6 A camera #2 = 12 V 1.25 A Camera #2 has better resolution so I think I'll probably use that. Any specific battery recommendations? Thanks. You don't get precise power from a wall wort, and a 12V battery us going to be fairly close, even more so than a vehicle which may hit 14+ volts charging.i One down side, as the battery discharges, at some point the device whill behave eratically. Professionally installed equipment has a low voltage cutout (LVCO), but you cvan make sure in this application the battery never runs down. Normal gel cells aren't designed for discharge cycles, and I'd lean torwards a premimum brand like panasonic, yusha, etc. If a few bucks extra isn't a problem, order from McMaster Carr. Most places are more likely to have 6V batteries than 12V, and you can wire two in series. The longest life would be a automotive sized deep cycle (trolling motor) bsttery. |
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Based on your last post, I think the easiest thing for you to do is to get a small, low-wattage 12V inverter and a battery to power the devices using their AC adapters. You could get better efficiency with a DC-only solution, but for something you're going to set up, use for a few hours, then take down -- especially since one of the devices requires 24V -- I wouldn't bother. A 75W inverter should be about right. 100W max. If you run a higher capacity inverter with a small load, your efficiency can drop substantially. 75W gives you plenty of headroom for the load you described (I estimate about 45W at 120VAC for the load you described [1.25A camera] including efficiency losses in the AC adapters). A gel cell rated around 18+ AH should take care of you for half a day. For recharging, get a 3-stage smart charger. |
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The only issues are nominal vs. actual voltage, and functional voltage range.
A fairly low car battery probably still gives 10V, which is probably fine for most 12V stuff. But 8 AAs on the low end can be as low as 7.2, which is probably NOT fine for a lot of 12V stuff. |
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Quoted:
You don't get precise power from a wall wort, and a 12V battery us going to be fairly close, even more so than a vehicle which may hit 14+ volts charging.i One down side, as the battery discharges, at some point the device whill behave eratically. Professionally installed equipment has a low voltage cutout (LVCO), but you cvan make sure in this application the battery never runs down. Normal gel cells aren't designed for discharge cycles, and I'd lean torwards a premimum brand like panasonic, yusha, etc. If a few bucks extra isn't a problem, order from McMaster Carr. Most places are more likely to have 6V batteries than 12V, and you can wire two in series. The longest life would be a automotive sized deep cycle (trolling motor) bsttery. I can attest to that. I blew out a power supply for an item (home-built weather station) that was pretty picky about it's input voltage. I save up wall-warts from anything that breaks or whatever, so I've got quite a collection built up, of varying output voltage. Went through that box with a multimeter to find a replacement for the bad one, and I was pretty astounded by the variance of output voltage from what was listed on the power supply. Anywhere from 1 to 5 volts in either direction; in fact about the only voltage I rarely saw was that listed on the device! 
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I can attest to that. I blew out a power supply for an item (home-built weather station) that was pretty picky about it's input voltage. I save up wall-warts from anything that breaks or whatever, so I've got quite a collection built up, of varying output voltage. Went through that box with a multimeter to find a replacement for the bad one, and I was pretty astounded by the variance of output voltage from what was listed on the power supply. Anywhere from 1 to 5 volts in either direction; in fact about the only voltage I rarely saw was that listed on the device! That probably due to most wall-warts under 1 amp are not regulated, they use the load from whatever device they are powering to set the voltage. The larger ones often say "regulated" somewhere on the little information sticker. |
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Quoted:
Thank you all. Should've put more info in the OP I guess ... For now, just need to run an IP camera & 802.11 network bridge off it. It'll sit about 1200-1700 yards downrange and serve as a target cam. Just parts I have laying around - network bridge = 24V 0.6 A camera #1 = 12 V 0.6 A camera #2 = 12 V 1.25 A Camera #2 has better resolution so I think I'll probably use that. Any specific battery recommendations? Thanks. as stated above, since you need 24 volts and 12 volts get an inverter plugged into car battery and plug those things into the inverter. those items will need about 29.4 watts max. call it 30 watts figure a 10% loss due to the inverter at the rated wattage, so a 100 watt inverter will use about 10 watts. so you will be using 30 plus 10 equal 40 watts max. off a 12 volt battery it will draw 3.3 amps which is nothing. I would not bother spending the extra money on a deep cycle battery If you run this setup you will be using 3.3 amphours, call it 4 amphours. any cheap car battery will handle that draw and charge cycle for years and years IMO. |
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however, I don't think you will get a wifi signal to go 1200 to 1700 yards without high gain directional antennas on both ends. 1700 yards is almost a mile. typically wifi is good for about 300 feet / 100 yards. without high gain directional antennas. Thanks. The AP/bridge I have is an 800 mW outdoor model with a 10 dBi directional antenna. Engenious ENH202. Haven't decided what to use on the display end of the link. Laptops have lousy antennas in them so I guess may need another bridge with an antenna. This pic was from last weekend. Some people were in our usual spot, so we shot off the other side of the ridge, about 800 yards to the opposite ridge. Line of sight way out in the desert, no other network noise ... I'm hopeful, anyway. 
Fun project regardless. 
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some such wall warts are AC as they are just a transformer. others have a rectifier built in so provide rectified DC but no smoothing of the pulses. others are real power supplies with rectification and filtering.
as long as the device is expecting DC to come in, it probably won't care much if it comes from a battery or a wall wart as long as it is the right voltage level and right polarity. some even have full wave rectifiers so they will work either polarity of incoming power. most have at least diodes built in to protect against reverse polarity damaging the device, but that is not a given. |
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800 mW and a 10 dBi antenna is plenty for the job, provided the display end can transmit back as successfully. You could easily make it work with less than 1/10th that. I ran the numbers for a 2.4 GHz link over 1,700 yards in southern California desert conditions. Assuming your bridge is a 1-piece unit (transceiver and antenna integrated with no coax loss in between), and assuming also a 10 dBi antenna at the display end, you have a minimum antenna elevation of at least 14 feet to limit out-of-phase ground reflections that degrade the signal-to-noise ratio. Sure, you have tons of signal, but the reflection noise will be equally strong, so get those antennas up there. For a 5 GHz system, you can get by with 10 feet antenna elevation. 10 dBi gives the antenna a 3 dB beam width of about 52 degress, so aiming will be a snap. With adequate antenna height, this setup should be good for better than 5.5 miles. But when the link is powered, keep your body parts at least 16 inches away from the antennas to avoid a radiation dose that's above the FCC recommended limits. So bottom line:
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Also, I want to reiterate my AC inverter recommendation to run the power supplies that came with the hardware. You have two different operating voltages listed and this is the easiest approach to satisfy both without having a much more complicated battery setup. Anything from an 18 AH gel cell to a car battery or deep cycle battery will do the job comfortably. |