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Old 10-30-2008, 07:26 PM   #1
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Connecting AC to a DC device


I have some security cameras which according to the spec sheet are designed to accept 12VDC at 3 amps. However, I purchased these second hand and the power supplies they came with output 24VAC.
The cameras seem to work, and according to the previous owner (an IT support firm) they have been running with those power supplies for over a year.

Are these power supplies slowly damaging the cameras, or did the IT company figure out some kind of work around?

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Old 10-30-2008, 07:35 PM   #2
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Connecting AC to a DC device


12 volts DC can be real close to 24 volts AC depending on what type of rectifier is used to change the AC to DC voltage.

Apparently it is close enough for your cams. Since they have lasted this long, I would suspect it is causing no damage. If it were, I would think they would have smoked long ago.


Last edited by nap; 10-30-2008 at 07:37 PM.
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Old 10-30-2008, 11:51 PM   #3
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Connecting AC to a DC device


Thanks nap. One thing though...
I would like to use an unused pair on a cat5e cable to get power to the cameras. I have checked the current load limit on 24 awg wire, and it appears to be around 3 amps in the open air. The camera's spec sheet claims the device uses 12VDC at 14 watts, so that should be around 2 amps. But if I use the 24VAC power supplies, does the amperage remain the same, half as much, or how do I determine the amperage -- preferably without using a multimeter?
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Old 10-31-2008, 02:48 AM   #4
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Connecting AC to a DC device


Cat5 isn't free-air, I would derate. Power over ethernet goes up to right around 12 watts, for reference. But that's conservative and assuming up to 100 meter runs.

Most likely the amperage is the same. I suspect the cameras have diodes and linear regulators inside them, or equivalent, which is why they can tolerate a power supply that is so much higher. This means they are just burning off the extra wattage as heat.

So most likely (if you measure you'll be sure) they are pulling about 1.2 amps at 24VAC the same as they would pull 1.2 amps at 12VDC, with the balance being burned off as heat. Measure with an ammeter to be sure.

Oh, you can get a cheap multimeter at wal-mart for like $5. A useful thing to have. Get one.

Last edited by Gigs; 10-31-2008 at 02:51 AM.
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Old 10-31-2008, 07:18 AM   #5
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Connecting AC to a DC device


Most CCTV cameras that I have installed are rated dual voltage and will accept either 12VDC or 24VAC and work just fine on either one. With the AC voltage you don't have polarity to worry about like you would with DC. I still tend to prefer running DC for most of the cameras, however I have one outdoor camera housing that has a small heater and a fan in it to control the environment inside the housing for the camera, this one requires 24VAC.

I would not run power over the Ethernet cable, especially if you are using it for anything other than powering the camera. Get the Siamese cable, it has power and video in the same cable; its what I used for all my camera installs. Usually its RG59 coax for video and 18/2 stranded wire for power; I believe there are some setup with RG6 if you really feel the need for the larger coax cable. Also, some Siamese cables include an extra set of 18/2 for a RS232/RS485 control line to control PTZ functions on the camera.
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Old 10-31-2008, 10:22 AM   #6
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Connecting AC to a DC device


Thanks Gigs and theatretch85. Taking your suggestion gigs, I did decide to use my multimeter to measure the amps, and it maxed out at 4.0 and idled at 2.4 on the 200m AC setting. I don't have much experience with measuring amps, but I am guessing that would be 4 amps, which would be too risky to run through 24 awg wire. So I guess I am stuck with 18 awg wire.

What you said theatretch85 did comfort me though. I think your experience and naps suggestion are enough to convince me that I don't need to buy new adapters for these cameras. I liked what I saw in the siamese cable, except for the coax. These are actually ptz IP cameras, so I don't think I could utilize the coax.
Ideally, I need to run cat5e and an 18 gauge 4 conductor wire -- two conductors for powering the camera, and two for the enclosure.

Anyway, I think you guys answered all of my questions, so thanks again for that. But if anyone has any further suggestions, I am all ears.
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Old 10-31-2008, 01:27 PM   #7
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Connecting AC to a DC device


I used speaker wire for my cameras. 16 gauge. I don't think you will need two power feeds. Why do you want two? If it's because they are different voltages, with the price of copper being so high it's probably cheaper to put a little regulator on the end for the second voltage.
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Old 10-31-2008, 04:09 PM   #8
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Connecting AC to a DC device


Quote:
Originally Posted by kimbokasteniv View Post
designed to accept 12VDC at 3 amps.
the power supplies they came with output 24VAC.
The cameras seem to work
Are these power supplies slowly damaging the cameras
If the transformers are outputting 24vac (at ~3A) to end up with 12vdc at 3A, there is a lot of power being lost inside the camera, about 36W. It may also be overstressing the camera power conditioning circuits.

But, if there is a switching supply in the camera it may be OK because they accept a wide range of voltages.
Check the camera nameplate data.

If this doesn't work, try to get some plausible answers from the cam. manu. on this point.

Measure the 24vac first so you have some hard numbers to back up your concerns. Maybe they use a center tap so you're only supplying 12vac to the camera.

Still, for long life, it's better to supply the minimum operating voltage to the camera.

Last edited by Yoyizit; 10-31-2008 at 04:11 PM.
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Old 10-31-2008, 10:41 PM   #9
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Connecting AC to a DC device


Well Gigs, the reason I want two conductor pairs(or think I need two), is because I figured running the enclosure and the camera off the same power supply over the same lines would burn something out. At least that is what the previous owner recommended, as in, that's how they had it wired up. Otherwise both the camera and the enclosure run on the same voltage.

And Yoyizit, I did go ahead and measure the output of the power supplies and it is approximately 24VAC. However... I opened the enclosure and pulled the camera out to see how it was wired, and well mystery solved.



After searching the part number (VLSMP2) and measuring the output, that board appears to be converting 24VAC to 12VDC. I suppose I should have disassembled the enclosures when I got them, but the units were rather expensive and I didn't want to fall victim of "fixing" something that was not broken. So I just assumed the wires on the outside of the unit were directly connected to the camera.

Anyway, I appreciate the help from everyone -- you guys were much more helpful than I expected.
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Old 11-01-2008, 10:36 AM   #10
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Connecting AC to a DC device


It looks like a switching power supply.
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Old 11-01-2008, 01:28 PM   #11
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Connecting AC to a DC device


Those 4 round black things stacked above each other on the right hand side of the circuit board are diodes. A diode will allow current to flow in one direction, and block it in the other. Since the polarity of AC power constantly changes, if it's blocked in opposite directions, the result is current flow in only one direction, AKA DC. The silver bands at the ends of these diodes are polarity marks. Current will flow from the band toward the diode, but not the other way. Notice that two of them face to the right, and two to the left. One set will be the DC positive, the other will be negative.

I can't see the other side of the circuit board, but I'd bet they're connected as a full-wave bridge rectifier. This is one of the simplest AC to DC converters that exists.

The 3 round things standing up on the far right hand upper side are capacitors. These are like batteries, sort of. They smooth out ripples in the DC after it has been converted from AC.

The two chips at the top are the voltage regulator circuit.

The round copper coil on the left side is called a choke. It's nothing more than copper wire wound around an iron core. A choke will further smooth out the DC power.

The two capacitors on the lower left are the final DC smoothing stage.

We now have a regulated DC voltage with minimum ripple to be fed to the camera video board.

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Old 11-01-2008, 02:47 PM   #12
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Connecting AC to a DC device


You dont need a second power wire set to power the camera and enclosure separatley. I have an outdoor enclosure I installed that has the heater plate and fan in it that runs on 24VAC and the camera in that enclosure is dual voltage and will accept either 12VDC or 24VAC. So I use the same power wire and put in a distro block in the camer enclousre to split out to the 3 devices that need the power. Its all 18 gauge wire in the siamese cable and its been installed and working for well over a year now.

How many conductors do you actually need for the PTZ control? Most cameras I have seen that are PTZ use an RS485 connection which is simply 2 wires. Thats what the second pair of 18 gauge wires would be for in the siamese cable.

Where is the video output on the camera? Is it mounted on the outside of this camera out of view in the picture?
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Old 11-01-2008, 05:05 PM   #13
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Connecting AC to a DC device


There is an advantage to using 24vac; doorbell transformers are short-circuit proof. They are also much bulkier for the power delivered.
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Old 11-01-2008, 09:51 PM   #14
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Actually micro, there's some ICs there. It's one of those newfangled "SMPS on a chip" deals, most likely.
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Old 11-01-2008, 09:52 PM   #15
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Connecting AC to a DC device


Micromind, I really appreciate the explanation. It was enough to motivate me to look up the circuit of a full wave bridge rectifier. I think I have to come to a basic understanding of it.

Theatretch85, one other concern I have with using a single line and power supply, is how do I know the power supply can handle it? Its claimed output is 24VAC 40VA. VA is watts, right? So a single power supply can output about 1.6 amps? This confuses me, since I am currently only using one power supply per device, and the enclosure is marked as 24VAC 54VA, and I know the camera pulls a max of 4amps at 24VAC, so unless VA stands for something other than watts, I am not sure how things are working at all -- unless these are "loose" ratings. Because of this, I am leery about connecting two devices to a single power supply. So, I am thinking that I would need to connect two power supplies in parallel (if that would even work), or buy a new power supply that outputs at least 6 amps. Or am I completely missing something important?
As for the ptz controls, well it is an IP camera so I do that all via the cat5e cable. And the video output is transmitted via the cat5e as well.

And Yoyizit, you were saying that such power supplies are short circuit proof. As in, after directly connecting the positive pole to the negative pole, the power supply will still function? The reason I ask, is because I appear to have broken one of my power supplies when I attempted to measure the voltage. Unfortunately, I had my probes connected so that they would measure amperage, so when I went to measure the output on the supply I created a short.

Just to clarify the equipment:
The cameras is an Axis 214ptz
And the power supply for the camera and enclosure is a MGT2440

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