12VDC or 24VAC?

[jack in oz 0]

Hi,

I just purchased a stand-alone 8ch DVR for use in my home. I will be using Cat5e with baluns for the video and power supply. I would like to build myself a dedicated power supply to power the 8 cameras. I'm still buying cameras as funds allow, but the ones I have (Samsung box) draw 6W. In that case I need a PSU capable of at least 50W. I should probably allow more for IR LEDS too.

 

Option A is to just put a 24V 50VA transformer in a box with 8 fused outlets. One problem I can see with that is the voltage of the TX may be too high if I don't have it loaded with 8 cameras. One advantage with 24VAC is that the current is halved and therefore the voltage drop over the Cat5e is halved.

 

Option B is to make a 12VDC power supply capable of 5A. I have a few 5A linear voltage regulator chips and some salvaged heatsinks, so that is a simple and cheap option too.

 

Other than those I've highlighted, what are the pros and cons of DC vs AC in a CCTV system?

Will IR LED illuminators run off 24VAC?

 

Thanks,

Jack

[Soundy 1]

Hi,

I just purchased a stand-alone 8ch DVR for use in my home. I will be using Cat5e with baluns for the video and power supply. I would like to build myself a dedicated power supply to power the 8 cameras. I'm still buying cameras as funds allow, but the ones I have (Samsung box) draw 6W. In that case I need a PSU capable of at least 50W. I should probably allow more for IR LEDS too.

 

Option A is to just put a 24V 50VA transformer in a box with 8 fused outlets. One problem I can see with that is the voltage of the TX may be too high if I don't have it loaded with 8 cameras. One advantage with 24VAC is that the current is halved and therefore the voltage drop over the Cat5e is halved.

At 6W, voltage drop will be almost negligible, either way. Going by this calculator, 24V at 0.25A draw, using only two pairs, and figuring a maximum 100' run (fair guess for a home install), will see only 0.2V loss... with 12VDC at 0.5A, it's a whopping 0.6V drop.

 

I've used far more 24VAC power supplies than 12VDC units, and never had an issue with "over voltage" regardless of loading. Keep in mind that transformers don't tend to be high-tolerance devices, meaning your output voltage could vary +/-5% anyway... nevermind fluctuations in the line voltage supplying it. Consider too that 99.99999% of 24VAC cameras will be stepping down and rectifying (and possibly regulating) that power internally anyway, so a couple volts either way REALLY doesn't matter.

 

Option B is to make a 12VDC power supply capable of 5A. I have a few 5A linear voltage regulator chips and some salvaged heatsinks, so that is a simple and cheap option too.

 

Other than those I've highlighted, what are the pros and cons of DC vs AC in a CCTV system?

Well, 12VDC cameras tend to be the most common... cheap ones that use a common power and video ground also tend to have ground-loop problems with baluns. 24VAC and 12VDC/24VAC dual-voltage cameras tend to be more common among "professional" models. Really, the only concern should be what type of cameras you'll be using.

 

Your 12V project would be a lot more work to put together, of course, and you'll see more power lost to heat dissipation.

 

Will IR LED illuminators run off 24VAC?

Some will... again, really depends on the units you get. If you're building your own, you can design them to use whatever source voltage you want.

[jack in oz 0]

Thanks for the response. I notice many of the Altronix power supplies have both 24VAC and 12VDC outputs. I might build a 24VAC PSU and leave room in the box for a 12VDC circuit board if I ever need it.

Cheers,

jack

[Soundy 1]

I've added a 1A or 3A 12V regulator board inside a 24VAC power can before, if I find the need for one or two channels of 12V power (whether for a camera, or something else, like a buzzer or door strike)... just feed one of the 24V channels into its input and I'm good to go!

[jack in oz 0]

Sounds good.

[heckufaguy 0]

Keep in mind, that depending on the camera, 12vdc cameras will draw up to twice that of what they will on 24vac. (.5 am on ac, vs 1 amp dc).

As you add cameras, if they'll run on both, AC tends to be easier to deal with.

[Soundy 1]

Keep in mind, that depending on the camera, 12vdc cameras will draw up to twice that of what they will on 24vac. (.5 am on ac, vs 1 amp dc).

Just for clarification: this fact is due only to the difference in voltage, not AC vs. DC. Current draw at 24VAC vs 24VDC would be effectively the same (there'd be slight differences depending on whether you're talking about a resistive vs. reactive load, and whether you're measuring the AC at P-P or RMS, but for real-world purposes, they're the same).