DC Power over coax

snowstreams

Hi,
I posted this question in the IOT forum Yi Camera but I thought I might post it here too, to see if I could get a better answer.
I have installed a usb powered outdoor security camera high on a wall beside an old satelite dish.
I couldnt easily get power to that area, so I decided to use the coaxial cable from the old satelite dish to supply power.

I have tried using a 12v 500mA DC power supply at one end of the 30m of coax, and then a 12v to 5v car usb adaptor at the other end.
This works to supply power for the camera in daytime use, but it turns off when it tries to start its infrared lights at nighttime.

So Im thinking of getting a different power supply.
Would i be better off getting a 24v supply or should I get another 12v supply with a higher current rating on it?

The camera takes 1A at 5v apparently according to the box.

2011

Assuming thy the 12V power supply is large enough:
The problem is the volt drop on the coaxial cable is too high. The volt drop on this cable is a function of the current flowing through the cable and the resistance of the cable. To deliver the power you need doubling the applied voltage will mean that the current will half, which will reduce the volt drop by the same proportion. So the answer is to use the 24V supply that you suggested.

Needless to say the 24V power supply will have to be large enough for the load.

snowstreams

Yeah I was thinking the higher voltage would make things easier.
I've no idea how much the losses are in the car adaptor either, but it is rated at 12/24v : 1A 5V so it should work better at 24v.

DublinDilbert

The big question is the car adapter USB adapter a switch mode converter or linear. I'm guessing if its rated for 12/24V @ 500mA its probably a switcher device. So increasing the voltage on the line will reduce the current been taken by the switcher (for the same power delivered to the load).

5v x 0.5A = 2.5V
12v x 0.2A = 2.5V

I'm not sure if voltage drop is the issue, the cross section of the coaxial conductors should be fairly big, you only need to push 200mA and can have about a 6V drop on the cable.

2011

DublinDilbert wrote: »

I'm not sure if voltage drop is the issue, the cross section of the coaxial conductors should be fairly big, you only need to push 200mA and can have about a 6V drop on the cable.

Depends on the coaxe. Some have quite a fine center core. If the screen isn’t made off well and strands break this can also increase the resistance quite a bit. Plus the run is 30m. I’m not sure how much the camera draws but (not a lot I would think) but I would assume less than 0.5 amps so the power supply should be large enough. However it seems that once the lid increases (due to LEDs) the increased load seems to be an issue which would occur when the volt drop is at a maximum.

snowstreams

The usb adaptor I'm using is fairly basic like this one below. My one is 1A output.


How does a switch mode converter work? Does the current increase in the output in a similar way to how an AC transformer works?
If so, then a small enough 24v supply should work well.

2011

snowstreams wrote: »

The usb adaptor I'm using is fairly basic like this one below. My one is 1A output.

The current drawn will depend on the load. 1A will flow when a resistance of 12 ohms is connected across it.

Ohm’s law applies:

Current = Voltage / Resistance

So if the total resistance is 24 ohms then a current of 0.5 amps will flow.
The total resistance = cable resistance + load resistance

2011

snowstreams wrote: »

How does a switch mode converter work?

It’s been a while since I studied power electronics so I will keep this simple as I have forgotten almost everything. Think of it as a transformer & rectifier combined into a highly efficient, light weight device that runs cooler than a transformer. AC in and DC out. Pretty much all modern laptops use them.

Does the current increase in the output in a similar way to how an AC transformer works?

For the purposes of this discussion see Ohm’s Law above.

snowstreams

Thanks I want to make sure that enough current was coming out at the car adaptor end into the camera.

My current setup is 12v on a 400ma AC power supply. So even if everything is efficient the car adaptor part isnt going to be getting enough power to supply 1A out.

12 x 0.4 = 4.8w then at 5v that would be 960mA output assuming 100% efficiency. so its probably more like 600mA coming out.

So I might try a larger 12v supply first before I go buying a new 24 supply.

I have a few old laptop power supplys around too that I might try using too. They seem to give out 20v though.

2011

Find out where the issue is through a process of elimination:
Step 1, test the assembly by seeing if it will work without the coaxial cable.