can we connect ac source series with dc source?

Status
Not open for further replies.

steve66

Senior Member
Location
Illinois
Occupation
Engineer
In theory, yes, sure. The resulting voltage would be the AC voltage riding on the DC voltage.

And the resulting current would still be the total voltage divided by the total resistance of the load.

But in reality, I wouldn't try this at home. Most batteries and DC supplies have a very low current rating, or they may not like the AC forcing current or votlage the opposite direction.

They could easily explode if connected to something like a load through 120V AC source.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
In theory, yes, sure. The resulting voltage would be the AC voltage riding on the DC voltage.

And the resulting current would still be the total voltage divided by the total resistance of the load.

But in reality, I wouldn't try this at home. Most batteries and DC supplies have a very low current rating, or they may not like the AC forcing current or votlage the opposite direction.

They could easily explode if connected to something like a load through 120V AC source.

Placing a rectifier bridge in the circuit would solve that problem, and it wasn't mentioned that doing so would be prohibited.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
120220-1524 EST

srinivasan:

Sure you can. There is no problem unless you do not do a good design job for the function you want to perform.

Examples:

The plate circuit in an audio amplifier has an applied DC supply voltage. Typically there is a resistance, but sometimes an inductor or transformer, between the B+ and the plate. Control of the tube grid modulates the plate voltage and you have an AC voltage superimposed on an average DC component.

A two wire system between a device and a remote device. For power at the remote device a DC voltage is applied to the wire pair. Superimposed on the DC is an AC signal for control or data transfer.

A 60 Hz transformer to a diode to a load resistance and there is a DC voltage with an add AC component.

A 60 Hz transformer, 24 volts output, in series with a lead acid 12 V car battery. This connects to a resistive load. The peak voltage of the 24 V AC is 33.9 V. This will cause negative current thru the battery. What is the problem? Part of the cycle the battery is a source of energy, and part of the cycle the battery is being charged. Current out of the battery is discharge, and into the battery is charge.

.
 

mike_kilroy

Senior Member
Location
United States
more examples...

ur tv mast mounted rf amplifier - dc supplied thru coax to it and rf signal comes back down same cable.

2.5vdc bias added to analog +/-1vac encoder signal so it works with 0-5vdc supplied op amps.
 

hurk27

Senior Member
The list can go on for ever.
like phantom voltage to power mic's in a PA system.

cable systems use a 60 volt DC piggy backed onto the cable trunk.

CCTV cameras piggy backing there power supply on to the video cable

almost all DBS satellite systems power there LNB's through the same cable the IF is sent down through.

And there are many more, like was said it only takes the proper coupling and decoupling of the AC to the DC, nothing more then a high powered low pass filter or high pass filter depending upon which end your looking at.
 

hurk27

Senior Member
The simplest forms of coupling is a DC blocking capacitor to remove just the AC component or inject the AC component, on the DC end just a iron core chokes tuned to block the frequency of the ac component from getting into the DC circuit, used at both the supply and load ends, for multiple frequency AC components, tuned band pass filters might have to be used.
 

hurk27

Senior Member
what is effect on the closed circuit current

After reading the OP (which I guess I should have in the beginning) I now realize what he wants to do.

You can get devices to do this, to piggy back the DC onto the coax, but there are some limiting factors, the big one is the length of the run because of voltage drop, I have used some from smarthome.com for 12 volt DC and cameras operating at standard composite video frequency, they worked well but after about 350' you would get DC bars in the video stream as the voltage drop kicked in, but the nice thing is you don't have to put the DC injectors at the DVR as they can be located closer to the camera at any convenient location you can locate a power supply.

I'll try to find some, mine were BNC to BNC connectors, but I think they were offered in F connection also.

Heres the one I used HERE

But looks like it has been discontinued:rant:
 
Last edited:

topgone

Senior Member
Correct me if I got it wrong here. I think you wanted to mix AC into a DC current flow.

That said, why do you have to physically "connect" when you can just superimpose an AC signal on the DC line by induction. This is basically the theory behind non-contact, switched spark initiation in Tungsten Inert Gas (TIG) welding machines. An HF generator output is routed into a coil turned around the output lead of the DC welding source and the arc is initiated when the torch head (negative terminal) comes near enough with the workpiece to be wielded (positive end).
 

hurk27

Senior Member
Correct me if I got it wrong here. I think you wanted to mix AC into a DC current flow.

That said, why do you have to physically "connect" when you can just superimpose an AC signal on the DC line by induction. This is basically the theory behind non-contact, switched spark initiation in Tungsten Inert Gas (TIG) welding machines. An HF generator output is routed into a coil turned around the output lead of the DC welding source and the arc is initiated when the torch head (negative terminal) comes near enough with the workpiece to be wielded (positive end).

If you read the OP, I don't think he needed that kind of AC signal for a CCTV camera:?

The OP is wanting to combine the composite video signal with a approximately 12 volt 110va DC supply onto one coax to eliminate having to run separate power supply wires all the way to his cameras, I think.

But you added one more good item that piggy backs DC and AC together on one set of conductors.
 
Last edited:

topgone

Senior Member
just i saw one question ac source with dc of RLC circuit .in your first reply itself i got a answer that capacitor will block dc ..

That's how we couple input signals to another stage in an amplifier; using coupling capacitors to do away with DC content of the input signal. I'm still not getting what your goal is!
 
Status
Not open for further replies.
Top