Mixing AC and DC

In theory, you should be able to send both the AC and DC over a single pair of wires, by putting a series-connected combination of a transformer and a (large) capacitor at each end (2 transformers and 2 capacitors). DC can be applied / taken off across the capacitors. The transformer winding will be essentially short circuit to DC and the capacitor will be a short to AC. In practice though the transformer's winding resistance and the capacitor's surge resistance will lead to some power loss.

Mixing AC and DC on the same line is possible.

DC and DC on the same line is a problem

AC and AC on the same line is possible.

Digital communications and control is used on Mains wiring to save from adding in extra wiring into already fabricated installations, the "data" is super-imposed onto the AC supply lines, and the mains lines become a combination of supply and data on the same pair of wires, check out "X10" with "Home automation"

For your TV Mast-head amp, you plug in a DC voltage into an adaptor inside, and the amp is up your TV mast (externally) there is a capacitor that blocks the DC component (power supply) from going in to your TV arial socket, and the DC flows up to your mast head amplifier, so you'll have a small signal that is your reception from the antenna and say about 12VDC on the same line.

The only stupid question is the one thats not asked ;o)

Hi snaketails and guest

My sketch came out bad but the intention was the AC and DC to be coupled in serial.

If I understand correctly you are both suggesting parallel is possible as well. Is that right?

(off topic) What do we call the output? 50/60 cycle DC?

About mixing DC with DC. On a DC traction train DC power is fed into the system at different locations. Or are the trajectories isolated from each other?

Also batteries can be joined in series or parallel. For example a flattery and a battery with a jumper cable (parallel) in a car.

Will 2 rectifiers solve the problem?

I cannot see the practical side of it you have generator making AC then add DC, and at the user split it again? DC is often used for electronics, AC and DC together would require mor Filtering IMO, i think it is just easier to make AC from DC on the user side.

Well if you add 100/110/200~240Volt (depends on the country) DC to an AC voltage then it is just a DC Voltage with a large ripple

An interesting question, I am not a hands on electrician but I remember as a marine engineer doing earth leakage tests. And the point that comes to mind is that different voltages could be identified, when there was two different voltage leakages. And as AC was a floating neutral, one gets to thinking that earth is carrying all the variations, and there is no pop and crackle? so I assume that the receiver of the different power readings is as important as the source. Like a radio signal they are individuals and not a mixture of the whole. Just some thoughts.

Regards JD.

Mixing AC and DC is done with signals, not with power supplies.

Modern instrumentation transmitters send digital information (in AC form) on top of a DC current signal.

Foundation Fieldbus instruments send their data (again in AC form) on top of their DC power voltage.

I thought the original question that was closed was about putting dc into an ac powered instrument?

Of course, with switch mode power supplies they can handle dc no problem, so I'm not sure why the few answers were all about pop, snap and bang!!!

John.

I noticed that Mr. Guest asked this question and shortly thereafter the CR4 admin "closed the question".

Then, the question reappeared but not in the same section.

I couldn't understand at first why people reacted the way they did the first time the question was asked (boom, boom, etc.) since ac and dc signals are mixed often. However, mixing ac and dc power sources may not be such a great idea or done often.

So a usual, either Mr. Guest's question was not particularly well framed the first time, or those CR4 members who replied tacked on additional unstated assumptions then replied as if the OP had asked their revised question.

Anyway, on the second go around, the "bang, bang" type of response seems not to be as strongly opined.

I also noticed today where a poster complained that he did not agree that another poster should have been given an off-topic score for what was good reply to a post. The CR4 moderator then came back and changed the off-topic rating stating that "I had a bad day, it was early in the morning", or something to that affect.

So yes, ac and dc signals can be mixed. (i.e. dc bias to transistor input in analog electronic circuit).

bang pop crackle? You've been - thunderstruck......

get it?

(little friday humor)

AC & DC power is mixed in the field windings of synchronous AC power generators. DC is applied to create the field, but AC is back-induced into the rotor by the rotating field magnetic developed in the stator. Also, many secure AC power circuits also have a small DC signal, called a supervisory, impressed upon them. If the circuit is broken, (or interrupted & re-routed), a DC relay drops out and causes an alarm.

In ungrounded or poorly grounded AC distribution systems, there is usually a DC "float" above earth potential. If the float is high enough, it can cause personnel injury through contact with a (supposedly) zero-voltage neutral conductor. Ground potential rise during lightning strikes has also been known to cause false tripping of differential protection relays.

AC & AC power is routinely mixed in Power Line Carrier and Broadband over Power Line systems. As long as the frequencies are far enough apart, the lower frequency is simply a carrier wave, regardless of the voltage or current. However, the entire system must be designed to handle the RMS value of the combined currents, and the heating which may be introduced at the resonant frequencies of sympathetic harmonics.

DC & DC mixing is never a good idea unless the voltages are similar and polarity is maintained. One source will always overpower the other. A good example is a simple battery and charger. Both the battery & charger are DC sources. When the charger output voltage exceeds battery voltage plus line drop, the battery is backfed (charged).

• If the charger output voltage was, say, double the battery voltage, the battery would be damaged by the excessive charge rate. In a lead-acid battery, excessive gassing would produce a dangerously explosive atmosphere.
• If the battery voltage exceeded the reverse breakdown voltage of the diodes in the battery charger, damage to the charger would occur.

I may be wrong here but I believe on the side of neutral with the DC voltage added, the negative cycle of the sine wave will be boosted by the voltage level of the DC source. So if the AC voltage is 100V peak to peak, and the DC voltage is 50V, wouldn't the resultant AC voltage be asynchronous, like 50V to neutral on the positive cycle, and 100V to neutral on the negative cycle?