# Supplying ac to a dc circuit

So a while ago, i was stumbled upon a post what will happen to the dc circuit when we supply ac to it. Lets assume the ac has the same voltage as the dc. We know that ac switches direcions so there will come a tome it is positive, zero and negative.

If the ac is in its positive voltage, the dc circuit will work fine. By the time it approaches zero voltage, no current will flow and so thr circuit is not functioning. Now what bothers me is the negative voltage applied. What really happens on the inside? Technically speaking. Lets say the dc circuit can be jus any device like a piano, laptop, etc

Thanks

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Impossible to say without knowing what the piano's circuit is. –  stevenvh Jul 8 '12 at 13:15
Hmm lets say a very simple dc circuit consisting of 3v battery and 1 resistor. What hpens when we pump ac to it? –  IvanMatala Jul 8 '12 at 14:14

Circuits designed for a particular DC voltage are very unlikely to work correctly with AC, even at the same voltage. There is a very good chance the circuit will be damaged by the negative half-cycles of the AC power.

There are some exceptions to this, but they aren't really "circuits". A incandescent lightbulb works fine with AC or DC, and so does a resistive heater. Beyond that, you're heading for trouble.

Some devices that require DC power may have protection built in so that they at least don't fry with AC provided or the DC power is hooked up backwards. This is usually in the form of a "idiot diode" in series with the positive DC power lead. It simply blocks anything negative so that it can't cause harm. Another approach is a fuse followed by a clamp. If the input voltage is backwards or too high, it shorts the power leads together and blows the fuse.

Cheap consumer devices that come with their own power supply are less likely to have protection. They also then come with a warning in the instructions to use only the provided power supply. If you hook up something on you're own, they are off the hook. That's a perfectly legitimate thing for a manufacturer to do when the final sell price needs to be as low as possible.

I meant to say this earlier, but had to quit and do something else. AC of the same voltage as DC will not only have large negative voltages, but also positive voltages greater than the DC level for part of the cycle. If the AC is a sine wave, the peaks will be at ± the square root of 2 times the RMS voltage. For example, "12 VAC" sine power will go as high as +17 V and as low as -17 V at the positive and negative peaks of the cycle.

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Then can u elaborate more on hw the negative cycles do damage? Tnx –  IvanMatala Jul 8 '12 at 13:54
@black: See the absolute maximum ratings of any IC as a example. They may say that you can apply from -300 mV to +5.5 V to the IC power pins without causing damage. -5 V will cause all kinds of junctions in the part to be reverse biased that weren't intended that way. High currents could flow, causing excessive heat, which destroys the part. Or, the voltage stress alone could damage reverse biased junctions and insulating areas. You don't know what can happen, but large negative voltages are very likely to cause permanent damage. –  Olin Lathrop Jul 8 '12 at 16:01
Ok tnx for ur answer. They are helpful. How about if the ac peak has the same value as rhe dc? This time ac will not exceed the dc in the positive range. –  IvanMatala Jul 8 '12 at 21:19
Your example of the battery in series with a resistor is atypical, in as much that the negative voltage might do less harm than the positive! If you would apply a positive voltage larger than 3 V there would flow a current from that voltage to the battery, and that's not a good idea if it's not a rechargeable battery. OTOH when the voltage is negative it's in series with the battery: the input's + connects to the battery's -, so the voltages add and there will be a current from the battery's + through the resistor to the input's -.