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# Do capacitors produce non DC-current?

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#### Resistanceisfutile

##### Member level 4
I saw this simple circuit online, its for an electronic fly swatter (note, I'm not trying to zap flies or humans, I'm just interested in the workings). I was a little confused because I've heard that transformers only work with alternating current, and the battery supply is DC. Then I remembered that on a voltage/time graph the gradient behaves like a sine wave. In a capacitor, the gradient behaves like a curve (exponential?) as it falls to zero.

1. Does this mean that a capacitor essentially generates non direct-current?
2. Can the output voltage of a capacitor be stepped up using a transformer?
3. Would the circuit work?

#### deepeshmishra

##### Member level 2
In this Circuit transformer is not supplied by DC.
After DC Batt. there is an RC network called RC Integrator which generates ac waveforms.

#### mrinalmani

Depending on the switching frequency, the circuit can work in two distinct mode.

MODE 1

If the switching frequency is less than the resonance frequency of the capacitor+transformer_inductor, The output will be a sine wave. Solve the differential equation for a series LC circuit where the the capacitor has an initial voltage. The output is a sine wave. But it is highly unlikely that the circuit will be working in this mode.

MODE 2

If the switch is closed only for a short time, and opened again for a large time, the following will happen...
1. When the switch closes, the transformer draws an input current from the capacitor.
2. When the switch opens, the current in the transformer is interrupted suddenly and this causes a large voltage spike. The 1K resistance on the secondary prevents current inrush when the air breaks down due to high voltage.
3. During the off-time, the capacitor also charges through R1.

Points: 2

#### chuckey

In the above explanation, mode 2 is the closest BUT this is what happens:- when the battery is connected the 22 MF capacitor charges up to the battery voltage via the 5K6. Whe the switch is closed the cap dumps its voltage across the low impedance primary winding of the transformer, which then causes an oscillatory voltage across it at a frequency tuned by the inductance of the primary and the 22 MF cap. This oscillatory voltage is stepped up by the turns ratio to a high voltage across the secondary. Because the oscillatory circuit has a low Q, it will only oscillate for a few cycles before all the energy that was in the capacitor is dissipated. The transformer then just gets a DC current of Vbatt/5K6 amps. when the switch is opened the capacitor charges up again in readiness for another shot.
Frank

mrinalmani

### mrinalmani

Points: 2

#### mrinalmani

The transformer then just gets a DC current of Vbatt/5K6 amps. when the switch is opened the capacitor charges up again in readiness for another shot.

And something else seems to be happening too!
Since the transformer had a small DC current that bled in through the 5K6 resistor, when the switch opens, this current will spike the transformer. So it appears to be working in both the modes, provided the switch is closed for long enough. What say Mr. Chuckey?

#### betwixt

##### Super Moderator
Staff member
You are technically correct Mrinalmani but the effect you describe will be insignificantly small because the current through the resistor is tiny. 5.6K limits it to no more than about 1.6mA with 9V supply, even less with a lower voltage. Breaking a current that small will not cause a significant 'spike' from the transformer.

Brian.

mrinalmani

Points: 2