Current and capacitors, calculator

 This calculator helps to compute current limiting capacitor for low frequency (below 60Hz) AC sources of voltage.
Take a look at this picture. If we need to limit current through the load R, we use the current limiting capacitor C.
In this case C will act as reactive current resistor.



If we know required current I and Voltages Vin and Vout, then  C = (4,45 * I) / (Vin — Vout)

Very common usage is a charging schematics for different batteries or cheap drivers for LED lamps.


In the schematics above input capacitor C act as current limiting device.
R1 is an extra current limiter to limit current surges during turning on and off
and as well as extra safety device. Imagine if C input would fail short. In this case
any load on the output side will see direct connection with all the power from Input
(for example 220V). This could lead to little fire work....Although we shall note few things:
1. This is inherently very unsafe schematics and load will be connected to high voltage.
When you handle this schematics or output load, it shall be disconnected from AC and it has to be in isolated enclosure.
2. All the capacitors connected to AC, will be charged and if this 220V AC source, it will be at high voltage charge even after you disconnect.
Thus shunting resistors has to be connected across the capacitors.
3. Capacitors have to be rated for 300V minimum.
4. Input capacitor shall be non-polarized.
This calculator quickly calculates approximate value capacitor C to limit currents for all sorts of different loads.
Capacitor calculator I (A)
Vin (V)
Vout (V)

Capacitance 0: uF


I is required current (for example LED needs 20mA or 0.02A).

Vin is voltage from input source. Very often it is used for 220AC

Vout is voltage on the other side of capacitor or another words on the output. Very often it can be LED in series. For example 4 LED in series is giving Vout =4x3.3V=13.2V

 Please note 1A=1000mA.This calculator uses Amps,

This schematics can be used for charging batteries too. This is not the constant voltage device. Unless you add a Zener diode to limit output voltage,  you will need to calculate TIME, when it has to be disconnected.  You have to disconnect this "poor man" charger BEFORE it reaches full charge. Also REMEMBER: ALL parts of this charger are connected to "Hot" wire in AC. Never touch any parts or even battery if it is connected to AC outlet.

Also remember never use it in wet environment, Moisture can create conductive paths and electrocute people or burn the schematics.


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