Capacitors and Resistors ?

[omdano]

I told the Electrician in the store about my needs and he asked me about Voltage for my capacitor and Wattage for my Resistor , what should i tell him ?
How can i decide the voltage and wattage ?

[MarkE]

Omdano, for ceramic capacitors you should choose a voltage rating that is at least twice the maximum voltage you expect that will appear across them in a circuit.  For Joule Thief circuits running from one AA cell, 16V will be good enough.  For a tantalum electrolytics the voltage rating should be at least 1.3 times the voltage.  On the LED side you should stay at 6.3V or above.  There is no harm in using a capacitor that has a higher voltage rating than you need.  It will just be more expensive and possibly bigger than you can get away with.  There are different kinds of ceramic capacitors.  You should use either NP0 or X7R for the timing capacitor, with NP0 preferred.  The alternative is a polyester film capacitor, but they are much more expensive.  Whatever you do, do not use a Y5V ceramic capacitor.

You can figure out the resistor wattage from Ohm's Law and a 2X safety factor.  P = 2 * duty_cycle^0.5 * V^2/R .  If you don't know the duty cycle then just assume that it is 1.  Nothing in a typical Joule Thief circuit is more than 10V, so any resistor 1.6K Ohm or bigger can safely be 1/8 Watt or bigger.  Anything over 800 Ohms can safely be 1/4W or bigger.  If you know the voltages more precisely then you can be less conservative.

Perhaps the most important thing in a Joule Thief circuit from a standpoint of making measurements will be what you do for your current sense resistor, or resistors.  A non-inductive current sense resistor with leads soldered right to the body for your oscilloscope connections and the normal circuit connections through the device leads is a good simple approach.  For examples see poynt99's you tube videos.

[omdano]

So i am safe using a 35v Capacitor (cermaic) and a 0.5 watts resistor on a circuit with low voltage input (200mV)

[MarkE]

Omdano, the input to the circuit may be 200mV but you also need to know what the boosted output is.  If it is driving an LED it will be a little bit more than 3V, with switching spikes that might be twice that.  35V is fine for the capacitor, and I would be very surprised if any of the resistors need to be a higher Wattage than 1/4W.  Post your schematic and I will tell you if anything needs to be changed.

[omdano]

Thanks buddy the Schematic is attached