Frequency calculation - Page 1
Forum section: electronics / beginners

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 25, 2015, 01:45:00 pm
content: Hi,How can we calculate (evaluate) the frequency of this kind of joule thief?JPD

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 25, 2015, 01:54:05 pm
content: The period seems to be 0.35 ms so the frequency is around 1/0.35 = 2.8KHz.Is that what you mean?

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 25, 2015, 02:45:34 pm
content: )How do you calculate the period?not 2pi flnot L/R

author: c4757p
author_rank: Super Contributor
timestamp: May 25, 2015, 03:21:10 pm
content: By viewing the plot and measuring it. The period/frequency in that circuit will be a bit hard to compute.

author: bktemp
author_rank: Super Contributor
timestamp: May 25, 2015, 03:21:57 pm
content: It is not easy to calculate the frequency, because almost everything in the circuit affects the frequency.The current in the inductor rises until the transistor can not longer supply enough current with the given base current.So a rough estimation of the transistor on period would be inductance * base current * transistor current gain / input voltage, but the input voltage also affects the base current.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 25, 2015, 05:48:44 pm
content: As the current rises in the Base Windings opening the collector and permitting more current in the Collector Windings.Thus, the flux rises in the Collector Windings and the Base Windings rising the current in the base of the Transistor and opening the collector a little more and allowing more current in the Collector Windings up to saturation of the transistor.Then the flux collapses...Is that the oscillation?The input voltage is one volt?My question was about there is no capacitor to calculate a thank oscillation.Let's say the resistor is 200 Ohms, the inductance 20uH: a time constant 1*10^-7 sec5 timesconstants: 5 *10^-7 sec ==> 2 000 000 HzCan we start from that?

author: bktemp
author_rank: Super Contributor
timestamp: May 25, 2015, 07:36:26 pm
content: Quote from: J4e8a16n on May 25, 2015, 05:48:44 pmThe base winding generates a voltage, not a current. This voltage adds or subtracts from the battery voltage and switches the transistor on and off.Wikipedia has a good description on how the oscillation works:http://en.wikipedia.org/wiki/Joule_thief#Description_of_operationQuoteIn your simulation, yes.QuoteIt is more complicated:First start with the base current:The turns ratio is about 3:1, so the base winding generates about 0.3V. This adds to the input voltage -> 1.3VThe base resistor is 280 ohms, Vbe is around 0.8V, that gives around 1.7mA base current. The ZTX690B has a hfe of about 900. This gives a peak current of 1.5A. Your simulation shows about 0.75. It is at least not completely off.The current rise in the 280uH inductor at 1V to 1.5A requires 0.42ms. Your simulation shows about 0.26ms.The off time depends on the load voltage. A white led has a Vf of about 3.1V. 3.1V - 1V supply voltage = 2.1V at the inductor. The current falls from 1.5A to 0A in about 0.2ms. This gives a total period of 0.62ms. Compared with the 0.35ms from you simulation thats pretty good. As I said, it is only a rough estimation using ideal components.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 25, 2015, 08:06:21 pm
content: Thank you very much for the infos and the link.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 01, 2015, 01:14:16 pm
content: Hi me again,What about the reactance? Xl = 2pi*(0.00028)(1/(0.062)) = 0.0283756755808 Ohms.Isn't it used?How do you calculateQuotethat time?Thanks again , there is really a lot to learn before being able to create a Joule Thief.JP

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 01, 2015, 02:27:21 pm
content: Quote from: J4e8a16n on June 01, 2015, 01:14:16 pmSorry to be so pedantic... but Significant Digits! False Precision! You cannot legitimately have more significant digits in your answer than you have in the _least precise_ quantity that goes into the calculation; in this case two sig digs are all you can really count on. In other words, 0.028 ohms. After all, your answer is saying that the value is _not_ 0.0283756755809 ohms, and _not_ 0.0283756755807 ohms..... and I've never seen a meter that could report a resistance value to the tenth of a trillionth of an ohm....(Don't mind me, it's just a perpetual bug-bear for me. I'm a relic of the slide-rule era and false precision is like a burr under my saddleblanket... )QuoteYou can approach the problem empirically by simply building a few JTs and varying component values to see their effect on the frequency measurements. The various JT circuits are basically "boost converters" and you may be able to analyze them from that perspective. But myself, I just build, test, vary components, re-test, etc.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: June 01, 2015, 04:19:37 pm
content: )In fact I just copy - pasted from the free microsoft mathematics calculator I use.I was questioningPosted by: bktemp« on: May 26, 2015, 05:36:26 AM »JP