Author Topic: Frequency calculation  (Read 3427 times)

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Offline J4e8a16n

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Frequency calculation
« on: May 25, 2015, 01:45:00 pm »
Hi,

How can we calculate (evaluate) the frequency of this kind of joule thief?

JPD
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Online PA0PBZ

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Re: Frequency calculation
« Reply #1 on: May 25, 2015, 01:54:05 pm »
The period seems to be 0.35 ms so the frequency is around 1/0.35 = 2.8KHz.
Is that what you mean?
Keyboard error: Press F1 to continue.
 

Offline J4e8a16n

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Re: Frequency calculation
« Reply #2 on: May 25, 2015, 02:45:34 pm »
:o)

How do you calculate the period?

not 2pi fl
not L/R

Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline c4757p

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Re: Frequency calculation
« Reply #3 on: May 25, 2015, 03:21:10 pm »
By viewing the plot and measuring it. The period/frequency in that circuit will be a bit hard to compute.
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Offline bktemp

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Re: Frequency calculation
« Reply #4 on: May 25, 2015, 03:21:57 pm »
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.
« Last Edit: May 25, 2015, 04:19:30 pm by bktemp »
 

Offline J4e8a16n

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Re: Frequency calculation
« Reply #5 on: May 25, 2015, 05:48:44 pm »
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 sec
5 timesconstants: 5 *10^-7 sec   ==> 2 000 000 Hz
Can we start from that?
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline bktemp

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Re: Frequency calculation
« Reply #6 on: May 25, 2015, 07:36:26 pm »
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 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_operation

Quote
The input voltage is one volt?
In your simulation, yes.

Quote
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 sec
5 timesconstants: 5 *10^-7 sec   ==> 2 000 000 Hz
Can we start from that?
It 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 -> 1.3V
The 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.
 

Offline J4e8a16n

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Re: Frequency calculation
« Reply #7 on: May 25, 2015, 08:06:21 pm »
Thank you very much for the infos and the link.
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline J4e8a16n

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Re: Frequency calculation
« Reply #8 on: June 01, 2015, 01:14:16 pm »
Hi  me again,

What about the reactance? Xl = 2pi*(0.00028)(1/(0.062)) = 0.0283756755808 Ohms.
Isn't  it used?

How do you calculate   
Quote
The current rise in the 280uH inductor at 1V to 1.5A requires 0.42ms.
  that  time?

Thanks again , there is really a lot to learn before being able to create a Joule Thief.

JP
« Last Edit: June 01, 2015, 01:39:07 pm by J4e8a16n »
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline alsetalokin4017

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Re: Frequency calculation
« Reply #9 on: June 01, 2015, 02:27:21 pm »
Hi  me again,

What about the reactance? Xl = 2pi*(0.00028)(1/(0.062)) = 0.0283756755808 Ohms.
Isn't  it used?
Sorry 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... )

Quote

How do you calculate   
Quote
The current rise in the 280uH inductor at 1V to 1.5A requires 0.42ms.
  that  time?

Thanks again , there is really a lot to learn before being able to create a Joule Thief.

JP

You 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.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline J4e8a16n

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Re: Frequency calculation
« Reply #10 on: June 01, 2015, 04:19:37 pm »
:o)

In fact I just copy - pasted from the free microsoft mathematics calculator I use.
I was questioning 
Posted by: bktemp
« on: May 26, 2015, 05:36:26 AM »

JP
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 


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