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Author Topic: Joule Thief behavior question.  (Read 20311 times)

Offline crowclaw

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Re: Joule Thief behavior question.
« Reply #30 on: November 12, 2013, 09:29:36 PM »


Also, can someone tell me if there is a way to measure the amp output of the circuit?  My meter just reads 0.0.  I'm assuming its because it cant keep up with the oscillation speed of the circuit.
Hi, ddredar and welcome.

Xee2 has answered your question well, if you wish to experiment further with these or similar circuits, I would strongly suggest you invest in an oscilloscope. You should be able to pick up a decent one reasonably cheaply which will give you the opportunity to analyse the waveforms and measure the signal amplitudes etc. Digital meters don't perform well when inputing these type of signals.
Regards Crow


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Re: Joule Thief behavior question.
« Reply #30 on: November 12, 2013, 09:29:36 PM »

Offline TinselKoala

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Re: Joule Thief behavior question.
« Reply #31 on: November 12, 2013, 09:55:49 PM »
That's right.

It appears that there is still some confusion about the LED ratings. If your package says "3.3 v, 28 mA" that means that the LED will produce its rated light output at a DC current draw of 28 mA. The LED is never a "direct short", in fact it is a non-linear load to the power supply. When it is supplied with a voltage below its "forward voltage" then it looks like an open circuit and no current flows. At the "fwd voltage" a current begins to flow, and at voltages slightly above the fwd voltage the LED begins to look like a short but with that fwd voltage drop included. To calculate the current through the LED you do an Ohm's Law calculation. I = V/R, for example. Around a circuit, the whole supply voltage is "dropped" (Kirchoff's circuit rules). But the LED drops the voltage some already, so your series resistance drops the rest.
(Supply voltage - LED fwd voltage)/series resistor = current in the circuit.
So if you have a LED with a 3.3 volt fwd voltage, and a 12 volt supply, and you want 28 mA in the LED (to get the rated brightness) you do R = V/I :
(12 -3.3)/0.028 = about 310 Ohms. 330 Ohms is a standard value for a resistor, so use that and accept a slightly lower current in the circuit.
The above is for the DC case.

Pulsed LEDs are the same... but different. The LED can withstand a lot more voltage if the pulse is kept very short. For example I have made some LED strobes to freeze motion of my MHOP pulse motor, and they are getting a full 12 volts pulse with no series resistance, but this is very short, so the LED doesn't fail. It also doesn't get very bright!

So the JT works like this: it makes high voltage pulses when not loaded. The capacitor-diode arrangement will charge the cap to the maximum output voltage of the JT, given enough time. When you hook up the LED, though, the voltage will only go as high as the fwd voltage of the LED, because then the LED looks like a "short" (clipping the voltage rise of the JT), and the light comes on. It might be very bright, but only for a short time, then the JT shuts off until it makes another pulse. The eye "integrates" the bright flashes into what looks like a constant light at some brightness less than the maximum. The eye is a very non-linear "brightness" measurement and what you perceive is a complex function of the true pulse brightness, the duration of the pulse and the time between pulses.

To measure the current during these very short pulses, the proper way is to use an oscilloscope to monitor the voltage drop across an inline resistor in series with the LED. A one-ohm resistor will show a voltage drop across it that can be directly converted to the current by Ohm's law. I = V/R, where the V is the voltage drop read on the scope, and the R is 1 ohm... so the volts indicated = amps. A 0.1 ohm resistor will disturb the circuit less, but then you need to be able to divide by 0.1 instead of 1 !
But this current only flows for a short time out of the total process. Say it flows for 1/3 of the whole cycle as shown on the scope. The _average_ current for the whole cycle  then is 1/3 of the value you have calculated based on the voltage drop across the "current sense resistor" CSR, sometimes called CVR (current viewing resistor, the term I prefer).

Nowadays, even cheap DMMs can be surprisingly accurate at "average" current and voltage measurements even in pulsed circuits. There are videos and documents available that show this to be true. Poynt99 has published a very good demonstration/test of this, but I can't find the link at the moment.

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Offline ddredar

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Re: Joule Thief behavior question.
« Reply #32 on: November 13, 2013, 07:38:43 AM »
Thank you all for the information.  Even though I don't completely understand this circuit, I do have a much better idea of what is going on.  Would any of you have a recommendation for an oscilloscope?  I will probably order it online due to the horribly over priced local electronics store.  Also are there any documents or threads you would recommend for reading?

Offline Legalizeshemp420

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Re: Joule Thief behavior question.
« Reply #33 on: November 13, 2013, 08:46:07 AM »
Thank you all for the information.  Even though I don't completely understand this circuit, I do have a much better idea of what is going on.  Would any of you have a recommendation for an oscilloscope?  I will probably order it online due to the horribly over priced local electronics store.  Also are there any documents or threads you would recommend for reading?
Analog or digital?  I personally would recommend a used analog 60mhz, or above, for 50-100 dollars.  Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.


Offline ddredar

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Re: Joule Thief behavior question.
« Reply #34 on: November 13, 2013, 06:05:13 PM »
Analog or digital?  I personally would recommend a used analog 60mhz, or above, for 50-100 dollars.  Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.


Oh my, please tell me where you are finding those prices.  When I look online, most of them are in the $600 to $5000 range.  I found a few in the $250 area, but not too many and they are not good for 60mhz or higher.

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Re: Joule Thief behavior question.
« Reply #34 on: November 13, 2013, 06:05:13 PM »
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Offline Legalizeshemp420

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Re: Joule Thief behavior question.
« Reply #35 on: November 13, 2013, 06:23:36 PM »

Oh my, please tell me where you are finding those prices.  When I look online, most of them are in the $600 to $5000 range.  I found a few in the $250 area, but not too many and they are not good for 60mhz or higher.
Ebay and I grabbed my 20mhz analog for 34 dollars plus 25 shipping (kinda bulky with a little weight to it) and it had all the original items + manual + original bags and box.  20mhz is fine but to really home in on some of the JTs I was working on I would need 60-100mhz and those can be had for 50-150.  Mind you it took me 3 weeks of a lot of bids and I finally grabbed mine via a snipe bid 3 seconds before the end.

Offline ddredar

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Re: Joule Thief behavior question.
« Reply #36 on: November 14, 2013, 04:32:08 PM »
Ebay and I grabbed my 20mhz analog for 34 dollars plus 25 shipping (kinda bulky with a little weight to it) and it had all the original items + manual + original bags and box.  20mhz is fine but to really home in on some of the JTs I was working on I would need 60-100mhz and those can be had for 50-150.  Mind you it took me 3 weeks of a lot of bids and I finally grabbed mine via a snipe bid 3 seconds before the end.

Looks like I will have to dust off the old ebay account login ans start searching. 

Think I'll go play some Planetside2 before I dive into that search. :P

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Re: Joule Thief behavior question.
« Reply #36 on: November 14, 2013, 04:32:08 PM »
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Offline ddredar

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Re: Joule Thief behavior question.
« Reply #37 on: November 14, 2013, 11:38:07 PM »
Analog or digital?  I personally would recommend a used analog 60mhz, or above, for 50-100 dollars.  Get a 100mhz analog scope for 75-100 and that is all you would need for most JT work.

So, why do you recommend analog over digital?  I'm old enough to know that in many cases analog is better than digital.  I just wanted to know your reasoning for this recommendation.

And if anyone has a different opinion, I'd like to know the reasoning behind that too.


Offline Legalizeshemp420

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Re: Joule Thief behavior question.
« Reply #38 on: November 15, 2013, 12:24:48 AM »
So, why do you recommend analog over digital?  I'm old enough to know that in many cases analog is better than digital.  I just wanted to know your reasoning for this recommendation.

And if anyone has a different opinion, I'd like to know the reasoning behind that too.
For quirky circuits like this I just think it handles it better for the price range we mere mortals can afford.  Have US $2k+?  If so then go digital but be sure to get a persistence vision model.

I'm an Analog type guy to begin with as I prefer analog for certain things since we aren't on quantum computers just yet.  0/1 and that is it but for an analog it isn't that simple since it can be off, on, and all sorts of states in between.  Some will argue in favor of digital and the one thing I will give digital is all of the extras thrown in it.  You get a frequency counter, a FFT, and math functions out the wazoo that the poor PURE analog loses against.  We also have the digital sampling that can get in the way and give you headaches on something like a RIGOL 1102E.

For me, personally, I have my 20mhz analog scope and love it but it has its limits due to being 20mhz but I want one more scope before I die and that should do it.  About 300mhz 5G/s persistence vision digital with quad channel.  Know the price of such a beast?  Last time I looked around 10K-15k-20K US dollars.  I think I will take my 36 dollar Analog for now. :) 

Offline TinselKoala

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Re: Joule Thief behavior question.
« Reply #39 on: November 15, 2013, 08:21:56 AM »
I prefer analog kit myself, but if I was going to buy a "first scope" today on a limited budget, and I had the computer already, I would probably get this one:
Hantek 6022BE
http://www.theoscilloscopeshop.com/item-hantek-6022be-pc-based-usb-digital-storag-oscilloscope-2channels-20mhz-48msa-s_221270085136_US_Hantek.html

Comes with probes and software for under 100 USD.

They also have a 40MHz version for a little more money.

Somebody gave me an old Link DSO that is very similar, and I had the perfect laptop for it, a ThinkPad 600e with parallel port. It is very useful around my lab, because I mostly work with low frequency stuff-- and I had no real use for the laptop any more, so it's a double bargain. I have three analog scopes too; one very popular and inexpensive analog scope is the Tektronix 2213a, a 60MHz scope with some nice delayed timebase features.
Watch out for Atten scopes... I don't trust them.

Almost any scope at all is better than no scope at all, though.

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Re: Joule Thief behavior question.
« Reply #39 on: November 15, 2013, 08:21:56 AM »
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Offline MileHigh

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Re: Joule Thief behavior question.
« Reply #40 on: November 15, 2013, 08:48:40 AM »
Oops.  Sorry, wrong thread.

 

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