Joule Thief

[freepow]

Thanks for your info. But the volts measured on my DMM are at least 800v+, also I can power very bright a 240v mains powered  (LEDx20-bulb). So it has to be more than 150v

[Pirate88179]

Thanks for your info. But the volts measured on my DMM are at least 800v+, also I can power very bright a 240v mains powered  (LEDx20-bulb). So it has to be more than 150v

Remember that a DMM is not a reliable measuring tool when working with JT's, especially because of the frequency ranges they operate in.  I have also heard that most scopes can not read the output accurately either.

How are we supposed to measure the output then?  I am really not sure.

Bill

[conradelektro]

@freepow

Hello Freepow !

Oscilloscopes usually measure Voltage in the range -35 Volt to +35 Volt. With a x10 probe the range extends to -350 Volt to +350 Volt.

To measure 800 Volt one needs a x100 probe (which would allow the range -3500 Volt to +3500 Volt).

You can overcome this problem with a voltage divider (resistive divider), see:

http://en.wikipedia.org/wiki/Voltage_divider

and the attached drawing.

Greetings, Conrad

[jeanna]

@ Jeanna, I used my scope on my JT-power supply and I have posted a complete drawing of the scope shot, its exactly the same as on the screen, can you help me with what the frequency would be and volts

the very top of the green lines went a bit over the top of the grid when the JT is at its best.

Oh dear,
I have such a different kind of scope.
Mine tells me right out what the frequency is in Hz as well as mS, but, I just read it off.
It also looks like a very low number of volts to me.
Sometimes if I have the resolution too "far away" it doesn't see what I actually have well at all.



When I use my dmm I could figure multiplying the voltage output by 20 so it really makes me thing this is not showing the story too well.

If each div is 5v then you have 12v then multiply by 10 and this is 120v.
That is not enough spiky volts to do what you are doing.

I am puzzled, and I will think about this.

Thanks for making the drawing.
I hope someone who uses a cro can help more.

I wonder if you can get more of the spikes to show, if the machine will give you a more accurate reading.?
jeanna

ps
a question

When you say the dmm measures 800v, are you rectifying the secondary output and collecting it in a cap and reading THAT on the dmm?
In my experience (and thanks to MK1) this is pretty much the same as the scope on volts.

[electricme]

@freepow,

Full marks for trying to meassure the hi voltage of the joule thief with your scope, however you need to make sure you can lower the green display line so it appears right on top of the horizonral center line before making measurements.

BUT, making Hi Volt measurements with your ordinary scope probe (or a DMM) should be a no no, as the insulation used while very good is not rated for anything too high, plus the fact if you pump to high a voltage into the front end of any measuring device, it will fail eventually.
(Hands up all those here who have written off their DMMs, yours truly included).

Silicon Chipp magazine (April 2010) had a very good article on how to build a Very High Voltage probe to be used on a DMM, please go here to view it, it has the Hi Volt Prove covered fairly well and costs $40 Australian.

http://webcache.googleusercontent.com/search?q=cache:TJx7Gc42TO0J:www.siliconchip.com.au/cms/A_111937/article.html+Silicon+Chip+Hi+volt+probe&cd=1&hl=en&ct=clnk&gl=au

I suggest you take a look at Jaycar for the kit, it  is not too expensive (it's on my list to buy) and it will give you piece of mind that you wont degrade the DMM, perhaps it could be used on your scope.
JPG is (C) to Silicon Chip a great Australian electronics magazine 

jim