Joule Thief

[jeanna]

Hi Jesus,
I am not sure I understand your question. It is just the same as measuring the resistance with a static resistor, except the legs are usually harder to see and reach.

There are usually 3 legs... I don't think I understand your question. please ask it again.

jeanna

[nievesoliveras]

Hi Jesus,
I am not sure I understand your question. It is just the same as measuring the resistance with a static resistor, except the legs are usually harder to see and reach.

There are usually 3 legs... I don't think I understand your question. please ask it again.

jeanna

@lady jeanna

If you read the @mk1 explanation on the posted graphic composition. He says that every time he changes the JT number of turns he also tune the base resitor, usually using a 1k pot trying tp get the highest voltage on the voltmeter. Keeping the voltage data and resistance at the base by checking the resistance used by the pot in the circuit.

Then my question is how do I check the resistance used by the pot in the circuit?

Jesus

[TheNOP]

... I quickly push x10 button on the meter.

then your scope know on what scale you are on, so the values are always displayed accordingly.
i just wanted to make sure.

your's is unlike most probes that have that switch on the probe itself
with most of those probes, the scope is not aware of the resistor change.

Why can't I just use lead or copper or aluminum foil for magnetic shielding?

because, nothing known can shield a magnetic field.
we can only redirect the lines of force.


the problem with cores is the frequency.
if the currents you try to push in them is AC, then hysteresis is also a problem.

less iron = need higher frequencies to compensate for poor coupling.
               higher frequecies mean more magnetic flux change per second this compensate for the poor coupling.

more iron = more eddy currents loss at higher frequencies, so it call for special cores construction.
                 steel insulated wire cores are the best, the smaller the wire the better it is.
                 the eddy current loss is in function of the square of the frequency and inverse square of the material thickness.
                 the higher the frequency and/or the thicker the core material(non insulated), the more eddy currents loss you get.


all max frequency for materials, you might find on the net, are for AC and are based on hysteresis limitations, not on eddy currents loss.

the JT play with plused DC, not AC, so hysteresis is a lesser problem.

METGLAS has a really low hysteresis curve, meanning it can work at very high frequencies, but its saturation point is really low( .2 tesla, iron is 2 tesla).

industries do not use steel wires for cores due to contruction cost.
they are making tradeoff between $$$ and efficientcy.

MuMETAL® WIRE is interesting indeed.



@all
why be affraid of eddy currents when your jt is not running at very high frequency ?

garden wire cores are cheap when you don't count the time it take to make.  

[Mk1]

@jesus

A pot has 3 leg , you will need to use leg 1 and 2 or 2 and 3 , you can test any pot for overall value between leg 1 and 3 , now if you use 1 and 2 take the resistance reading between 1and 2 , if you use 2 and 3 then check resistance between those two.

You find first the spot where the voltage for the pickup coil is at his highest .


Mark

[TheNOP]

How do i check the resistance used by the pot on the circuit?

the best way is to take it out of the circuit then measure it.

an other way is to go with ohms law, but the spikes might get in the way.
R = V / I