Joule Thief 101

[tinman]

Brad:

Yes, I was part of the mass confusion here and I looked back in the thread.  In post #1191 I speculated that there was a collector-base breakdown.  Then in post #1199 PW confirms an emitter-base breakdown and congratulates me on getting it right.  I mistakenly think he is confirming my theory.  Then in post #1219 you congratulate me for getting it right.  I even make a posting about the collector-base junction capacitance potentially starting an avalanche that leads to the collector-base junction breaking down and nobody says anything.

I thought the collector-base junction would break down before the emitter-base junction because the collector would be at higher potential than the emitter.  I have long forgotten about the nuances about the inner guts of a transistor nor did I look at a datasheet.  Needless to say, reverse current going through the base could theoretically come from the collector or the emitter.

So if you and PW have been talking about an emitter-base junction breakdown, then I apologize because I thought that the focus was on a collector-base junction breakdown.  Only much more recently did I start thinking about a possible emitter-base junction breakdown.

MileHigh

Ah-ok,i see what happened now.

I just got all setup to look further into this emitter/base breakdown,and then seen you talking about collector/base breakdown. So went out to the workshop,and started looking for this collector/base breakdown,and posted my finding's. But now i know we are still talking about the emitter/base breakdown,i can continue on with that

Brad

[MileHigh]

Gosh Miles!

That sort of a response is troubling indeed!

Let us hope that you're not going into a "breakdown"
mode and that perhaps a little rest might restore your
vitality and sense of reason?

The Tin Man makes perfect sense to me.

I actually don't believe your last statement.  You just want to step in for your typical little cameo and with a barely concealed smile on your face say something out of kilter that is in tune with the whole "Dr. Strangelove" atmosphere in the thread.

One can imagine a reedited Dr. Strangelove where a few times in the film they cut to some freezing sailor standing on watch on a navy ship way above the arctic circle.  The freezing sailor doles out some quixotic "pearls of wisdom" that add to the whole unreality of the scene.  Blame that behaviour on the effects of the chemtrails.

Thank you for your cameo.

[Pirate88179]

I actually don't believe your last statement.  You just want to step in for your typical little cameo and with a barely concealed smile on your face say something out of kilter that is in tune with the whole "Dr. Strangelove" atmosphere in the thread.

One can imagine a reedited Dr. Strangelove where a few times in the film they cut to some freezing sailor standing on watch on a navy ship way above the arctic circle.  The freezing sailor doles out some quixotic "pearls of wisdom" that add to the whole unreality of the scene.  Blame that behaviour on the effects of the chemtrails.

Thank you for your cameo.

Holy crap!  I just had a visual of Slim Pickens riding a Joule Thief circuit dropped from a B-52.


I should get more sleep.


Bill

[picowatt]

The scopes of base/collector junction show 20 something volts so one side of the scope probe (ground or probe) had to receive 0 volts (or near 0) while the other side had to receive the higher volts to produce that scope shot. So where did that 0 come from while the transistor is on (closed).

Measurement of a voltage potential is always made relative to some point of reference.

In this circuit, when the transistor is turned on, electrons flow from the negative terminal of the battery (BATT-) to the transistor's emitter, from the emitter thru the base/ L2 and collector/L1 paths and then back to the positive terminal of the battery (BATT+).

Where the scope reference (probe "ground" clip) is connected to the circuit determines the polarities (and values) of measured voltages.

In this circuit, when the transistor is turned on, the base terminal is approximately 0.7 volts less negative than the voltage at the emitter (which is the same as saying the base is 0.7V positive with respect to the emitter). 

If the scope reference is the BATT+, when the transistor is turned on, the voltage at the base will measure approximately -0.8V and the emitter will measure -1.5V (assuming battery voltage is 1.5V).  The voltage measured at the battery side of L1 and L2 will measure zero volts (excluding any Vdrop in wires).

If the scope reference is the BATT-, when the transistor is turned on, the voltage at the base will measure approximately +0.7V and the voltage at the emitter will measure 0.0V (again, excluding Vdrop in wires). 

Both measurements are correct, it is all relative.  This is why the measurement reference point must be specified for us to make sense of the measurements, particularly with regard to scope captures.  Note how TK provided his reference as being the emitter in the scope capture (image) he recently posted.  As well, Tinman typically provides a schematic with measurement and reference points indicated.

Use of the word "ground" is rather ambiguous, as there are actually some instances where that literally means connected to ground, such as a ground rod driven into the dirt.  However, it is often just used to mean the measurement reference point, a circuit common, etc, and not necessarily meaning actually connected to an Earth ground.

PW 

[tinman]

 author=MileHigh link=topic=8341.msg478907#msg478907 date=1459261299]
Brad:

In post #1296 I say this:

Why didn't you put the CVR for the collector trace on top of L1 so that you would not disturb the equipotential between the LED anode and the collector because preserving that equipotential might be important?

This is your reply in #1297:




<<< It was the best of times, it was the worst of times.  Learn something new.  >>>

Oooh, I could take that as an insult.  Wattsup would have an issue with your tone, better watch out.



MileHigh

For the very simple reason that we are looking for your current flow through the collector junction,into the base junction,due to junction breakdown.
You ask the dumbest of questions some time's.

Yes,the last bit-and the likes of,will not be included in my replies from now on.

Take a look at the attached schematic and weep.  Is this not clear enough for you, "Why didn't you put the CVR for the collector trace on top of L1?"

As my answer above,and a little more for clarity.
If we put the CVR on top of L1,we would be seeing the current flowing through the LED,battery,and then back to the CVR on top of L1. As we wanted to see only the current flowing into the collector,through the collector/base junction,and then out through the base of the transistor during the OFF period,i placed the CVR on the collector to view only this current we were looking for.

If you look at the scope shot below,you will see why i placed the CVR at the collector,and what i was looking for.
The scope shot below is with the CVR on the emitter,and there you can see the spike i was looking for,in regards to this breakdown between the emitter and base junction.
Both channels are inverted,so as it is easier to match it with the reverse current flow out of the base,that has a positive potential.

I hope that clears that up.


Brad