Joule Thief 101

[MileHigh]

Brad:

Thank you PW.
I hope MH reads this,as he thinks this is some sort of death spike,and is not normal in a JT circuit operation. To what capacitance are you referring to?. Is the junction capacitance enough in the 2n3055 to allow(carry the current) for this spike to appear ?.

But what about some other possibilities?  It looks like a pretty significant pulse to me.  Who knows I could be wrong, but the point is to explore various options.  When I saw the spike it looked highly unusual to me.  I think there could indeed be another mechanism at play.

MileHigh

[MileHigh]

Indeed i have--many,and all are having the same issue.
Some have upgraded there firmware to the rigal firmware,but if it dose not take the first time,then it's bye bye scope-there is no returning to the original firmware. A few have lost there scopes trying this upgrade,so i am not taking the chance.

I am confused about the math trace business because the purple traces in your attached scope captures look like perfectly clean math traces except for the fact that it looks like they are upside-down, and I can't be sure of your "virtual ground reference" for the purple traces.

If the purple traces are indeed upside-down, then it would be trivial to set them right-side-up and dim out the other two traces and then you are good to go.  Am I missing something?

MileHigh

[tinman]

Brad:

But what about some other possibilities?  It looks like a pretty significant pulse to me.  Who knows I could be wrong, but the point is to explore various options.  When I saw the spike it looked highly unusual to me.  I think there could indeed be another mechanism at play.

MileHigh

OK MH
Where would we start?
Perhaps the answer can be found by working out how the current can flow from emitter to base during the off time of the transistor?,as we know how it travels from base to emitter during the on time.

I am not sure what capacitance PW is talking about that is in relation to this reverse current flow spike as yet. But at a guess,i can see only two that exist. 1-being the small junction capacitance between the base/emitter junction,and 2- the winding capacitance between L1 and L2.

Perhaps it works like one of the joule ringer circuits that lasersaber had there,where the coil that is connected only to the base(L2) could be open at the other end,and the other connection was made via winding capacitance,in stead of a physical connection between L1 and L2.


Brad

[picowatt]

To what capacitance are you referring to?. Is the junction capacitance enough in the 2n3055 to allow(carry the current) for this spike to appear ?.

When Q1 turns off and L2 collapses, the current flow thru L2 and the LED also induces a voltage across L2, which is the negative voltage portion of the base waveform.  During that off time, whenever the base drive voltage is below the Vbe forward voltage, the base leg is basically an open circuit because Q1's base is not conducting.  Under DC conditions, there would be a bit of reverse leakage current, but this would only be a few microamps at most.

As we have seen in your first video, as the variable base resistor's value is decreased, the negative going portion of the base waveform increases in amplitude slightly. The base resistor was only changed about 500 ohms or so.  We know that the reverse leakage current of Q1's base is fairly low.  Under DC conditions, we not expect to see any significant change in the reverse biased voltage observed at the base of Q1 by changing the resistance in series with the base by only 500 ohms (think of the reversed bias base as being a several megohm resistor forming a voltage divider with the L2 and base resistor).

The fact that the reverse voltage at the base does vary noticeably when the resistance is changed indicates that there remains a significant load on the waveform observed at the base of Q1 during the Q1 off time while the base is reverse biased.  This can only mean that there is a significant load at the base with respect to AC conditions (far greater than the megohms expected under DC conditions).

Keeping in mind that the negative going portion of the base waveform is a rather fast transition (edge) containing significant high frequency components, the most likely explanation for the amplitude of that portion of the base waveform changing as the base resistor is changed is due to there being a significant capacitance loading the circuit at the base of Q1.

Some of this capacitance may be probe related, which is why it is very important to use your scope probes set to their 10X position (assuming they are 1X/10X switchable).  More so important than increasing the probe's DC resistance to 10meg is the reduction of probe capacitance (AC reactance) while in the 10X mode.

That said, however, it is very likely that there is a significant amount of junction capacitance between the base and CE junctions of Q1 (a 2n3055 has a fairly large die area).  During the fast transition portion of the waveform, the base waveform is going negative while the collector waveform is going positive.  I would expect Miller capacitance alone (collector to base capacitance) to present a significant load to the otherwise open base during this time.

The base current trace became very spikey when you attached the scope ground to the base, as you were forcing the rest of the circuit to change potential with respect to the scope ground during that fast transition.  This greatly greatly increased the stray capacitance and current seen across the 10R CSR at the base during the fast negative going edge of the waveform.

When the CSR (base current) is properly viewed, I would expect to see a very narrow spike (as in your recent CH1-CH2 attempts) as any base capacitance is charged, followed by a period of little to no current flow, a smaller. slower (more rounded) spike at Q1 turn on, and then the actual base current of 2ma or so as Q1 remains on until turn off.

Indeed i have--many,and all are having the same issue.
Some have upgraded there firmware to the rigal firmware,but if it dose not take the first time,then it's bye bye scope-there is no returning to the original firmware. A few have lost there scopes trying this upgrade,so i am not taking the chance.

Have the scope owners that successfully changed their software seen the math issue fixed/resolved?

PW

[MileHigh]

Here is what I thought was happening with respect to the big negative current spike:

When the transistor switches off, the potential at the L2 feedback coil was going way below ground potential, perhaps to -15 or -20 volts.  The potential at the L1 coil also raises to start to push current through the LED.  My assumption is that under these conditions the N-P junction between the collector and the base was breaking down, and current was punching through the junction and the Joule Thief transformer was briefly shorting itself out.  After enough energy was burnt off, then the shorting would stop and the rest of the L1 discharge would go through the LED.

When Brad lowered the base resistor you could see the negative spike getting larger and deeper, indicative of a bigger "punch through" of the N-P junction between the collector and the base.

If this was indeed the case, then reducing the number of turns in the L2 feedback coil would reduce the negative potential on L2 and then the "punch through" would stop happening and all of the energy would go into the LED instead.

But now I am not so sure because the time base on the scope shot is 10 microseconds per division and the width of the negative spike is only a few microseconds.  So it could be just a junction capacitance effect from the N-P junction between the collector and the base also, I am not certain.  However, my gut feel is still going to go with a punch-through, I will go out on a limb.  PW is here so his comments will most likely clarify this issue.

MileHigh