No-this is a different JT i was testing at the time of your request,so rather than swap back to the other JT,i simply did the same thing on this JT to give you examples of what i can and cannot do with the scope as quickly as i could.

No they cannot-the math trace switches off as soon as you switch of one channel.

Brad

Tinman,

Please do not consider any of my posts as "requests".  I am only trying to offer measurement suggestions.

If someone wants to argue about whether changing Ib also changes Vce or Ic, it would only seem logical to figure out how to accurately measure those values with minimal disturbance to the operating conditions. 

I have no dog in the hunt one way or the other. 

Did everyone see where LIGO recently announced the detection of gravity waves using a measurement accuracy of 1 part in 10²¹?  I believe I read where that is equivalent to being able to detect the width of the Milky Way changing by the width of a pencil eraser.

Talk about measurement precision, amazing...

PW

That is what I said when I stated:

"



Which circuit is this that you are measuring?



I think you are getting close...

PW

ADDED:  What does the math trace look like with the channels set at 2V/div?

If you can display the math trace, forget about inverting one of the channels and just display a math trace of CH1-CH2..."

That is(as i stated) what the last scope shot was showing,and the configurations used.

It would be a bit easier if you would stick to one set of measurements/operating conditions/circuits until we can figure out what the problem is.

That was from the original circuit--sorry i forgot to mention that.
I will stick with this circuit from now on-the one the rest of the testing has been done on so far.

I like how you dimmed the CH1&2 traces.  Try increasing the VPD for both channels.  You'll want as much signal displayed for each channel as possible without clipping the scope's front end.  As I said earlier, increase the 10R to 100R if necessary.

As stated in my last post with the scope shot of the noisy math trace-that is across a 100 ohm resistor.. If i raise the VPD to 5v,the wave forms are clipped at the top of the screen-they go out of screen capture.

What i meant by obtaining the voltage drop without using the math trace,is to calculate the values manually,as the math function on this scope is just junk with very limited function.
No one has been able to work it out yet--even Poynt spent some time with me on skype via video,and still no luck.


Brad

You could try measuring across the 1K resistor instead of the added CSR for more difference signal.



I do now recall you, Poynt, and TK as well, discussing this issue in the past.
 
PW

In your reply 1181, the capture posted there looks like you could easily increase the VPD to 2V/div.

Yes-i guess i could fit the whole waveform in the screen if i drop each channel down a few divisions.

In that capture, are you using a 100R resistor at the base?

Yes

Have you returned to the original circuit as per post 1168?

Yes

Keep in mind that other than the leading edge of the negative going signal at the base during turn off, where I would expect to see a significant spike as capacitance is charged, you are only looking at around a 20mv signal for 2ma of base current with a 10R resistor.

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 ?.

Your scope was a pretty popular model, have you searched out forums looking for math trace related discussions?

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.

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

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

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.



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.



PW

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.

Thank you PW for clearing this up.

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

To those i have spoken to,yes,there math is fully functional now since the firmware update.
But it has to be done right the very first time. If the power happens to go out when doing the upgrade,then you throw your scope in the bin.
One guy said he now gets channel drift on his scope since the upgrade,and has been unable to fix it.
I cannot afford a new scope ATM,so i am not going to chance it,as it seems to be only about a 90% success rate--and im bound to be one of the other 10%


Brad

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

MH,

Keep in mind that the very "spikey" scope shot was captured while scope ground was connected to Q1's base.  Also keep in mind that the observed waveform during that test is actually indicative of current, not voltage.

In previous scope shots, the negative going base waveform was not nearly as "ugly".  If we were dealing with the base reverse breakdown voltage, which to be honest I did not consider, I would think the waveform would have a consistent voltage level at which it is clamped fairly hard.  Also, no current would flow until the waveform reached whatever that breakdown voltage actually was.  The observed spike appears to happen immediately during the transition.  Still, I am willing to consider the possibility.

It is a shame the differential measurements cannot be made a bit cleaner.

PW   

MH,

Keep in mind that the very "spikey" scope shot was captured while scope ground was connected to Q1's base.  Also keep in mind that the observed waveform during that test is actually indicative of current, not voltage.

In previous scope shots, the negative going base waveform was not nearly as "ugly".  If we were dealing with the base reverse breakdown voltage, which to be honest I did not consider, I would think the waveform would have a consistent voltage level at which it is clamped fairly hard.  Also, no current would flow until the waveform reached whatever that breakdown voltage actually was.  The observed spike appears to happen immediately during the transition.  Still, I am willing to consider the possibility.

It is a shame the differential measurements cannot be made a bit cleaner.

PW

I could make the differential measurements cleaner by switching back to the 100 ohm resistor,but it would be without the math trace--we need to forget about the math function on my scope.

Brad