Is joule thief circuit gets overunity?

[poynt99]

A rather obvious test would be to swap the probes between channels. If the noise follows the probe to the other channel, then you know the probe is bad. If it doesn't, then you know the scope's noisy channel is bad.

Lawrence, very easy to do as per TK's suggestion above.

[ltseung888]

A rather obvious test would be to swap the probes between channels. If the noise follows the probe to the other channel, then you know the probe is bad. If it doesn't, then you know the scope's noisy channel is bad.

Yet again I ask. How was the large voltage discrepancy noted during the probe compensation discussion resolved? What caused it, how did you correct it, and can we be assured somehow that it is in fact now corrected?

@TK

I just followed your advice and swapped the probes.  The result is the same.  So the problem most probably lies with the scope.

One of the boards you refered to showed unusual high voltage output and hence high COP.  My mistake was to return it to Mr. Zhou together with the "rejects".  His people "recycled" some components without a thorough investigation.  The rejects were replaced by good ones. 

Now I am keeping the rejects.  I do not have the expertise to indentify the problem.  So they are just being stored until I can find the right person to check them out.  If you are interested, I can send you some of these rejects and you can tell us what might be wrong with them.

[picowatt]

Is there some reason why the DC offset issue can't be tested right now, at home, using a known low-voltage source?

For a first test, take a 10 megohm resistor and a 100K resistor and connect them in series. Hook both scope probes from one oscilloscope to the junction of the two resistors. Hook up a AA battery,  positive pole to the 10 meg free end and negative pole to the 100K free end. All probe references to the negative pole. Use an accurate DMM to measure the voltage between the junction of the resistors and the negative pole.

Now look at the voltages seen by each probe/channel combination. Obviously they should all be the same, and the same as the DMM measures. If there is an offset problem it should show up as a difference in the readings of the channel vs. the DMM. Mark your probes and swap them around on the scopes and channels..... 4 probes and 2 channels per scope means 12 tests per scope.  This might actually take you half-an-hour to perform the whole series.

(Check all probes are at 10x attenuation and that all channels are also set for 10x attenuation probes. Do not use the "default setup" or "auto" functions of the scope; set each channel for DC coupling, no BW limit, zero baseline on the center graticule (overlap both traces so that you can see any differences) and the appropriate vertical amp setting (20 mV/div if possible). The timebase setting shouldn't matter since you are measuring DC here. Set the trigger at the zero baseline.)

TK,

This is a good idea, however, the divider resistance should be lower to reduce errors.  With 10meg probes (10X probes and 1meg scope input) and using the 10meg/100K dividerstring, probe loading will produce way too much error.  Better to use a 1K and 10R resistor in series with the free ends across the battery to minimize errors due to probe loading.

Better yet, using 100R and 1R would allow his test measurement to be made with the probe connected to a resistance similar to the CSR's and in a similar voltage range.  A fresh AA battery can handle the ca. 15ma load of the 100R/1R string for an time adequate to make the measurement, or he could use his DC supply set to 1.5V.

PW

[ltseung888]

Is there some reason why the DC offset issue can't be tested right now, at home, using a known low-voltage source?.......

@TK

I do not have an accurate "known low voltage source " at home.  Since the problem appears to be with the Atten Scope and at least one of them is still under warranty, I shall get the manufacturer to check it out with their experts.

All my three Atten Scopes showed the crossing 0 reference line behavior.  If it were a bug, the manufacturer would be and should be interested.  They need to fix it or provide a solution for their customers.

[poynt99]

All my three Atten Scopes showed the crossing 0 reference line behavior.  If it were a bug, the manufacturer would be and should be interested.  They need to fix it or provide a solution for their customers.

Can you borrow an analog scope? If so, check your current trace and let us know what you find.