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ed, I didn't use the center tap.
Not sure what steve would suggest, but did you try the R3 increase as per my post #5?
hi alec, i'am going to change that resistor tonight and will post back. will the change increase /decrease freq?
I'll try the resistor first as that is easier to get at. thanks guys.
ok alec, changed r3 to 200k. the output voltage dropped dramatically, now output at test leads is 245 mv. whoopee!! the side effect however is that as steve noted the gain of the second op amp has to be increased. I think the resistor between pins 6&7 needs to be adjusted. theres a 39k there now. what value would you suggest?
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Sir pete g . . . . . . .
Part 1 . . . . .
Vice that loop gain alteration of the U1A op amp oscillator circuit , a less circuit invasive and altering procedure would have been to insert a drop resistor between the osc op amps output and C2 coupling capacitor.
Initially try using 220 ohms and work your resistor value upwards until you get exactly the osc level value that you want.
Part 2 . . . . .
Short the test leads together to get maximum coupling into the U1B signal amplifier to see if a full scale meter deflection can be produced by the detected DC and having R11 or R12 to their minimum resistance position.
If not, then the U1B loop gain R8 and R13 resistors values need to be shifted to get a bit more than maximum deflection . . . . then, it being brought back down with R11- R12 adjustments.
That regulated supply reduction down to 5VDC . . .holds stable readings for a long time from that 9V supply battery.
73's de Edd
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Sir pete g . . . . . . .
Part 1 . . . . .
Vice that loop gain alteration of the U1A op amp oscillator circuit , a less circuit invasive and altering procedure would have been to insert a drop resistor between the osc op amps output and C2 coupling capacitor.
Initially try using 220 ohms and work your resistor value upwards until you get exactly the osc level value that you want.
Part 2 . . . . .
Short the test leads together to get maximum coupling into the U1B signal amplifier to see if a full scale meter deflection can be produced by the detected DC and having R11 or R12 to their minimum resistance position.
If not, then the U1B loop gain R8 and R13 resistors values need to be shifted to get a bit more than maximum deflection . . . . then, it being brought back down with R11- R12 adjustments.
That regulated supply reduction down to 5VDC . . .holds stable readings for a long time from that 9V supply battery.
73's de Edd
.
.
hi edd, thank you for the input. actually, the u1b is running on a reg 9v. does the gain of u1b go up with a higher value resistor? I opted not to use r8,r13 and one 20k pot for zero adjust. the gain now with the r3 change just barely moves the meter pointer with the leads shorted.
That's way above what the schematic shows. Rather than increase that value it might be easier to slap another resistor (say 1k) in parallel with R7.
hi alec, the original schematic for this esr meter had a 39k across pins 6&7.
changing R3 to 200k does not directly affect the amplitude. It has the effect of changing the frequency.
you may be measuring a lower voltage for several reasons:
1) the transformer is less effective at the higher frequency
2) your measuring equipment is having problems due to the higher frequency.
3) Stray inductances and capacitances are having an effect on either the voltage or the measurement.
Another major issue is that ESR measurements are typically done at 100kHz. Whilst it's not extremely critical, doubling it is certainly not going to give you results comparable to those in datasheets out taken by others.
you may be measuring a lower voltage for several reasons:
1) the transformer is less effective at the higher frequency
2) your measuring equipment is having problems due to the higher frequency.
3) Stray inductances and capacitances are having an effect on either the voltage or the measurement.
Another major issue is that ESR measurements are typically done at 100kHz. Whilst it's not extremely critical, doubling it is certainly not going to give you results comparable to those in datasheets out taken by others.
hey steve, well, you're right about the freq. it went up a lot. the problem now is I can't get enuf gain out of u1b. the result is I can't zero the meter. I tried putting another 1k across r7, no change in gain.
hevans1944
Hop - AC8NS
Did anyone notice that if you visit this link that there is a little box at the bottom right side of the page that lets you select your language of choice? There are also some really nice links on that page that are well worth the time to visit. The link to the very expensive commercial version has some nice PDF page links on it too.
The diodes across the test leads are there for protection (to discharge a charged capacitor), as @pete g stated in his post #14. At low excitation levels, the diodes should not conduct very much.
The excitation voltage is supposed to be sinusoidal and the amplitude is determined by the transformer turns ratio for the custom-wound transformer on an Amidon EA-77-188 core. The excitation voltage is attenuated somewhat by the ten ohm resistor placed across the secondary winding. You could probably attenuate it even more by decreasing its value, but this may affect the oscillator driving the primary.
The last thing you want is "square waveish" excitation, although this can be used with a large series resistor and an oscilloscope to directly measure ESR. PITA to set up and capacitor should be removed from circuit to test. See this page for more info on that.
The Radio Shack transformer may be a poor substitute at the nominal 50 kHz excitation frequency, but @(*steve*)'s suggestion to use only half of the primary will have the effect of increasing the excitation voltage whereas the original objective was to decrease it.
I think I will try to build this thingy in an empty Altoids box without a meter and use it with an external DVM. The commercial product claims 5 mV rms excitation at 100 kHz, so I think I will start from there.
Sounds like a really fun and useful project. I can take that puppy to Mendelson's and try-before-I-buy some of their dirt-cheap NOS electrolytic capacitors.
The diodes across the test leads are there for protection (to discharge a charged capacitor), as @pete g stated in his post #14. At low excitation levels, the diodes should not conduct very much.
The excitation voltage is supposed to be sinusoidal and the amplitude is determined by the transformer turns ratio for the custom-wound transformer on an Amidon EA-77-188 core. The excitation voltage is attenuated somewhat by the ten ohm resistor placed across the secondary winding. You could probably attenuate it even more by decreasing its value, but this may affect the oscillator driving the primary.
The last thing you want is "square waveish" excitation, although this can be used with a large series resistor and an oscilloscope to directly measure ESR. PITA to set up and capacitor should be removed from circuit to test. See this page for more info on that.
The Radio Shack transformer may be a poor substitute at the nominal 50 kHz excitation frequency, but @(*steve*)'s suggestion to use only half of the primary will have the effect of increasing the excitation voltage whereas the original objective was to decrease it.
I think I will try to build this thingy in an empty Altoids box without a meter and use it with an external DVM. The commercial product claims 5 mV rms excitation at 100 kHz, so I think I will start from there.
Sounds like a really fun and useful project. I can take that puppy to Mendelson's and try-before-I-buy some of their dirt-cheap NOS electrolytic capacitors.
hi all, b4 this thread ends, I just want to say how much I enjoyed it. thank you to all who contributed. I really learned a lot. thanks again. till next time. peteI'm suffering Mendelson's withdrawal syndrome.
