Testing Windings: Tools & Gear for Appliance Repair

[jim hardy]

Or is the problem that the TL082 doesn't swing close enough to the +ve rail? If so, which characteristic on the datasheet details better rail-to-rail figures?

this is with 15 volt supply

i don't see any load on it, so looks like it should go reliably within 3V of rail , maybe 1.5

 

[Tom.G]

If the right-hand op-amp is in open-loop, do I just need a higher OLG op amp?

Or is the problem that the TL082 doesn't swing close enough to the +ve rail? If so, which characteristic on the datasheet details better rail-to-rail figures?

@jim hardy in post #15 had a good insight about the Common Mode range of IC2. Try replacing the Diode between IC2-8 and IC2-5 with 4.7K resistor and see if that helps. It could cause an extra count in the LED display though.

Other than that, no direct answer yet. Need more data. How about some measurements and a couple scope traces?

For all the 'scope measurements, the inductor chosen should have the highest ringing frequency available that counts at least two rings.

• To start, please measure the "+9V Supply."
  
• With the 'Test Leads' shorted could you measure the voltage difference between IC2-5 and IC2-6 for verification? I expect 128mV. That is the minimum ringing voltage that will switch the OpAmp output.

For the 'scope measurements, please document both Vert and Horiz scale factors including probe attentuation (x1, x10, ?) and, if not obvious, where the Zero level is on the traces.

Use External trigger from Q1 Collector. If the Chop frequency of the 'scope is not high enough for good photos, set 'scope for Alternate trace. In addition to levels, I'm trying to get relative timings here.

• A dual trace scope shot of IC2-6 and IC2-7 with the 'Test Leads' open. Also document which, if any, LEDs light.
• A dual trace scope shot of IC2-6 and IC2-7 with a good inductor connected. Also document which, if any, LEDs light.
• A dual trace scope shot of IC2-7 and IC3-15 with a good inductor connected. Also document which, if any, LEDs light.
• A dual trace scope shot of IC2-7 and IC3-3 with a good inductor connected.

The purpose of scope measurement with 'Test Leads' open is to see what the maximum No Ringing voltage is at the OpAmp input. I expect this to be between 700mV and 1.5V. This would guide us in possibly changing the 128mV input offset of the OpAmp.
The purpose of scope measurements on IC3 is to check if the RST pulse is interfering with counting.

 

[jim hardy]

Or is the problem that......

Clarification of post 34:

When Q1 turns ON it yanks RESET pin high through C3. That sets counter to zero.
It won't start counting again until that pin goes LOW.
That won't happen until C3-R13 bleeds down to VIL threshold and we're not sure what that threshold is.

IF
RESET takes 47 usec to decay down to VIL (10,000 picofarads X 4.7 K ohms = 47 usec time constant)
THEN
your ringdown is over by the time RESET goes away.
and counter stays at zero.

That's the only reason i can see for it to count slow ringdowns but miss fast ones.

Worth a look ? Ch 1 on RESET, trigger on + transition and watch the decay
ch2 on clock ?Or try a 102 in C3 ?old jim

 

[jim hardy]

I may be confused as to when counting occurs - Q1's ON or OFF transition...

 

[Tom.G]

I may be confused as to when counting occurs - Q1's ON or OFF transition...

OFF.
According to the data sheet: http://www.ti.com/lit/ds/symlink/cd4017b.pdf
The RST input is positive logic, active high = RESET.
Input pins are diode protected to 10mA for anyone input at a time.

 

[jim hardy]

The RST input is positive logic, active high = RESET.

And it's capacitively coupled...

When Q1 turns ON it yanks RST high through C3,
That resets the counter to zero and inhibits counting clock pulses
RST decays back to LOW with 47 usec time constant.
Q1's ON transition also raises voltage at junction R10 R11 C5 by one diode drop (0.6V?) which drives CLOCK low through R10, C5 C4 and opamp's inverting input...
After RST bleeds back down to LOW, CLOCK will follow whatever the coil does, pesumably it'll ring .After 47 microseconds RST gets back down LOW and enables counting. If ringing persists that long it should count.

When Q1 turns OFF, it drives RST below 0 volts and the 4017's internal protective diodes give C3 a discharge path. So no reset on OFF transition.
It also drops voltage at junction R10 R11 C5 back to zero driving CLOCK high. It should count rings again ?

I'm awaiting 'scope traces to understand better.

 

[Guineafowl]

Sorry, lads, I'm 'away from bench' at the moment. Your posts are nonetheless much appreciated - I'll get to it soon!

 

[Guineafowl]

OK, back at bench.

+9V supply is 9.12V from bench source.
IC2-5 to IC2-6 is 114 mV with test leads shorted.

2ms/div, CH1 is top trace IC2-6 0.5V/div x10. CH2 is IC2-7 1V/div x10. Zero volts for each channel is two divs either side of from centre line. Test leads open.

If I do the same measurement, but AC couple CH1 and alter to 50mV/div x10, you can see the signal better:

As usual, with leads open the 0 LED is lit and the 1 LED is pulsing.

 

[Guineafowl]

IC2 6 and 7 with good inductor. 9 LED is lit. Both channels AC coupled or no sensible signal can be seen:

1ms/div, CH1 IC2-6 50mV/div x10, CH2 IC2-7 10mV/div x10.

 

[Guineafowl]

Good inductor connected. 9 LED lit.

1ms/div
CH1 top DC coupled, 0.1V/div x10. IC3-15.
CH2 bottom AC coupled, 10mV/div x10. IC2-7.

 

[Guineafowl]

Good inductor connected. 9 LED lit.

1ms/div.
CH1 top DC coupled, 5mV/div x10. IC3-3.
CH2 bottom AC coupled, 10mV/div x10. IC2-7.

 

[Guineafowl]

Worth a look ? Ch 1 on RESET, trigger on + transition and watch the decay
ch2 on clock ?

No-ring inductor connected, LED 0 lit.

0.5ms/div.
CH1 top DC coupled, 0.2V/div x10. RESET.
CH2 bottom DC coupled, 0.5V/div x10. CLOCK.
Ext trigger on Q1 collector as before.

Same settings, but good inductor. LED 9 lit.

 

[Guineafowl]

Or try a 102 in C3 ?

Done. With open leads, 0 lit and 1 pulsing as usual.

With shorted leads, 0 lit.
Good inductor reads 9.
Bad inductor (smps secondary) reads 2.

1ms/div
CH1 top DC coupled. 5mV/div x10. RESET.
CH2 bottom AC coupled. 5mV/div x10. CLOCK.

I think the 102 has done it.

 

[Guineafowl]

A quick test of the vacuum motor field coil, which previously read 4, now reads 5 with a 102 in C3. It appears the faster RESET decay is allowing a ring or two more to be counted.

A lot of scope shots posted here - I hope you don't feel too swamped.

 

[jim hardy]

Post 45
IC2 7 is clock?
IC3-15 is RESET ?
So in post #45, RESET is yanked HIGH by Q1 turned ON and returns low well before ringdown is completed. Looks like that's when it starts counting clocks?
When Q1 turns OFF , RESET (IC3-15) 15 is clamped to -1 volt by IC3's internal diodes.

Post 46

CH1 top DC coupled, 5mV/div x10. IC3-3.
CH2 bottom AC coupled, 10mV/div x10. IC2-7.

So CH 1 is the "0" LED lit briefly? I can't quite tell whether it's aligned over first clock pulse on ch2, but RESET should light it about same time clock pulses start and it looks like that happens... .
CH2 is clock again ?

Post 44 IC2 pin6 swings only ± a diode drop, as expected. Pin 7 swings full range

Thanks, i feel a lot better now about the circuit. It makes sense . And it's clever.

I thought of you and this thread the other day. I was checking polarity of windings in an AC motor that a friend is rebuilding.
Put small amount of DC through it and verified its four poles were N-S-N-S with a Scout compass.
Found out you can overdo it and re-magnetize the compass backward so it points South...old jim

 

[jim hardy]

A lot of scope shots posted here - I hope you don't feel too swamped.

I see you posted several while i was typing that first reply.

THANK YOU . You know engineers, it's always "More Data" .

Glad to hear speeding up the RESET decay time helped.

Sorry I'm so slow - I've learned your circuit by its signal names so had to swap back and forth to relate them to scope traces with pin numbers. But that's fine it is unambiguous.

Thanks - that's a cool device you have there.

 

[Guineafowl]

Post 45
IC2 7 is clock?
IC3-15 is RESET ?
So in post #45, RESET is yanked HIGH by Q1 turned ON and returns low well before ringdown is completed. Looks like that's when it starts counting clocks?
When Q1 turns OFF , RESET (IC3-15) 15 is clamped to -1 volt by IC3's internal diodes.

Post 46

So CH 1 is the "0" LED lit briefly? I can't quite tell whether it's aligned over first clock pulse on ch2, but RESET should light it about same time clock pulses start and it looks like that happens... .
CH2 is clock again ?

Post 44 IC2 pin6 swings only ± a diode drop, as expected. Pin 7 swings full range

Thanks, i feel a lot better now about the circuit. It makes sense . And it's clever.

I thought of you and this thread the other day. I was checking polarity of windings in an AC motor that a friend is rebuilding.
Put small amount of DC through it and verified its four poles were N-S-N-S with a Scout compass.
Found out you can overdo it and re-magnetize the compass backward so it points South...old jim

All your assumptions seem correct.

If @Tom.G agrees, I think I'll add some little decoupling caps to the TL082 as per datasheet, and build the lot up on some Veroboard.

Regarding your motor testing, I think I know of a device that might help you...

If you have a spare moment, I'd love to hear your opinion of the new gadget I bought - hand-cranked megger - thread in this forum.

 

[Tom.G]

If @Tom.G agrees, I think I'll add some little decoupling caps to the TL082 as per datasheet, and build the lot up on some Veroboard.

Go for it!

My brain was having a hard time sorting out the timing on the scope traces, many of them looked...strange?... I finally realized that the photos were of multiple traces (operating cycles) of the circuit overlapping in the photos. That's why for instance in post #45, the photo showed IC3-15 going negative before going positive; that just did not compute! I'm glad Jim and you got it sorted out. Looks like you've got a cool piece of test equipment!

I'm kind of busy this week and probably next, so will be spending limited time online. I will be checking progress here but with limited participation.

p.s. To increase sensitivity (get more counts) for poor inductors, try doubling R8. You may want to cascade another IC3 stage w/LEDs if you do that.
p.p.s. Nice call on changing C3 @jim hardy

 

[jim hardy]

Does your scope offer choice in horizontal sweep controls between ALT and CHOP ? Chop gives unambiguous timing relationship between traces whereas ALT depends on what's selected for trigger

Yes the heavy mid trace line is a giveaway it's multiple traces.

NIce Job by all here !

old jim