Goal: Build a DIY tester similar to a commercially available device for testing TV transformers, in particular the LOPT, or flyback transformer.
Strategy: Repeatedly, at 10Hz, apply a 600mV, 2ms pulse to an inductor in parallel with a capacitor, and count the peaks of the damped LC ringing that results. Disregard peaks below a threshold. Use a counter to capture the rings and LEDs to display the result. Correlate results with know good and bad LOPTs.
Background: There's a commercial tool for this, about $55. The schematics and explanation for the commercial device are given in these pdfs.
First generation View attachment Ring tester 1.pdf
Second generation View attachment BLUE_rt_assembly_manual.pdf
Experiments: After reading how this worked, I realized my breadboard from a previous project already had the ICs on it I could use to create a similar circuit. The original circuit used a LM393 for the oscillation and ring detection functions. My 556 could replace the the oscillator. My op-amp could replace the darlington transistor used to amplify and square up the rings for counting. My 4017 counter could replace the shift register, with the added benefit of lighting just one LED at a time for lower power consumption and less current-limiting resistors.
One issue I didn't quite expect was that the positive pulse from the 556 timer, although it is the right sense, is not big enough to trigger the 4017 timer reset and to ring the inductor. So the transistor and/or one of the two op-amps is used to boost it.
I initially used an LM358 for my op-amp, one half to boost the 556 output and the other half to amplify the rings. This worked, but with two oddities: First, I had to extend the reset pulse from 5µs to over 20µs in order to get the 4017 to respond. And I could only get a ring result with very large inductors that gave slow ringing. I had no problem getting 10 rings or more, but not within the 2ms mentioned in the original paper. I had one transformer that rang with about 2ms between pulses, so only 1 ring would register within the 2ms window.
It was obvious the LM358 was too slow, so I switched to the TL082, since it's available at Radio Shack and is much faster than the LM358.
This had it's own quirks - the TL082 can sense to the positive rail but not the ground rail like the LM358. The transistor in the final circuit shouldn't really be necessary but I simply could not get the TL082 op-amp to perform well as a non-inverting amp for the 556 signal. So I used it as an inverting amp and let the transistor invert it again. This gives a nice solid reset and ringing.
Results: The circuit gives a full 10-count for some small coils I had in the junk box and, most importantly, a 10-count for the LOPT primary I built it to test. The larger transformers in my box give lower counts, but I was able to use my 'scope to see that this was because the pulse current is removed after 2ms, long before the coil is energized. The "off" ring fades out quickly.
Strategy: Repeatedly, at 10Hz, apply a 600mV, 2ms pulse to an inductor in parallel with a capacitor, and count the peaks of the damped LC ringing that results. Disregard peaks below a threshold. Use a counter to capture the rings and LEDs to display the result. Correlate results with know good and bad LOPTs.
Background: There's a commercial tool for this, about $55. The schematics and explanation for the commercial device are given in these pdfs.
First generation View attachment Ring tester 1.pdf
Second generation View attachment BLUE_rt_assembly_manual.pdf
Experiments: After reading how this worked, I realized my breadboard from a previous project already had the ICs on it I could use to create a similar circuit. The original circuit used a LM393 for the oscillation and ring detection functions. My 556 could replace the the oscillator. My op-amp could replace the darlington transistor used to amplify and square up the rings for counting. My 4017 counter could replace the shift register, with the added benefit of lighting just one LED at a time for lower power consumption and less current-limiting resistors.
One issue I didn't quite expect was that the positive pulse from the 556 timer, although it is the right sense, is not big enough to trigger the 4017 timer reset and to ring the inductor. So the transistor and/or one of the two op-amps is used to boost it.
I initially used an LM358 for my op-amp, one half to boost the 556 output and the other half to amplify the rings. This worked, but with two oddities: First, I had to extend the reset pulse from 5µs to over 20µs in order to get the 4017 to respond. And I could only get a ring result with very large inductors that gave slow ringing. I had no problem getting 10 rings or more, but not within the 2ms mentioned in the original paper. I had one transformer that rang with about 2ms between pulses, so only 1 ring would register within the 2ms window.
It was obvious the LM358 was too slow, so I switched to the TL082, since it's available at Radio Shack and is much faster than the LM358.
This had it's own quirks - the TL082 can sense to the positive rail but not the ground rail like the LM358. The transistor in the final circuit shouldn't really be necessary but I simply could not get the TL082 op-amp to perform well as a non-inverting amp for the 556 signal. So I used it as an inverting amp and let the transistor invert it again. This gives a nice solid reset and ringing.
Results: The circuit gives a full 10-count for some small coils I had in the junk box and, most importantly, a 10-count for the LOPT primary I built it to test. The larger transformers in my box give lower counts, but I was able to use my 'scope to see that this was because the pulse current is removed after 2ms, long before the coil is energized. The "off" ring fades out quickly.
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