Hi all,
Well, after building not one but TWO LC meters, I am quite satifised with the results.
First: **broken link removed**
Second: **broken link removed** (measures ESR too!)
Ultimately, they measure 'relative' capacitance... relative to a known value. Since they both rely on calibration (first one should really use an accurate calibration cap) in order to get <2% accuracy, I should probably use a precision cap, such as a silver mica, or perhaps a tight spec polystyrene cap to calibrate these. It *should* be a one-time process, perhaps recalibrating my meters every so often.
Whilst I could order some relatively expensive 1% (even seen some 0.3% silver mica's) before I do so, as a little 'test' I was thinking about a very accurate way to measure absolute capacitance... one that doesn't require a precision capacitor in the first place - only precision resistors, and the ability to measure time periods accurately. (read: microcontroller).
Seems a bit 'chick and egg' to design a way of measuring capacitance....for a capacitance meter. But the difference is, the above meters are for convenience, and speed, with a nice simple readout. For a 'one-time' measurement, I will be happy to manually do the maths, either with a pen, or MS Excel, it will be a quick prototype with no bells and whistles.
Since I know the infamous NE555 insideout, that looks like a good start. It's internal resistors aren't exactly precise, so I was thinking of using the same principle, but with LM393 comparators, and precision resistors (0.1% 10k's... I have lots for some reason).
Basic idea: Fully discharge the cap we're testing. Start charging via another precision resistor at VCC (same supply as the resisitor ladder used for the comparator references). First comparators thresshold is at VCC/3. Second at VCC/6 - just like the 555 timer. This takes VCC out of the equation since it is relative. Instead of charging/discharging the cap whenever each threshold is reached (keeping the cap voltage between VCC/3 and VCC/6) - a microcontroller simply measures the time between the two thresholds. The cap can charge way beyond the VCC/6 threshold, but is discharged (to at least VCC/10) between tests. The propagation delays of the two comparators should be more or less the same. And will cancel when the period is measured. ((Tth2 + CompDLY) - (Tth1 + CompDLY)) . Where Tth1 and 2 are the threshold times.
This has all been done before, and definately not as accurate as a bridge, but I'm hoping for +/-0.5% accuracy here. With 0.1% resistors, and a microcontroller with a timer running at 20MHz (AVR) does anyone think this is achievable?
As I said, its more of a novelty/academic problem, as I could just order some precision caps for calibration. But all it requires is precision resistors, and an accurate time measurement (20ppm with crystal oscillators). Could prove to be a good 'starter' to accurately find the real value of a capacitor, which can then be used for calibration. Not designing a full circuit, just a 'knock-up' for single use. Any takers?
Blueteeth.
Well, after building not one but TWO LC meters, I am quite satifised with the results.
First: **broken link removed**
Second: **broken link removed** (measures ESR too!)
Ultimately, they measure 'relative' capacitance... relative to a known value. Since they both rely on calibration (first one should really use an accurate calibration cap) in order to get <2% accuracy, I should probably use a precision cap, such as a silver mica, or perhaps a tight spec polystyrene cap to calibrate these. It *should* be a one-time process, perhaps recalibrating my meters every so often.
Whilst I could order some relatively expensive 1% (even seen some 0.3% silver mica's) before I do so, as a little 'test' I was thinking about a very accurate way to measure absolute capacitance... one that doesn't require a precision capacitor in the first place - only precision resistors, and the ability to measure time periods accurately. (read: microcontroller).
Seems a bit 'chick and egg' to design a way of measuring capacitance....for a capacitance meter. But the difference is, the above meters are for convenience, and speed, with a nice simple readout. For a 'one-time' measurement, I will be happy to manually do the maths, either with a pen, or MS Excel, it will be a quick prototype with no bells and whistles.
Since I know the infamous NE555 insideout, that looks like a good start. It's internal resistors aren't exactly precise, so I was thinking of using the same principle, but with LM393 comparators, and precision resistors (0.1% 10k's... I have lots for some reason).
Basic idea: Fully discharge the cap we're testing. Start charging via another precision resistor at VCC (same supply as the resisitor ladder used for the comparator references). First comparators thresshold is at VCC/3. Second at VCC/6 - just like the 555 timer. This takes VCC out of the equation since it is relative. Instead of charging/discharging the cap whenever each threshold is reached (keeping the cap voltage between VCC/3 and VCC/6) - a microcontroller simply measures the time between the two thresholds. The cap can charge way beyond the VCC/6 threshold, but is discharged (to at least VCC/10) between tests. The propagation delays of the two comparators should be more or less the same. And will cancel when the period is measured. ((Tth2 + CompDLY) - (Tth1 + CompDLY)) . Where Tth1 and 2 are the threshold times.
This has all been done before, and definately not as accurate as a bridge, but I'm hoping for +/-0.5% accuracy here. With 0.1% resistors, and a microcontroller with a timer running at 20MHz (AVR) does anyone think this is achievable?
As I said, its more of a novelty/academic problem, as I could just order some precision caps for calibration. But all it requires is precision resistors, and an accurate time measurement (20ppm with crystal oscillators). Could prove to be a good 'starter' to accurately find the real value of a capacitor, which can then be used for calibration. Not designing a full circuit, just a 'knock-up' for single use. Any takers?
Blueteeth.