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Circuit Question regarding Capacitor

Dave Hamernik All Messages By This Member #14432 Hi /g/w4dccqa/files/AC%20to%20DC.pdf This circuit shows a rectifier to convert DCC square wave AC to DC so that it can be measured with a DC volt meter.? I don't doubt that the circuit design is wrong, I am new to circuit design and am seeking a better understanding.? Should there be a resistor upstream of the capacitor in series to regulate the charging of the capacitor?? If a resistor is not needed, then why? Thank you. -- Dave Portland, OR
Michael Rozeboom All Messages By This Member #14433 开云体育 On 2023-11-16 3:58 p.m., Dave Hamernik wrote: Hi /g/w4dccqa/files/AC%20to%20DC.pdf This circuit shows a rectifier to convert DCC square wave AC to DC so that it can be measured with a DC volt meter.? I don't doubt that the circuit design is wrong, I am new to circuit design and am seeking a better understanding.? Should there be a resistor upstream of the capacitor in series to regulate the charging of the capacitor?? If a resistor is not needed, then why? Been a long time since I've built a rectified voltmeter... The resistor allows current to flow back the the rectifier, and you measure the voltage across the resistor. You could also just use an AC voltmeter. Good ones should work acceptably well, and you'll just be making comparisons. It won't be completely accurate, but if you only want so see if there is voltage, and compare voltage at one point to another, it will work. If you really want accuracy, a voltmeter designed for DCC frequencies will be needed, or an oscilloscope. -- Michael Rozeboom Team Amiga
Dave Hamernik All Messages By This Member #14436 That makes sense. But is a resistor needed between the rectifier and the cap? I thought that a resistor would be needed to regulate the cap’s charging.? thanks.? -- Dave Portland, OR
Don Weigt All Messages By This Member #14438 The cap and rectifier will present a near short circuit momentarily when first connected to the DCC signal. Once charged, the current will be low, so no problem. Adding a resistor between the rectifier and capacitor will reduce the voltage out by somewhat more than the ratio of that resistor divided by the sum of the two resistances, since rectified DCC isn't quite continuous. For a metering circuit, the capacitor should be a small value, so not much of a short, and the resistance high, also not much of a load. In this case, there's probably no harm in not limiting the inrush current to the capacitor. The cap's impedance may do a good enough job by itself, also, every source, even a DCC booster, has some impedance, as does the wiring, so it's not a perfect source, or perfect short circuit load, so the initial peak current won't be infinite. Don W -- Don Weigt Connecticut
Dave Hamernik All Messages By This Member #14442 Thanks. -- Dave Portland, OR

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