![]() This situation will only last for a short while. What I think your first simulation shows is that at start up while the 10uF capacitor is still charging, the input does go negative. Volts = Analog input pin on Arduino dedicated to Voltage measurements. The secondary winding voltage is 4.6VAC rms. AREF = 2.5VDC, supplied by a linear regulator. Additionally, I threw in a MOV to help with permanent excursions above 120VAC. For example, using a 10V-rated chip for the 1.25VAC signal that I'm trying to pass here.Īnyhow, an updated diagram is below, the parameters haven't changed, but the values of the capacitors and resistors have based on prior feedback. a 25V-rated chip or whether one should select a rated voltage that allows the same amount of reverse flow as it did forwards. I wonder whether one should select the capacitor voltage rating on the basis of the above to survive the reverse bias, i.e. There is a chart there also showing the reverse current as a function of voltage for an unspecified chip. The diode DR has a bend at approximately 10% of the capacitor’s rated voltage to describe the real change of capacitor’s VA curve In the reverse mode, tantalum and niobium oxide dielectrics are modeled by a diode DR and resistor RD integrated in theĮquivalent circuit diagram. On the Tantalum and Niobium side of things I found the following statement from AVX in this document: I can order a bi-polar Aluminum electrolytic capacitor from Digikey for less than a dollar but they're only offered as through-hole components. Tantalum caps in this range seem less expensive, ditto for Aluminum Electrolytic but how to verify that the cap can handle the reverse bias? That is, how to orient the thing and to determine that the thing is going to be safe and not explode the way tantalum and aluminum electrolytic capacitors are wont to? Thanks again for the insights.Īh, but for just $1.65, a 100uF, 10V ceramic capacitor can be yours in single quantities. The capacitive-coupled circuit certainly also has appeal thanks to the grounded output and costs only marginally more to implement. With the right voltage divider on the secondary and so on, the first circuit is appealing because of its proven performance in past board iterations. I wager you'd have to be more knowledgable about the topic than I presently am.Īnyhow, which circuit to go with is a good question at the end of the day is a good question. People apparently use Aluminum electrolytic capacitors in this type of circuit, what I wonder is what the reverse bias would do to it over time. I must be missing something because the ESR of the types of caps I usually consider only add another Ohm or two per their spec sheet.Īs far as capacitor types go, are there preferences one should follow? I was thinking of going with a 100uF 10V ceramic cap as there are no issues with reverse bias then and the cost is only a dollar more vs. Thing is, when I plug in the above formulas, I get a reactance of 26 not 60 Ohms as expected with a 100uF cap 60Hz. That I got my X c reactance wrong is just typical of me, it's been a while since I took my last EE course. I thought I'd explore the capacitive-coupled circuit topology because it's allegedly better at protecting the Atmel. Even though the combined resistance of the two resistors on the secondary of the transformer were very close to the total resistance of the bias circuit, the circuit works. The resistors I used on the secondaries were 100 Ohm (whose output fed into the Arduino) and 2.2k for most of the voltage drop. What is interesting in the light of Grumpy_Mikes earlier comments is that the previous voltage divider on the secondary side was fed by a nominal 6VAC rms wall wart, whose unloaded output was closer to 7.2VAC rms. The previously posted circuit topology has worked for me in the past - I just got my resistor value ratios wrong in my haste to post here. Alternatively, I think your previous circuit was OK, because the capacitor from pin 5 of the transformer to ground means that one end of the transformer secondary is virtually grounded anyway.īoth of you, thanks for the suggestions and formulas. If you want to stick with that circuit, I suggest you increase the two 1K resistors to 22K each and increase the capacitor to 100uF (impedance about 60 ohms 60Hz). The 27nF capacitor is way too small, its impedance at 60Hz is around 100K ohms.
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