| Electronics > Beginners |
| AC measurement circuit question |
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| Tom18991:
Ian, your helpfulness knows no bounds. Thank you for all that! I had to get LTSpice to see the file, so apologize for the delayed response. The prospects of programming an internal comparator gives me chills so I gravitate toward your last suggestion with the LT1716. :) I'll breadboard that circuit and report back on how it did in the real world. The specs on the LT1716 are very helpful in that it can handle the full range of expected input within its design limits (presumably why you selected it...). So it would just be quirks of nature I'd need to worry about, which I assume is why you added the Zener. There's no magic to the 15v rating on that diode, right, other than that's around the max of the voltage it will likely see? Given the comparator's specs, if I put in a 10v or 20v Zener (as may be handy), is there any real down side other than how frequently it flows? On the LT1716 circuit you show, I assume the the "Pulse" line is just one of the two leads from the VR sensor, with the other lead connected to the ground plane at the bottom of your schematic? What keeps the negative half of the pulse from bringing the whole ground plane to negative 15v? |
| schmitt trigger:
If you really would like to go the analog way, use a dual opamp and create a precision rectifier. Here is the LTspice file of a circuit I commonly use, three resistors, a diode and a dual opamp. The opamp shown in the .ASC file I chose out of the LTspice library for a quick run. Choose any suitable opamp whose input common mode range accommodates your signal swing. |
| Ian.M:
The Zener serves two purposes - it protects the LT1716 against gross input positive over-voltage and also clamps its input to stop it going more than 0.7V negative, again to keep within the comparator's input voltage ratings. The Zener voltage is non-critical - anything in the range 5.6V to 30V would work equally well. I'm assuming one side of the sensor can be grounded. If not, the problem just got a *LOT* more complicated. What else s the sensor being used for? If it also goes to a tacho circuit you need to consider that as well to ensure compatibility. It isn't going to be dragging ground anywhere, as the 'hot' end (pulse in the sim) is connected via a 10K resistor which limits the max current to a few mA so unless some plonker arc welds on the engine block or manifolds without disconnecting the electronics it should be fine. If the sensor already has one side grounded at the engine, and your circuit is mounted and grounded elsewhere, a different approach with a differential OPAMP will be required as ground bounce could be misinterpreted as a valid sensor pulse |
| Tom18991:
If the neg goes more negative than the zener's value, won't it break down and flow well below .07 neg? The VR sensors are not much more than a coil that produces an ac voltage on it's two leads -- so when you ask if one side "can be grounded," I guess I'd say sure, but I'm not entirely sure what will happen when I do. I can tell you that the original circuit I posted on this thread did work to produce a relatively smooth positive only DC value, and that circuit had one side of the VR sensor grounded, so seems like it should be ok. I'll try LT1716 approach as soon as I can get the parts and report back. Thank you again. :) p.s., I've welded on chassis before. Usually disconnect the battery and make sure the ground clamp is very close to the weld and very secure. :) Call me a plonker... |
| Ian.M:
Well below its Zener voltage a Zener behaves identically to an ordinary silicon diode, conducting in the forward direction (anode positive) and blocking in reverse. When passing a few mA its forward drop will be in the 0.6V-0.7V range. Is the VR sensor coil dedicated to your need to detect 3V pk-pk or does it also feed anything else? If the former, you can certainly ground one end, preferably at your PCB. You may want to put a 1K resistor in series with its ground with 0.1uF across the resistor at your PCB to prevent damage if the sensor wiring is ever shorted to 12V. If you put the ground clamp right next to where you were welding, on the same part, and also disconnect the battery, you are not a plonker. The problems happen if a plonker puts the ground somewhere other than on the part they are welding, or if they weld very close to light clusters, sensors etc. without disconnecting them so there's enough voltage drop between the weld and the ground clamp to cause some of the current to flow via the wiring loom. |
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