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Rather new to electronics, I'm trying to design some of my first circuits. One of them includes a bridge rectifier feeding into an opamp used as a comparator. However, I'm a bit perplexed as to why LTSpice shows the input voltage as consistently negative, regardless of where in the sine wave the input voltage component is at/should be. I've tried reversing the polarity of the voltage component, reversing the leads that feed the bridge, and the like but seem to get the same result. The input voltage is above zero at about +1V (I'm assuming due to 2X the voltage drop of the diodes used) for part of the cycle - inverting the concept of a diode "voltage drop". Is the voltage really driving negative or is this an artifact of LTSpice modeling? In either case, is there a way to change this?

enter image description here

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    \$\begingroup\$ It reports the voltage relative to GND. Look where you put GND. \$\endgroup\$ – PlasmaHH May 20 '16 at 12:54
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    \$\begingroup\$ You built your bridge rectifier upside-down, connecting vrout to the negative dc terminal. \$\endgroup\$ – W5VO May 20 '16 at 12:59
  • \$\begingroup\$ @PlasmaHH and W5VO You were both correct - I'd had tried changing both ground and outputs before, but never at the same time. Thanks for the pointers toward the issue - I'll do some more thinking. If one of you wants to make this an answer I'll mark it as such. \$\endgroup\$ – JimMSDN May 20 '16 at 13:08
  • \$\begingroup\$ So, a follow-up question: is there another way to see both the positive and negative halves of the input sine wave other than what I've done, which is to measure at both my Vrin1 and Vrin2? \$\endgroup\$ – JimMSDN May 20 '16 at 13:11
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    \$\begingroup\$ When you probe (measure) Vrin1, what the program plots is the voltage between Vrin1 and ground (which is actually 0 V (zero volts)). So if you would plot Vrin1 - Vrin2 you would plot the voltage across V1 which will have positive and negative values. \$\endgroup\$ – Bimpelrekkie May 20 '16 at 13:58
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To answer your follow-up question Jim, if you move the ground to Vrin2, Vrin1 will be a sinewave at 0v dc offset. But there is another way to see the sinewave, and that is with mathematical plotting.

  • Right-click the chart, choose "Add Trace".
  • In "Expression(s) to add" type in: V(vrin1)-V(vrin2)
  • Shorthand: click V(vrin1), press -, click V(vrin2)

Note that the voltage source is ideal by default, so has no resistance, thus will produce a perfect sine wave. Expect real-world measurements to never match simulation. So here is added 1Ω of series resistance, and a capacitor chosen with real-world parasistic values (a good 25v Nichicon): enter image description here

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    \$\begingroup\$ Even shorter shorthand hand - click Vrin1 and hold and drag to Vrin2 and release. \$\endgroup\$ – efox29 May 20 '16 at 15:34
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    \$\begingroup\$ A good observation - I've been starting with idealized circuits (which are easier for me to understand at this point) and then I've been adding in more "real world" effects once I'm comfortable I understand what's going on. All of this is leading towards my first (semi) practical design: a "voltage breaker" for keeping the speed of a Lionel model train (with its high-amperage draw and frequent full shorts) below a pre-set speed/voltage. I have a 3YO son, ya know... \$\endgroup\$ – JimMSDN May 20 '16 at 16:50

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