# Why am I having totally different outputs with the identical input signals of two different sources? [closed]

The question is about the above Schmitt trigger indented to convert AC signals to pulses. As seen, I use V2 as input in the simulation.

The circuit above works both in LTspice and breadboard when I use a function generator for the input. So far so good.. But if the input is coming from a "permanent magnet AC generator" things are not the same. Let me summarize the situation:

The above circuit works for a given input signal by a function generator let’s say the input is set to: 1V amplitude 100Hz sinusoidal

Then I adjust the output of the permanent magnet AC generator to 1V 100Hz sinusoid as well

Now we have a function generator and a permanent magnet AC alternator where both output identical signals

But the above circuit doesn’t work and acts weird when the input is from the permanent magnet AC alternator. It works fine when it is from the function generator.

For an empirical test I removed R8 and C3 in the above circuit and the circuit works this time for the permanent magnet AC alternator input.

My questions are the following:

1-) What is the difference here between the function generator and the permanent magnet AC generator(alternator), I mean why the same input signals result different outputs. (My guess was maybe the inductance of the inner windings of the generator is coupling with the caps?)

2-) If my guess is the real reason V2 can represent the function generator but not the alternator? And if so, how can I model this kind of input in LTspice.

edit: Sometimes randomly I get some output instead of A as B:

## closed as unclear what you're asking by Daniel Grillo, hkBattousai, brhans, jippie, EcnerwalFeb 27 '16 at 18:03

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• The difference between the signals only you can tell by having a look at them with a scope, we can not guess how the waveform looks like – PlasmaHH Feb 24 '16 at 11:45
• I first set the output of the permanent magnet AC generator by hooking it up to a scope. But when I couple it as input to the above circuit, something happens which does not happen in the function generator case. – user16307 Feb 24 '16 at 11:50
• and we should now not only guess how the waveform looks before but also after it is connected instead of you showing it to us? – PlasmaHH Feb 24 '16 at 11:51
• depending on the actual changes you observe, it is not impossible, but without knowing what the changes are by observing the waveforms anything is guessing. – PlasmaHH Feb 24 '16 at 11:59
• And next time, please pay better attention to the tags you add. It's incredibly annoying if your "ltsoice" and non-committal "inout" go down in history unnoticed and need even more attention later. – Asmyldof Feb 24 '16 at 12:22

## 1 Answer

What is the difference here between the function generator and the permanent magnet AC generator(alternator), I mean why the same input signals result different outputs. (My guess was maybe the inductance of the inner windings of the generator is coupling with the caps?)

One difference might be if the signal generator's output is AC coupled - allowing the DC voltage to float - while the permanent magnet AC generator is a short to ground at DC.

At 100uF your input capacitor is very large (1uF would be plenty at 100Hz) and probably an electrolytic type. Leakage in this capacitor could upset the comparator bias voltage and prevent it from producing an output signal.

If a polarized capacitor is connected backwards it will have very high leakage. In your circuit the input voltage is lower than the bias voltage, so the capacitor's negative (-) terminal should go to the input.

• Thanks. Just trying to understand your comments and want to retry on breadboard tomorrow. I'm building this for the permanent magnet generator. The frequency input coming from the permanent magnet AC generator will vary between 4Hz to 20Hz and the amplitudes could vary from 100mV to 2V. I can use a supply voltage Vcc 12V or 5V. So next time in my circuit I will remove R3 and C3 and replace C1 with a 1uF where the negative end of C1 will be connected to the input. What do you think about it? – user16307 Feb 24 '16 at 17:19
• For 100mV at 4Hz I would use 10uF. 100mV is close to the lower limit of this circuit. – Bruce Abbott Feb 24 '16 at 19:20
• would 10uF work also for up to 20Hz? – user16307 Feb 24 '16 at 19:23
• yes. A larger capacitor passes lower frequencies better, but also works just as well at higher frequencies (at least up to several Megahertz, when inductance starts to have an effect). – Bruce Abbott Feb 24 '16 at 22:18
• Hi again i tested and it works now for 4 to 20Hz and 1 to 10V inputs which is fine. Vcc is 24V. But sometimes my DAQ indicates very high pulse frequency deviations. It happens for example 2 3 times in 30 minutes. Sudenly one output pulse is much higher than it is expected in a steady rotation. Could it be noise? I use breadboard and BNC cables. How can I reduce noise on yhis circuit? – user16307 Feb 25 '16 at 13:01