Speed controller problem making no sense

[gazzagoalie]

Basically, I have a comparator comparing the output of an inverting amplifier with 0V (so when the output of the amplifier is >0V, the comparator output is high, and vice versa). However, when I try this, the output is around -7V when the input is <0V, and around -6V when the input is >0V. I can't understand why, as the comparator works when it isn't connected to the circuit.
Any ideas?
Thanks

[typhoon_driver]

Re: Speed controller problem making no sense

circuit diagram?

[gazzagoalie]

Re: Speed controller problem making no sense

The signal into the comparator is only between + and - 50mV, but I have tried using a non-inverting amplifier, which did exactly the same.

[gazzagoalie]

Re: Speed controller problem making no sense

anyone?

[daliad100]

Re: Speed controller problem making no sense

Looks to me like you're using the non inverting amplifier as a buffer. Try a voltage follower instead that's what it's designed for.

[gazzagoalie]

Re: Speed controller problem making no sense

What I've shown in the diagram is an inverting amplifier, which is working fine. I have tried putting a voltage follower between the inverter and the comparator, which didn't solve the problem. Ive also tried putting a non-inverting amplifier between the inverter and the comparator, and the output of the amplifier was the same as what i am getting out of the comparator.

[daliad100]

Re: Speed controller problem making no sense

[attachment=0:390if79c]circuits.png[/attachment:390if79c]
Top left: 0 crossing detector
Top right: Inverted 0 crossing detector
Bottom left: Schmitt trigger (set R2 to 0 to set thresholds to 0v)
Bottom right: Inverted Schmitt trigger (ditto)

Both inverted circuits will do what you want them to do (from what I can tell, although I don't know why you'd want an inverted signal)

The Schmitt trigger ones will be better as they are far less susceptible to noise. You will get a little bit of circuit loading but can fix this with a normal voltage follower in the input.

[gazzagoalie]

Re: Speed controller problem making no sense

The idea is that I have a summing amplifier centering the input from my reciever on 0v. This inverts the signal, so I have the inverter to get it the right way round again. The comparator compares the signal to 0v to control a multiplexer, as I only want to know the magnitude of the signal in this bit. So, if the signal is positive, it will go straight through, and if it is negative, there is another inverter, so the resultant output of the multiplexer will be 0v.
I know I could just connect the output of the summing amp to the comparator, but I have tried this and it didnt work either.
I will draw up a full circuit diagram and scan it in on Friday, as it will explain it better.
Thanks for the suggestions so far though.

[gazzagoalie]

Re: Speed controller problem making no sense

Circuit diagram

[gazzagoalie]

Re: Speed controller problem making no sense

Bump