Hey guys, we were shown a circuit in class recently for ac rectification using an opamp.

Looking this circuit up on the net found this:

WHich makes perfect sence to me.
The problem I am having is the variant we got in class. The problem circuit can be seen as an attachment to this post.


The problem I am having is the opamp tries to make the voltages at both inputs the same. So the inverting input should nearly become zero volts. For As it will still be fractionally positive (once in balance) the output of the opamp, in my mind will be -0.7 volts, otherwise the diode would not conduct. In my notes when the input is positive the output is 0. I can see how this occurrs in the above schematic but not in my attachment.

Thanks!

 

The two circuits are identical, except for where the output is taken from (and the missing input resistor in the attachment, which I assume will actually be present). If one of them works, the other one obviously won't.
The attachment is the incorrect one.

 

Thanks, I fixed the schematic in the attachment. Yeah I think the one my lecturer gave us is wrong too. And yes I was also aware that they are essentially identical. I just wanted to confirm that it was indeed the wrong one before I asked him again about it again as he seemed so confidant that it was fine.

 

A quick simulation of the two circuits.

As you can see, the output from your lecturer's schematic is not what is desired.

 

Thanks SgtWookie!

I am so downloading that program that you have used tonight! It looks like an extremely handy program.

Thanks, I thought I was going mad there with that simple circuit!

 

You might still find Circuitmaker Student floating around, but it hasn't been supported in years - and you can't add components to the library in the student version. I just use it for "quick and dirty" simulations.

Try Linear Technology's LTSpice instead.
http://www.linear.com/designtools/software/
It's free, you can add as many models as you want, and there are support groups for it that have generated quite a few models; including TTL and CMOS.

You'll be far better off starting with LTSpice.

 

You might still find Circuitmaker Student floating around, but it hasn't been supported in years - and you can't add components to the library in the student version. I just use it for "quick and dirty" simulations.

Try Linear Technology's LTSpice instead.
http://www.linear.com/designtools/software/
It's free, you can add as many models as you want, and there are support groups for it that have generated quite a few models; including TTL and CMOS.

You'll be far better off starting with LTSpice.

Will do!
Thanks for the advise

 

If you ever have problems with LTSpice or other programs, you may also try NL5. It does basically the same, however it can reliably work with ideal componenets, so you don't need to select specific diodes and OpAmps, just use very "generic" stuff. This might be useful for studying basic electronics, control theory, etc., just like schematic you discussed in this post. NL5 is free up to 20 components in the schematic.

Thank you,
Alexei.

 

If you ever have problems with LTSpice or other programs, you may also try NL5. It does basically the same, however it can reliably work with ideal componenets, so you don't need to select specific diodes and OpAmps, just use very "generic" stuff. This might be useful for studying basic electronics, control theory, etc., just like schematic you discussed in this post. NL5 is free up to 20 components in the schematic.

Thank you,
Alexei.

Not to denigrate your program, Alexei (I haven't tried it), but LTspice also has ideal, generic, and simplified component models, and it's free with no limit on components. That's not to say that your program doesn't have useful features that are absent in LTspice.

 

Thank you for courteous note ! Usually I hear more "categorical" opinions...

That's why I said: "If you ever have problems with LTSpice...", because I've heard about problems simulating circuits with ideal components (specifically - switching power supplies) not only with LTSpice, but with Tina, MicroCap, Workbench: just because of Spice-based algorithm of those simulators. However, you may never experience such problems at all, then any simulator is fine!

Thank you,
Alexei.