Octopus Schematic

[bryan]

All:

Going to put together this simple variant of a "Octopus" circuit, but for some reason it doesn't look right to me. Wonder if the LM317 would not start oscillating without caps on the inputs and outputs, but adding them would it not skew the results of any test.

http://www.next.gr/meter-counter/meters/Simple-Curve-Tracer-Circuit-l14760.html

Cheers and thanks

 

[MrAl]

Hi,

It looks like the device is being driven by pulsating DC rather that pure DC. That means it's output will go high then low, then high again then low agin, etc.

 

[cowboybob]

Hi,

It looks like the device is being driven by pulsating DC rather that pure DC. That means it's output will go high then low, then high again then low agin, etc.

AC if switch S3 is closed.

 

[MrAl]

AC if switch S3 is closed.

Hi,

Yes, thanks, i assumed that he was talking about when the switch was open because he was talking about the regulator being in operation.

 

[throbscottle]

That looks a nifty gadget - I might build one

 

[bryan]

Yes, if S3 was open. Wonder what the purpose of pulsing DC would be for component testing?. I assume the LM317 is acting as a current limiter with the way the pot is configured?

 

[throbscottle]

Think about it. If you have a normal type oscilloscope - one that doesn't remember where the trace has been, the component curve will be there and gone in a single sweep - not what you want! So if you have a pulsed input it can draw the same trace again and again and again so you can actually see the curve.

 

[bryan]

Thanks, but isn't that what the AC component of the schematic is accomplishing? A 50/60 hz sine wave

 

[Nigel Goodwin]

Yes, if S3 was open. Wonder what the purpose of pulsing DC would be for component testing?. I assume the LM317 is acting as a current limiter with the way the pot is configured?

Pulsed DC is still AC - will it pass though a capacitor? YES, will it pass through a transformer? YES - it's AC.

But in any case it's NOT pulsed DC anyway, it's still AC centred around zero - that's what the bridge rectifier is for (DC on the LM317, AC on the 'output' of bridge).

Personally I'd probably add small capacitors to the LM317 - 0.1uF or something similar?

 

[MrAl]

Hi again,

Nigel, Yes! That is not wired as a bridge rectifier it's wired as an AC current control (switch open). That makes it pulsing DC but dual polarity.

When the AC input voltage starts to rise, the LM317 starts to limit current to the output.When the AC input switches polarity the LM317 still limits current but now it appears to the output as a switch in polarity.

Using the horizontal and vertical like that means it traces the device curve, although the dwell time is not the same for every test point (not always a problem).

 

[Nigel Goodwin]

Hi again,

Nigel, Yes! That is not wired as a bridge rectifier it's wired as an AC current control (switch open). That makes it pulsing DC but dual polarity.

It's still a bridge - just used in a slightly unconventional way

I do wish people wouldn't use the term 'pulsing DC', it's AC - not DC at all, it just causes confusion - and trying to add a further by inventing 'pulsed DC dual polarity' is even worse.

Incidentally a similar method was used in light dimmers decades ago, using a thyristor and a bridge, back before TRIAC's.

 

[MrAl]

Hi Nigel,

Well as you know we need ways to describe things to people who dont know what they are yet and it helps to create 'handles' to these circuit objects so we can refer to them without going over the definition again and again.

I was taught long time ago that we call what comes out of a bridge rectifier (for example) "pulsating DC", simply because it contains a large DC component plus AC components, but even more simply because that's what comes out of a bridge rectifier. In other words, if we always call it "pulsating DC" then everyone would know what we were talking about right away without more explanation.

Calling it DC alone isnt right, and calling it AC alone isnt right, because it contains both DC and AC components. We need a way to distinguish it from pure AC or pure DC without going into these kind of time consuming details, so we use a term that is descriptive.

I guess i call it "pulsing DC" sometimes but i was actually taught many moons ago to call it "pulsating DC". A little different but not much.

If you still want to call it just AC then that will cause confusion when someone puts a DC meter across it and even without any filter caps it will read a DC voltage. It's still up to you though of course.

 

[dr pepper]

With S3 open the pot will control the amount of clip on the ac waveform, presumably so you can limit the o/p voltage.
Clipping the o/p a lot will make the scope trace very bright at the ends.