can someone explain this?

[m3vuv]

hi all,im building this,i just wondered what d1 is for?

[langwadt]

protect the led, leds can only handle something like 5V in reverse

[TimFox]

Also, it helps keep the voltage waveform across R1 roughly sinusoidal by allowing current in both directions through C1 & R2.

[m3vuv]

 Well im trying to build this on some vero board,ive come to the stage of adjusting the voltage pots,the issue is i connect 1 of my probes to one leg of say r3,where do i clip my other meter probe,it says measure from the red octopuss probe and the black,the black probe goes to ground but without a scope commected i cant see how it can ground to anything,surley the points to measure against is one leg or the wiper of r3( depends how its wired) and the transformer lower leg?,i cant see how the octopus black lead can reference anything as connected to the scope its at mains ground,can anyone explaine as im stuck on setting this thing up,cheers m3vuv .

[langwadt]

 what is the schematic supposed to do?

[m3vuv]

its a 3 position octupuss component tester/curve tracer.

[Renate]

Without D1, D2 is not going to light at all.
You can't force DC through a cap.
(A resistor for D1 would work, but a diode is better.)

Unless I've got my head screwed on wrong (which might be true), won't this generate curves "upside down" from what you would normally see?
I want to see _/‾ for normal resistive/diodey type stuff.
Your circuit will show ‾\_
Does your scope have an "invert channel" button for the Y axis?

This is a very old style curve tracer.
The plus side is that it will show interesting O's for capacitors or inductors.
The minus side is that the slew rate is not constant.
If I were to build something like this, I'd want something more modern that could also step the base/gate voltage/current for a family of curves.

[m3vuv]

it int a cure tracer as such,more for checking transistor and diode junctions etc,sort of poor mans huntron tracker!,im not fitting the led cap and diode etc as i dont need an indication when its on,im trying to fit it into an old laptop power supply case so room is tight.

[m3vuv]

well i built this and it works,check out the pics testing a diode,the verticle part of the trace looks strange,ie like 2 lines not one,anyone have any idea why this is?

[Renate]

Yup, you've got a phase shift.
You know Lissajous patterns? You can visualize them as a 3D phenomena.
If you moved your position (rotated this imaginary 3D world) the two lines would line up.

Take the horizontal signal from your tester and feed it into both inputs (the same way).
You should get you a nice, clean, single line at a 45 degree angle.
If you don't, there is a problems with your 'scope.
If you do, there is a problem with your tester.

Actually, with the whole design of that tester there is no way to get the dynamics of the two channels the same.

Since your scope only has Invert Ch2 (like most 'scopes) and the Y is on Ch1 and the X is on Ch2 (like  most 'scopes) the only way for you to get a normal display is to use Invert Ch2 and use your 'scope upside down!

[m3vuv]

i dont think i will go to those lenths,its good enough for what i need it for!,i wonder why my huntron tracker didnt do the same?,actualy i dont think this dso has an invert function,at least i cant see one!.

[Renate]

No?

[m3vuv]

well spoted,i need to go to specsavers!