Curve Tracer circuit question

[RichardM]

Hi All,

I have decided to build a circuit tracer as a learning experience. Details are here

https://www.changpuak.ch/electronics/Curve_Tracer_advanced.php

I have put a board together and thought I would try and test the step generator part of the circuit that uses the third (lowest in schamatic) transformer in series (primary) and I think the secondary is also in series but has the centre shown wired to ground ? I am not seeing any voltage on any of the regulators ? If I take the rectifier outputs and feed them to a breadboard with an LM7812 and input capacitor I see a nice regulated +12v at the regulator output ? I am very much a learner but it appears it may be a ground issue ? I haven't wired up the other rotary control knobs etc nor have I connected the transformers for the other paers of the circuit.

I have a dual 12v secondary transformer instead of the 9v in the schematic. Some oscilloscope outputs are illustrated on the attached image.

Why is there a ground shown in the centre tap position ? Is this circuit ground ?

Any help appreciated. I have some additional PCBWay circuit boards (located in Aus) if someone else wants to give this curve tracer a go, and help me along the way

Richard

[ledtester]

Just to be sure, in each your scope measurements of (1), (2) and (3), where are you placing your probe and ground clip?

[ledtester]

Why is there a ground shown in the centre tap position ? Is this circuit ground ?

Yes, the center tap is circuit ground. The circuit is creating +12 and -12 power supplies relative to the center tap. It's a standard way to create a +/- power supply, e.g.:

https://theorycircuit.com/dual-power-supply-circuit/

[RichardM]

I don’t have the scope in front of me but from memory :

For 1 I place the probe tip on the + output from the rectifier and ground clip on ground of 7812
For 2 I place the probe tip on the + output from the rectifier and ground clip on - output of rectifier
For 3 I place the probe tip on the output side from the dude and ground clip on input side

In regards to the ground connection at center of secondary output, this is not connected to circuit ground, nor is it when I build this on the breadboard.


Richard

[RichardM]

Why is there a ground shown in the centre tap position ? Is this circuit ground ?

Yes, the center tap is circuit ground. The circuit is creating +12 and -12 power supplies relative to the center tap. It's a standard way to create a +/- power supply, e.g.:

https://theorycircuit.com/dual-power-supply-circuit/

Thanks I will read more on this

[RichardM]

Ok, so if I dont connect the center tap to a circuit ground on this side of the schematic I dont have a circuit ?

[ledtester]

Ok, so if I dont connect the center tap to a circuit ground on this side of the schematic I dont have a circuit ?

You won't have a working power supply.

[Terry Bites]

https://www.jammarcade.net/simple-component-tester-a-k-a-octopus-curve-tracer/

Nothing as simple as an octopus.

[MathWizard]

Yeah OP you would want to have the 2 secondary output's wired more like this below, and pay close attention to the polarity of the windings, you want basically 2 batteries stacked on each other, so in essence 0V +12 +24V, or -12V 0V 12V once you call the center tap the common point.

Also pay very close attention to the diode and capacitor polarities, and remember, just considering the peak voltages, it's just 2 batteries stacked constructively together, and so no one should blow up their cap's, if the bottom of the transformer was called 0, and the middle was called +12V

[RichardM]

Thanks all

I have a functional curve tracer. Works really well actually, best on the analog scope but also very good on the digital scope.

As it turned out there were a couple of issues on the pcb, first the centre tap was not grounded as per above, then the silk masks for both bridge rectifiers were reversed. Its been a good learning experience.