Lab VI & Lab VII: Diodes and Rectification

[DISCLAIMER: I am pretty sure that I included erroneous explanations of certain elements of this lab due to my lack of complete comprehension of them. So if you find something that’s wrong, I would very greatly appreciate it if you could let me know . :) ]

The objective of this lab was to learn about diodes and their role in rectification.

Intro
The purpose of this lab was to become more familiar with arming circuits and mainly to understand the role of diodes in rectification circuits.
The basic idea behind rectification is to turn AC current into DC current.
We followed the lab from Panitz notes (found on PhD. Koch’s github page: https://github.com/mxbenitez/2012-Physics-308L-Lab-6-Diodes-etc/blob/master/Panitz%20notes_diodes,%20rectification,%20voltage%20stabilization.pdf)
and progressively altered the first simple half-wave rectifying circuit by adding more diodes, a capacitor, changing up the wiring,etc. While arming the circuits we analyzed the signal output through one of the 1kOhm resistors.

Materials
430kOhm resistor
1kOhm resistor
1N4002 diode
1n5242 zener diode
Oscilloscope probes
Heathkit Breadboard
Jumperwires
Multimeter

WEEK I

Setting up
The lab started out with a small introduction to the wiring of the transformer. There’s a nice thing to note about the transformer on the heathkit: it has a center tap. Anthony explained that this means that the two taps on the edges are 180 degrees out of phase with eachother (this comes in handy in circuit ii). Also, if you measure the voltage across the center tap and an outer tap the voltage will be 15V; if you measure across both outer taps the voltage will be 30V.

(i) Half wave rectification
 

                                Images 1: Half-wave rectification circuit

As you can see, the first circuit had both resistors and a diode connected in series. I measured the voltage across the 1kOhm resistor and saw the effects of the diode: it was only allowing the positive direction flow (the original wave has two directions + and -)– hence the half-wave rectification. I wondered about the voltage across the second resistor, so I  measured the output across the whole circuit and saw that it doubled.

                        Image 2: Graph of the half-wave rectification

 

(ii) Full wave rectification

                                 Images 3: Full-wave rectification circuit

In the last circuit , half of the signal was just being wasted. In this circuit we connected a second diode in parallel with the other one, which converted the whole wave into one, in this case, positively directed wave– hence the full wave rectification. However, this is only possible because the outer power supplies are 180 degrees out of phase! So the wave going through the second diode is making up for half of the wave that was missing from the half-wave rectification. If the outer power supplies were in phase, we’d just get the same half rectified wave. I wondered if the amplitude would double, had their power supplies been in phase. From what I understood, it turns out that the amplitude wouldn’t double because the diode has a very straightforward trait: it’s open or closed and when it’s open that’s as far as it goes. So a combination of two diodes wouldn’t change this aspect of the signal.

                         Image 4: Graph of the full-wave rectification

(iii) Filtering

                           Images 5: Filtering Circuit

The main purpose of the last two circuits was to convert AC into DC current (from what I understood). PhD. Koch explained to me that this was achieved but the there was still some AC on the DC– this is the AC coupling. To filter the AC out we introduce the capacitor in parallel with the resistors.This creates a low pass filter. Which you can observe in the next few pictures…

                               Image 6: Graph of filtering

Image 7: Using the “fourier transform” function of the oscilloscope to observe the high frequencies. This is without the capacitor.

Image 8: Fourier transformation again, this time with the capacitor. Notice how the frequencies that remained are lower than in image 7.

 

 

WEEK II (day i)

(iv) Zener Diode


                                           Images 9: Zener diode

In this circuit, the zener diode was connected in parallel to the resistors. When the zener diode is introduced, it keep a steady voltage AKA the zener voltage for a wide range of current. To determine the zener voltage, I referenced the zener diode’s datasheet:
http://www.datasheetcatalog.org/datasheet/HitachiSemiconductor/mXwuzrw.pdf

Its zener voltage was 12 V + or – 5.

                    Image 10: Zener diode

 

(v) Light Emitting Diode

                                 Image 11: LED in series

I introduced an LED in series with the resistors. My understanding of this was that the LED  minimized the variance of the zener voltage from +- .5 to +- .2, so the zener voltage was minimized to 12. +- .2 . An observation PhD. Koch pointed out was that  without the zener diode the LED would flicker (although it was a very small flicker, undetected by my eyes).

                                        Image 12: LED

 

The day of the lab I was feeling pretty mentally exhausted, so I decided to come in the next day and finish up the last circuit.

WEEK II (day ii)

(vi) Full Wave Bridge

                                 Image 13: Full Wave Bridge

The outcome of this circuit is to serve the same purpose as in circuit iii. The difference in this circuit is that it’s yielding the same result as if the transformer didn’t have a center tap; the addition of the diodes make up for this. PhD. Koch pointed out that these kinds of circuits are more common since most typical transformers don’t have a center tap.

When the circuit was all armed and ready and connected to the oscilloscope and measuring across the 1kohm resistor, a problem came about. The expected waveform was supposed to be similar to image 6 but instead was illustrating a waveform which was all over the place and not steady. A zener diode was added to the circuit to make the voltage more steady, but it didn’t change the waveform.

PhD. Koch predicted that the reason why the waveform was so messed up was because there were two grounds: one on the oscilloscope (can see this in image 6, between CH1 and CH2) and one within the circuit.This was a problem because in connecting the leads of the oscilloscope to measure across the resistor, the oscilloscope was forcing one of the resistor ends to be grounded. When this happens, the lead draws a bunch of current through the circuit. This was visible via two weird behaviors in the circuit: the little shocks that were observed when connecting the leads to the resistor and the increase in temperature of the diodes.

To fix this we got an old school Tektronix 2445 150Mhz oscilloscope which wasn’t grounded. The older oscilloscope was connected across the same resistor and the waveform came out steady with a voltage of around 12 V, which was what it was supposed to be. (!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!)

Image 14: Full Wave Bridge connected to an un-grounded oscilloscope

 

Common Problems and Solutions
1. Wiring. This was one of the biggest problems while arming the circuit. The solution is simple: keep in mind that the breadboard is connected vertically, not horizontally. Also, distinguish what is in series and what is in parallel. Once things are broken up this way, it’s easier to arm the circuit.

2. Measuring current across circuit elements. The reason why I hardly have any numbers at all is because I went about measuring circuit elements the total wrong and dangerous way. I took a multimeter and measured the current across one resistor, both resistors, a resistor and a diode, etc. Don’t do this!!!! It turns out you draw a bunch of current from the circuit which damages components like Zener diodes (of which I ruined ~3) and you can shock yourself. The right way to do it is by connecting the multimeter as an element in the circuit. For example, to measure the current across the zener diode and the resistor, you wire up the multimeter in series with the resistor and the zener diode.
By doing it this way I was at least able to get one reading for circuit iv:
current (in dc)= 0.007 amps

Toolbox
Oscilloscope Basics

 

Credits
Images– the circuit diagrams were taken from the Panitz notes, while Images 1-12 were taken with Anthony’s camera phone and Image 13 was taken with PhD. Koch’s camera phone. PhD. Koch and Anthony helped clarify the vast majority of questions I had.

EDIT
Repetition of the full-wave rectifier.

PhD. Koch said that someone had tried the lab on monday with the old oscilloscope but said that they weren’t able to make it work. :(
New approach: Oscilloscope differential.
I wired up the circuit (with reference to John’s photo on this lab: http://j2phenom.iheartanthony.com/wp-content/uploads/2012/03/wpid-1330991533062.jpg) and attached the leads.

                     Image 15: Full wave rectifier circuit

I pressed math menu on the oscilloscope and changed the operation to subtraction.  First I attatched the leads of the inner wire (one to CH1and other to CH2)and measured across the 1kOhm resistor. I got a wave; the diodes weren’t getting hot.

Image 16: Difference between channels yields the correct waveform

 

Then I connected with the outer wire and I got a flat line. With the outer wire the diodes started getting hot.  Also, I observed that it doesn’t make a difference if you do CH1-CH2 or CH2-CH1, at least graphically I didn’t notice one.

Image 17: Correct way of connecting the leads

This was wrong though because, PhD.Koch said that you can’t connect the grounds across the resistor because it’s bad. So he switched it over to the non-ground wires (inner wires) .

Image 18: Note the equal scales in CH1 and CH2, the Cursor 1 voltage and the flat waveform at the very top

Then made sure to have the same scale in CH1 and CH2 on the oscilloscope. Then turned on the cursor and put it over the SUBTRACTED wave (which was flat!!!) and said that it was 14V which is what we want.