A Tester For Zener Diodes

Zener diodes are used for several purposes, from providing a reference voltage, to protecting sensitive circuits from being destroyed by the wrong input.

Today, I will show you how these diodes work and how to build a simple circuit to measure their most important characteristic, the reverse breakdown voltage. To know more on this topic, please watch the companion video posted on YouTube.

A zener diode looks like a regular diode and actually behaves as such when directly biased (positive voltage on the anode).

However, when inversely biased (negative voltage on the anode), a zener diode behaves in a completely different way.

Let’s take a look at its characteristic I-V diagram:

zener_characteristic

You can see that in the region of direct (or forward) bias, the zener behaves just like any diode. It also seems like in the inverse bias it behaves like a regular diode.  However, there is a big difference between the two.

For a regular diode, the reverse breakdown voltage is very high, in the order of 100V or more, sometimes much more. Such high that you never think at it when you use regular diodes, and you assume that with inverse bias the diode just does not conduct electric current.

For a zener diode, instead, the reverse breakdown voltage is low, in the order of one or two digit volts. Therefore, it is very easy in an electronic circuit to bring this kind of diode to reach the condition when it will start conducing electric current even if inversely polarized.

We actually exploit this behaviour to create reference voltages, or to provide a protection against unwanted voltages at the input of certain circuits, or a ton of other things.

The behaviour of a diode depends in fact upon the way it was fabricated, and in particular upon how it was doped. Regular diodes are lightly doped, while zener diodes are heavily doped. Depending on the amount of doping on both the P and the N side of the junction, the reverse breakdown voltage changes. That way, manufacturers can create zener diodes within a large range of breakdown voltages.

Problem is, manufacturers often don’t put the value of the breakdown voltage on the body of the components. Instead, they put some internal code or, sometimes, nothing at all.

So, if you had a number of such diodes on your workbench, how to distinguish them from one another?

Meet the zener tester.

It is a device that allows you to measure the reverse breakdown voltage, so you know if the diode works and what that voltage is.

How such a tester works? From the I-V diagram above, you can see that the characteristic of the zener diode is an almost vertical line when polarized in the reverse bias region. For any current value in that vertical line, the voltage is always the same and corresponds to the breakdown voltage. So, if we circulate a current at any point of that vertical line, we can measure at the terminals of that diode its breakdown voltage.

The zener tester I’m showing you today does just that: forces a current into the zener diode so we can measure the value of the breakdown voltage. We choose this current in such a way that it is high enough to stay away from the point where the characteristic is not linear, but low enough to avoid dissipating inside the diode a power that the diode itself cannot handle.

The following link allows you to download an archive containing the schematic of such device, along with the OpenSCAD code to 3D print the box for the device.

zener_diodes_tester_files

In the schematic you’ll see that I used a ready-made boost converter and a digital voltmeter. Here are the links to the store where I bought them. Of course you are free to use any other equivalent component. It will work as well.

BOOST CONVERTER

DIGITAL VOLTMETER

Please make sure to watch the YouTube video that completes the information I provided in this post. Between the two, you should have a complete view of the design of the device and should be able to build it.

Happy experiments!

 

DC Electronic Load V.3

Back in August 2018, I presented a DC electronic load on my YouTube channel (V.2). For that, I used an old 2N3055 transistor in a Darlington configuration with 2 more transistors to be able to get enough gain to use it.

100W_el_load_v2

Although declared useful for 100W, I was never able to make it work at those powers due to the limited dissipation capabilities of the power transistor and the heat sink. The max power dissipation I could have from that device was about 20W.

Today, at the anniversary of that presentation, I have created a new version of of the DC electronic load. This new version is based on a MOSFET that can work alone as a load, adjusting the current only through an appropriate voltage on its gate, avoiding the need of having a Darlington circuit with multiple transistors.

The schematic of this new version of the DC electronic load is based on a single MOSFET capable of driving the necessary current, up to 5A and a voltage divider connected to the battery, providing the appropriate voltage to the gate of the MOSFET.

electronic_load-v.3

In order to make it work correctly, the trim-pot RV1 needs to be tuned to obtain a voltage of 1.5V on pin 3 of the potentiometer that regulates the amount of current flowing through the MOSFET, which provides a better use of the multi-turn potentiometer that regulates the actual value of the current.

A combination of digital voltmeter and am-meter, like in the previous version of the DC load, takes care of providing information about the power supply under test.

The device is powered through a 9V battery and it is connected in such a way that the voltage is measured through the yellow wire of the digital voltmeter, wile the current is measured putting the am-meter in series with the MOSFET, with the thick red wire on the source, and the thick black wire toward the negative connector, through a 5A fuse that is used, mostly, to protect the am-meter itself against currents too high of those it can handle.

I made a new case for this new version of the DC load. The main difference is the location of the heat-sink, which is now located on the back panel rather than the top of the device. The new heat-sink is also attached to the back panel through 4 separators, which allow for a better air flow and cooling of the unit when it is used for long period of times.

Here is an OpenSCAD view of the box and the corresponding code to create it.

v3_box_view

$fa=0.5;
$fs=0.5;

//main section
rotate([180, 0, 0]) translate([0, 10, -2])
{
// front panel
difference()
{
cube([150, 80, 2]);
translate([27.5, 40, -1]) cube([45.8, 27.7, 4]);
translate([52, 20, -1]) cylinder(d=6.2, h=4);
translate([120, 60 , -1]) cylinder(d=9, h=4);
translate([108, 20 , -1]) cylinder(d=9, h=4);
translate([132, 20 , -1]) cylinder(d=9, h=4);
translate([3,3,0.5]) linear_extrude(height=2) text(“eleneasy.com – DC load – 25W max.”, size = 6);
translate([36,18,0.5]) linear_extrude(height=2) text(“off”, size=5);
translate([61,18,0.5]) linear_extrude(height=2) text(“on”, size=5);
translate([109,44,0.5]) linear_extrude(height=2) text(“current”, size=5);
}
translate([102.25, 15, 0]) cube([2, 10, 2]);
translate([126.25, 15, 0]) cube([2, 10, 2]);
translate([111.75, 15, 0]) cube([2, 10, 2]);
translate([135.75, 15, 0]) cube([2, 10, 2]);
translate([12, 20, -36]) cube([27, 2, 35]);
translate([39, 4, -36]) cube([2, 18, 35]);

// left panel
translate([0, 0, -60]) cube([2, 80, 60]);

// right panel
translate([148, 0, -60]) cube([2, 80, 60]);

// bottom panel
translate([0, 0, -60]) cube([150, 2, 60]);

// top panel
translate([0, 78, -60])
{
cube([150, 2, 60]);
}

// screws supports
translate([2, 2, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([138, 2, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([2, 68, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
translate([138, 68, -58]) difference()
{
cube([10, 10, 58]);
translate([5, 5, -1]) cylinder(d=2, h=16);
}
}

// back cover
translate([0, 10, 0]) difference()
{
cube([146, 76, 2]);
translate([5, 5, -1]) cylinder(d=4, h=4);
translate([5, 71, -1]) cylinder(d=4, h=4);
translate([141, 5, -1]) cylinder(d=4, h=4);
translate([141, 71, -1]) cylinder(d=4, h=4);
translate([73, 38, -1]) cylinder(d=40, h=4);
translate([126, 60, -1]) cylinder(d=12.5, h=4);
translate([130.5,51,0.5]) rotate([0, 0, 180]) linear_extrude(height=2) text(“5A”, size=5);
translate([(146-55)/2, (76-50)/2, -1]) cylinder(d=4, h=4);
translate([146-(146-55)/2, (76-50)/2, -1]) cylinder(d=4, h=4);
translate([146-(146-55)/2, 76-(76-50)/2, -1]) cylinder(d=4, h=4);
translate([(146-55)/2, 76-(76-50)/2, -1]) cylinder(d=4, h=4);
}

Assembling the circuit is pretty straightforward, and it is done partially in the air and partially  on a perforated board.

We just need to make sure we provide the cables with the right thickness for the current we need to support.

In my case, I used stranded cables with an 18 gauge. These cables are necessary between the thick am-meter cables, the MOSFET source and drain, and the external connectors.

Every other connection can be made with 22 gauge cables.

And finally, the heat-sink should have a resistance of 0.82 Centigrade degrees per watt or less, to prevent the MOSFET from becoming too hot. Note that this will not save the MOSFET in case you draw a current too high. The product between the current and the voltage as provided by the measurements display must never exceed 25W, and the current should never exceed 5A, or the MOSFET will burn.

The tuning is done by measuring the voltage between the terminal 3 of the potentiometer and the ground, with the circuit on, but not connected to any external power supply. The trim-pot has to be adjusted such that the measured voltage equals 1.5V, which is just below the minimum voltage necessary to make the MOSFET conduct current. This way, when turning on the apparatus with the potentiometer all the way to the counter-clockwise position, there will be no current. Then, moving the potentiometer in the clockwise direction, current will start flowing.

Testing of the unit is done attaching it to a power supply that provides different test voltages while we adjust the current with the multi-turn potentiometer on the DC load unit. Just make sure not to exceed 25W of power at any given time. Doing so could damage the MOSFET itself.

I plan to use this DC load in all my future projects that require a power supply of 25W or less, to test the power supply itself. Besides checking that the power supply works fine, you could also check that the ripple of the output voltage does not exceeds your requirements. That can be done connecting the power supply output to an oscilloscope while the DC load draws the current.

And finally, here are a couple of picture of the finished device.

20190816_112540.jpg

20190816_112614

Happy experiments!

 

Behind The Scenes Of The Theremin Design

How I design my electronic circuits and prepare the videos to show them to you.

20180915_153351

Did you ever ask yourself where I get the schematics of the Theremin circuits and other gadgets that I present on my YouTube videos? The answer is simple: I do some research on books, on specialized magazines and on the Internet. I see solutions created by other people, if any, and then I think about what would better work for my case. Sometimes it ends up to be a modification of something that somebody else did, maybe for a totally different purpose. Sometimes, I just use the general idea to create something different, new, my own design that is more appropriate for my needs.

20181213_105525

Either way, I usually build a number of prototypes of what I need, then I take some measurements in lab, then I start making further modifications to my original design, until I obtain exactly what I am looking for.

20181213_105610

Also, more often than not, I figure that the circuit I am testing is too sensitive to certain parameters of the circuit itself. Maybe is a capacitor which value needs to be adjusted a little bit, or a connection between two or more components that causes issues because of capacitive or inductive coupling with other components. That is when I try to change my design to reduce such sensitivities, so that the circuit can be assembled by anyone with the exact same results as mine. And this is what is called engineerization, or adjusting the design for mass production.

20180915_153412

And sometimes, to do so, it is not enough to test the single circuit. Instead, I need to connect the circuit with other pieces that have to work together with it, and see if further unwanted interactions happen, so that I can eliminate them or, at least, reduce them so that they become negligible.

20180915_153256

Sometimes this process goes fast, sometimes takes a long time. And that’s why my videos are not published at fixed intervals. Unfortunately, since this is done only as a hobby, I don’t always have enough time to dedicate to my project, so days go by until, finally, I am done. Then I finalize my schematics, I build the last prototype and the final product and, in the process, I also record all these activities so I can end up making a video out of them.

20180915_153313

Then the video editing process starts and, once the video is finally ready, I release it on YouTube for you to watch it.

One day I will be able to do this full time. Who knows, maybe when I retire. Or, maybe, if you all give me a hand, this could become my new full time job (donations, donations, donations). We’ll see.

Thank you for reading this article. And, as usual, happy experiments!