Posted on Wednesday, May 8, 2019

7 MOSFET Myths and Misconceptions Addressed

Let's set some things straight

by James Lewis

The most popular AddOhms video is my short tutorial on MOSFET basics. In the years since I posted the video, people have sent me many questions. While answering those questions I’ve learned quite a bit as well. For example, in that video, I say that Vgs is the threshold to turn on the MOSFET. Well, it turns out, that is not entirely true. It is the threshold to turn it off! Oops. Minor point with a subtle difference, but a common MOSFET misconception.

In this post, I dispel that and other common myths and misconceptions around using MOSFETs. As with all engineering tips and tricks, this post is not a definitive guide to FETs. Instead, it is meant to be a guide to help you ask the right questions to design in the correct part.

1. Misconception: You don’t need resistors on the gate

Back when I made the AddOhms episode, I added a resistor to the MOSFET’s gate pin. Of course any time a resistor is shown in a schematic, people get worried about what complicated formula is needed to determine its value. For slow switching applications, like below 10 kHz, the resistor value doesn’t matter. Something in the 100 to 1000 KOhm range is fine.

P-Channel with series gate resistor

So if the value does not matter, why have one? The gate of a MOSFET is a small capacitor. And what happens when applying a voltage to a capacitor? It starts charging.

Resistor-Capacitor Charging Curve: Voltage and Current

The initial current is very high. It slows down as the capacitor charges. That initial current rush, also known as in-rush current, can be a problem. Even though it is a short time, there is a significant current surge which can damage an I/O pin. Depending on the size of the MOSFET’s gate capacitance, it may not be necessary to include that resistor. I wish I could say to “just” add it any time you use a MOSFET. If there is a high switching frequency, say 100 kHz or higher, then you have to worry about the RC charging curve created by the resistor and the gate capacitance.

Classic, vintage, or retro computer systems are well documented on sites like Wikipedia. Their historic position is well known. Their schematics are even published from original documentation. But how useful are those schematics in their current form? (Spoiler, not much.) Presented at KiCon 2019.

Download Slides (8mb)

Posted on Wednesday, April 10, 2019

Low side vs. High side transistor switch

Why do these both exist, how do they work, and when do you use them?

by James Lewis

A common task for a transistor is switching a device on and off. There are two configurations for a transistor switch: low side and high side. The location of the transistor determines the type of circuit and its name. Either transistor configuration can use a BJT or MOSFET.

In this post, I draw the configuration for both transistor types, talk about which requires a driver, and explain why you would use either. If you are new to transistors, check out the resource links at the bottom. I have a couple of videos I made and some from element14’s The Learning Circuit which do a great job introducing transistors.

On April 26th and 27th, the first year of KiCad KiCon kicks off in the Windy City. Join me and 26 others for talks about the open source electronics CAD tool. The list of speakers is impressive. There are many names which I follow on social media and some I recognize from the KiCad team. For example, Wayne Stambaugh is the KiCad project leader and has one of the keynote talks.

You can see the full list of KiCon talks here. And tickets are available here.

Bald Engineer’s Apple IIgs KiCon Talk

Here is the description for my KiCon talk.

While documentation exists for 1970’s and 1980’s calculators and computers, unfortunately they exist in bitmap formats. As I started converting parts of the Apple IIgs schematic to KiCad, I realized something. There are benefits to “preserving” historical schematics in a living, active, and open format. In this talk, I talk about my experiences in converting scanned PDFs into KiCad, the project behind that motivation, and to encourage help from others to preserve history with KiCad.

So what is it? Well, several months ago, I did a couple of Apple IIgs hardware live streams. I have a project in mind for the MEGA-II ASIC. But before I could move forward on the project, I wanted a modern version of the IIgs schematic.

While schematics for classic 8 (and 16-bit) computers are readily available, they are usually only in PDF format. Studying the design is like reading a book. While I am glad the PDFs are available, I would like to be able to do actions like a search.

In my talk, I’ll explain why we should be converting these classic schematics into an open format. Along the way, I’ll take the audience through my journey of using KiCad for this project. In the end, I’ll be asking for help to convert other classic computer schematics.

Where is KiCad KiCon 2019?

The location for the conference is mHUB in Chicago, IL. If you’re able to attend in person, I look forward to meeting you. If you’re not able to travel, I fully expect either a live stream or recorded versions of the talks to be available.

Learn more about KiCad KiCon 2019

Date:	April 26, 2019—April 27, 2019
Event:	KiCon 2019 - Preserving history with KiCad
Topic:	Preserving history with KiCad
Venue:	mHUB
Location:	965 W Chicago Ave Chicago, IL 60642
Public:	Public
Registration:	Click here to register.

The concept of high end varies depending on what you are talking about and who you talk too. In this Evaluation Engineering Evaluation Engineering article the author discusses high-end oscilloscopes. I am mentioned several times in this article as part of my day job at Rhode and Schwarz. There I am a product manager for oscilloscopes in North America. We have scopes that range from 50 MHz to 8 GHz.

For a little bit of context let me explain how this type of article works. The author, or editor, reaches out to some field experts. Each person is asked to fill out a written interview form. From there the author compiles the responses into a story like this one. This process is always nerve racking to me. I always worry I’ll misquote a specification or make a major typo. I don’t get see the article until it is published.

If you aren’t familiar with high bandwidth oscilloscopes, I think you will still find some value in reading about my favorite test tools.

Read the full article

Date:	March 25, 2019
Appearance:	High-end Oscilloscope report on Evaluation Engineering
Outlet:	Evaluation Engineering
Format:	Magazine

Surface Mount Rework Tools – Workbench Wednesdays

element14 Presents on YouTube
2019-03-27

In this element14 Workbench Wednesdays episode, I review tools provided by Weller which are suitable for surface mount soldering. Through-out the soldering series, I have been using mini-projects to see how the gear works. Making this particular video was special to me. The subject was a TI-85. Back when I was a kid, one of my first soldering projects was to replace a capacitor in the TI-85. At the time, all I knew is that the change would make it run faster. I didn’t know why I just knew it worked.

Today, I now know that capacitor was part of an RC oscillator for the Z80 CPU. It clocked the processor. By putting in a lower value, such as 2.2 or 4.7 pF, the calculator would speed up. The trade-off, of course, is that it means the batteries drain faster! But hey, before someone created Zshell, this was the only way to make Breakout run fast.

Of course, the focus of the episode is the gear from Weller. So please, hit-up element14 and check that stuff out. You can also find the polls I mention at the end of the video there.

Watch, Comment, and Vote on element14

If I were a professional actor, I would feel typecast at this point in my career. Whenever someone wants to talk about Arduino, Oscilloscopes or Capacitors, they call me! In this case, element14 asked me to do two videos on how to replace multilayer ceramic capacitors (MLCCs) with Polymers.

Polymers are an interesting capacitor type. What is usually called a “polymer” is better a called a “polymer electrolytic.” The reason for that detail is the word “polymer” describes the cathode layer and not the dielectric.

For more details, why not check out this episode of element14 Presents’ The Learning Circuit! If you have questions about these capacitors, head over to element14 and leave me a comment there.

e14’s TLC #40

Date:	February 13, 2019
Appearance:	Polymer Capacitor Introduction on e14s The Learning Circuit
Outlet:	element14 Presents' The Learning Circuit
Format:	Vlog

Soldering Tool Upgrade Paths

element14 - Workbench Wednesdays
06-FEB-2019

Right after the digital multimeter, or DMM, a soldering iron is a must-have tool for electronics work. Like most tools, there is a vast variety of options available. In this episode of element14’s Workbench Wednesdays, I look at a range of instruments from Weller. They offer everything from a cheap $10 “fire starter” (sorry, it is what we call them!) all the way up to a full-blown surface mount rework station.

Whether you don’t have a soldering iron or you have a $100 station, this video will show you options to consider when thinking about an upgrade.

After you watch the video, head over to element14 and tell me for you favorite solder tips! (Or your most burning questions!)

Send James your Solder Tips

Answering BJT questions with Karen on element14’s The Learning Circuit

element14's The Learning Circuit
2019-01-02

Over on element14, Karen hosts The Learning Circuit. It is a tutorial show geared towards learning STEM basics. So far she has covered subjects like soldering, diodes, and how to make a DIY electromagnet. She did a great job on introducing BJTs and how they work. While I thought she provided a clear explanation of the internal workings, some members of the element14 community still had questions.

She invited me on to revisit BJTs and transistors to (hopefully) clarify the connection between how transistors physically work and how to use them.

Posted on Wednesday, November 21, 2018

Connect pins with KiCad Bus, Labels, and Global Labels

When to use them and why in a KiCad Schematic

by James Lewis

When your schematic has a large number of related signals, it is helpful to group them. In its schematic editor, KiCad has a few tools to help. Your end-goal helps determine which tools to use. For example, do you need a KiCad bus or a label? In this post, I explore how you can define signals, group them, and reference them across schematic sheets.

Up until recently, I did not need to use a bus or multiple sheets. However, the Apple IIgs project I’m working on is too large for a single page. In a KiCad live stream, I looked at how to create busses and connect them. In a separate tutorial, I will show how to work with multiple sheets in KiCad.

tldr; KiCad does not require the use of a bus to connect signals together. Wire labels already provide that connection. A KiCad bus offers two things: 1) a visual representation and 2) an easier way to create global connections (across sheets.)

Lastly, if you are not familiar, KiCad is an Open Source eCAD tool. Although I have used others, this one currently my preferred platform.

KiCad Bus, Label, and Wire

Before jumping to how to use a bus, first, we need to start with the basics. KiCad connects nodes with a “wire” element. KiCad gives each wire drawn a unique name unless it connects to an existing node. The user can override the name by adding a label.

Wire and Label Example