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Quorten Blog 1

First blog for all Quorten's blog-like writings

Picking a MOSFET with proper support for gate voltage

quorten

2020-03-29

Categories: raspberry-pi  
Tags: raspberry-pi  

MOSFETs… yes, they have so many gate voltage threshold parameters. How do you navigate them if you, say, want to be able to control a MOSFET with a 5 V signal? Here’s how.

  • Look for “drive voltage” as this is indicative of how.

  • “Gate threshold voltage” is the minimum voltage that you’ll barely get any MOSFET switching behavior.

  • Max gate voltage is, of course, the maximum voltage you can put on the gate.

MOSFETs are kind of like BJT transistors, they’re not perfect binary switches. When there is less voltage, the source-drain path is more restrictive to the flow of current than when there is more. That’s what the range of gate threshold voltages is indicative of, and the restrictiveness to current flow is indicated by the varying resistance between the source and the drain.

20200328/DuckDuckGo mosfet drive voltage
20200328/https://electronics.stackexchange.com/questions/367274/what-is-drive-voltage-for-a-mosfet

Good Wikipedia info on switched-mode power supplies

quorten

2020-03-28

Categories: raspberry-pi  
Tags: raspberry-pi  

Switched-mode power supplies, switched-mode power supplies. Gosh they can be difficult to understand. So again I go looking back at Wikipedia, and of course I should have known. The Wikipedia article on switched-mode power supplies is a good table of contents for all variations of the switched-mode power supply design.

20200328/https://en.wikipedia.org/wiki/Switched-mode_power_supply

In particular, I’m using a SEPIC switched-mode power supply design, and this article has really helped me better understand how it works.

20200328/https://en.wikipedia.org/wiki/SEPIC_converter

Some more good Digi-Key articles

quorten

2020-03-28

Categories: raspberry-pi  
Tags: raspberry-pi  

Here are some more good Digi-Key blog articles that I found. First, some recommendations on using low-pass filters with analog inputs for the sake of anti-aliasing. Of course, to some extent depending on your particular application, this may be a judgment call depending on whether you want to have that particular anti-aliasing effect at the expense of lower resolution.

20200327/https://www.digikey.com/en/articles/the-basics-of-anti-aliasing-low-pass-filters

Second article, ways to improve the driver circuit for a piezo buzzer. Wow, now this is interesting… so the basic driver circuits don’t give you the best output, but if you want some more “Hi-Fi” like output, you can use some of these more complicated and expensive driver circuits.

20200327/https://www.digikey.com/en/articles/techzone/design-techniques-to-increase-a-piezo-transducer-buzzer-audio-output

But, the ultra article discovery, RISC-V development boards? Wow, that’s becoming a bigger and bigger thing so it looks.

20200327/https://www.digikey.com/en/articles/how-to-get-started-with-risc-v-based-microcontrollers

The trick to directly connecting a magnetic buzzer without flyback diodes

quorten

2020-03-27

Categories: raspberry-pi  
Tags: raspberry-pi  

Quite some time ago, I bought an Inland electronics parts pack from Micro Center, and in the instructions for using the included buzzer, it simply said to connect it directly to your microcontroller. Sure, that makes sense, it could be a piezoelectric buzzer so it wouldn’t generate inductive back EMF, hence it wouldn’t need a flyback diode. Well, anyways, I did just that, directly connected it to my Raspberry Pi Zero, and it worked fine… but upon closer inspection, it seemed to clearly be a magnetic device, not a piezoelectric device. Weird, how did this work just fine without damaging my Raspberry Pi? Was it because it was sufficiently low current that the back EMF if generated could have caused no damage?

Sort of, that’s not quite it. After some careful thought, the real reason why this works okay becomes clear. The magic is in the fully buffered termination in the pull-up GPIO output. When you switch the GPIO output off, the Raspberry Pi provides a path to ground on the GPIO output, rather than leaving the output “floating” as an “open collector” design would. Now, the magnetic coil inside the buzzer wants to keep the current flowing, and that is absolutely no problem in this case. Since both ends of the coil are connected to ground, a current can keep flowing in the coil any way it pleases, and since there is no additional resistance on the coil, it will do so at its desired current, which will actually end up generating a back EMF voltage equal to the original supply voltage.

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Field programmable analog arrays and arrays of analog switches

quorten

2020-03-26

Categories: raspberry-pi  
Tags: raspberry-pi  

Field programmable analog arrays, mixed signal field programmable gate arrays. Yes, yes, these would be the ultimate ideals in having reprogrammable hardware, but they are hard to come by.

20200326/https://en.wikipedia.org/wiki/Field-programmable_gate_array
20200326/https://en.wikipedia.org/wiki/Field-programmable_analog_array

Alas, there is an easier and more commercially abundant solution. With some lateral thinking, of course it makes sense. I’ve seen the “bilateral switch” solid-state analog switching chips that team together two separate analog switches into one chip. Why not make such a chip that teams together a grid of such switches on a matrix of wires? Indeed, I’ve found such a chip right here. You can get either an 8x8 or 16x8 grid, and every single switch is individually accessible via an address bus.

20200326/DuckDuckGo programmable interconnect grid
20200326/DuckDuckGo programmable analog grid
20200326/DuckDuckGo programmable semiconductor switch array grid matrix
20200326/https://www.digikey.com/product-detail/en/microchip-technology/MT8809AP1/MT8809AP1-ND/4309754
20200326/https://www.digikey.com/product-detail/en/microchip-technology/MT8816AP1/MT8816AP1-ND/4309764
20200326/https://www.digikey.com/product-detail/en/microchip-technology/MT8816AF1/MT8816AF1-ND/4309763
20200326/https://www.microsemi.com/document-portal/doc_download/127034-mt8809-datasheet-sept11

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Can you use a soldering iron to reflow solder paste? Yes.

quorten

2020-03-26

Categories: raspberry-pi  
Tags: raspberry-pi  

Solder paste, it sure is a nice material to work with, but is doing reflow soldering in an oven the only way to work with it? Can you just reflow it using a soldering iron? Indeed, you can!

Please note that if you are soldering through-hole components, using a soldering iron with traditional wire-type solder still is the best way to do that.

20200326/DuckDuckGo can you use a soldering iron to reflow solder paste
20200326/https://www.quora.com/How-do-you-use-solder-paste-with-soldering-iron?share=1

Understanding inductance calculation based off of geometry, and enlightenment on the readymade inductor market

quorten

2020-03-26

Categories: raspberry-pi  
Tags: raspberry-pi  

Inductors, inductors. The sophisticated looking “donut rings” that taunt electronics hobbyists. You wnat to get beyond the basics and get into building your own switched-mode power supplies, and this requires an inductor, ideally a “donut ring” inductor. Okay, so how do you approach getting one of those? Some people buy their toroidal inductors, others wind their own. That being said… I want to know how you wind your own.

The Wikipedia article on inductors contains some good equations for calculating inductance based off of geometry.

20200325/https://en.wikipedia.org/wiki/Inductor

So, let’s try out an example with the dimensions of one toroidal inductor ferrite core that I found on Digi-Key.

20200325/https://www.digikey.com/product-detail/en/tdk-electronics-inc/B64290L0618X038/495-3861-ND/1830191

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Beware of ground loops when using Schottky diodes to polarity protect multiple power supplies

quorten

2020-03-25

Categories: raspberry-pi  
Tags: raspberry-pi  

Schottky diodes, yes, your primary go-to when you want to protect a potentially battery-powered system from polarity inversion, right? Yes, indeed, if you have one power supply voltage input, just use one Schottky diode and you’re done. But what if you have a board with two power supply inputs? For example, you might have both 5 V and 3.3 V logic components on your board, and assuming you do not use an on-board switched-mode power supply, that means you’ll have two power connectors. So, just use two Schottky diodes.

Hold on. Now, this is where you need to scrutinize your specific wiring. Often times in logic circuits with both 5 V and 3.3 V power supplies, a “common ground” return path is used for both voltage potentials. Assuming the worst in a prototyping environment, you might connect a voltage source across the two different potential levels. So, how is this problematic? Well, if your circuit design is botched, you might have been assuming that current would never be able to flow across that path and somehow have some short across that path. Like, “Hey, I don’t care whether this circuit gets 5 V or 3.3 V, just give me either power supply that is available.” So you wire the 5 V and 3.3 V together just like a “Y” connection, and, just like that, you’ve got a short.

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Decisions on choosing a supercapacitor

quorten

2020-03-22

Categories: raspberry-pi  
Tags: raspberry-pi  

Okay, so now that I understand how a capacitor works better, let’s try my hand at selecting a supercapacitor. How do you know what parameters are needed when selecting a supercapacitor? Remember, the capacitance is an indicator of how long your capacitor can supply a particular voltage before the voltage sag becomes too great for your computer to keep operating off of the remaining power. For simplicity, rather than using a boost switched-mode power supply and a smaller value supercapacitor, we can just use a larger value supercapacitor. Besides, it’s rechargeable and it’s short-term charge, so it’s not as essential to maximize the running time on a single battery or charge.

C = Q / V
V = Q / C
Q = C * V

C = 1 F
V = 5 V
Q = 5 C

C = 1 F
V = 3 V
Q = 3 C

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Explaining n-channel and p-channel MOSFETs

quorten

2020-03-22

Categories: raspberry-pi  
Tags: raspberry-pi  

MOSFETs… n-channel MOSFET, p-channel MOSFET, how do you use them? What do they mean? An n-channel MOSFET is turned on when you supply a positive voltage to the gate, this allows positive voltage current to flow from drain to source. (The source provides a source of electrons.) A p-channel MOSFET is turned on when you supply a path to ground to the gate, this allows positive voltage current to flow from source to drain. (The source provides a source of holes.)

Yes, so in essence, control of MOSFETs is very much like PNP versus NPN BJT transistors: an n-channel MOSFET is like an NPN transistor, a p-channel MOSFET is like a PNP transistor. And, therefore, what I have previously written about current paths for BJT transistors is also somewhat applicable for MOSFETs too… just that not nearly as much current needs to flow through the gate of a MOSFET for it to operate.

It’s hard to fish that information out of Wikipedia, and there are definitely easier ways to explain for electronic circuit designers. You don’t need to know the full theory in this purpose.

20200322/https://en.wikipedia.org/wiki/MOSFET
20200322/https://en.wikipedia.org/wiki/NMOS
20200322/https://en.wikipedia.org/wiki/Field-effect_transistor#n-channel_FET

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