If you search the internet for 12 volt to mains AC inverter designs, the chances are you’ll encounter a simple circuit which has become rather ubiquitous. It features a 4047 CMOS astable multivibrator chip driving a pair of MOSFETs in a push-pull configuration which in turn drive a centre-tapped mains transformer in reverse. Not a new design, its variants and antecedents could be found even in those pre-Internet days when circuits came from books on the shelves of your local lending library.
[Afroman], no stranger to these pages, has published a video in which he investigates the 4047 inverter, and draws attention to some of its shortcomings. It is not the circuit’s lack of frequency stability with voltage that worries him, but the high-frequency ringing at the point of the square-wave switching when the device has an inadequate load. This can reach nearly 600 volts peak-to-peak with a 120 volt American transformer, or over a kilovolt if you live somewhere with 230 volt mains. The Internet’s suggested refinement, a capacitor on the output, only made the situation worse. As he remarks, it’s fine for powering a lightbulb, but you wouldn’t want it near your iPhone charger.
If this video achieves anything, it is a lesson to the uninitiated that while simple and popular designs can sometimes be absolute gems it must not be assumed that this is always the case.
Continue reading “Afroman And The Case Of The Suspect Inverter”
[Afroman] is back again with another great tutorial video on the basics of electronics. This time it’s zener diodes.
Page three or four of every ‘beginners guide to electronics’ covers a diode as, “a component that only allows current to flow in one direction.” This is true; a diode only allows current to flow in one direction. However, like any depth of knowledge, the dialectic of diodes quickly turns to a series of, ‘but..’ and ‘however…’ statements.
A zener diode is like a normal silicon diode, where a forward biased diode will pass current with a ~1 volt drop. When a zener diode is reversed biased, there’s a different voltage drop, annotated as Vz on the datasheet. When reversed biased, current cannot flow across the diode unless the voltage is above Vz. This is what makes zeners useful for a bunch of applications.
[Afroman] goes over a few of the most useful applications of zeners, including a diode clamping circuit. This circuit will clamp the voltage to a maximum of Vz, helpful when you’re feeding a signal into an analog input. This voltage clamping circuit can be used in some interesting applications. If you feed a sine wave or other signal though the circuit, you can clip the signal.
Zeners can also be used as a very crude, low current, low accuracy power supply. If you’re looking for a voltage regulator for a microcontroller that’s impossibly easy and you’re all out of 7805s, pick up a zener. It’s not the basis of a good power supply, but it does work.
An accelerometer is the ubiquitous little sensor that tells your tablet when to flip orientation or informs the brain of your quadcopter how closely its actual actions are matching your desired ones. In a quick three minutes, [Afroman] explains what is inside an accelerometer and how they work.
It turns out the tiny devices that report acceleration in one, two or three dimensions are not powered by magic complicated mechanisms but very simple Micro Electro-Mechanical Systems or “MEMS.” MEMS are similar to copper/silver/gold-wired integrated circuits except in a MEMS circuit conductive silicon is used and they actually physically move, but only just a bit.
The secret is in creating microscopic capacitors along a weighted lever that flexes in response to changes in velocity. When the plates flex the distance between them changes which alters the capacitance. This translates physical motion into voltage which can then be interpreted by the rest of your circuit. The chemistry behind MEMS is interesting too.
This Christmas when your laptop’s power cord clotheslines your cousin’s kid, your hard drive has a chance of parking the head (on the drive, not on the child) between fall and impact and preventing damage (to the drive, not to the child) because of an accelerometer. If bad roads cause you to drift into the ditch, it is an accelerometer that senses the crash and tells your airbag to deploy before your body hits the steering wheel.
The MEMS market is exploding right now and for us hackers in particular, Wearables are looking to be a big part of that growth.
Once again, [Afroman] is here for you, this time breaking down electrolyte and the terminology behind batteries.
Volts and Amps are easy mode, but what about Amp hours? They’re not coulombs per second hours, because that wouldn’t make any sense. An Amp hour is a completely different
unit podcast, where a 1Ah battery can supply one amp for one hour, or two amps for 30 minutes, or 500 mA for two hours.
Okay, what if you take two batteries and put them in series? That would double the voltage, but have the same Ah rating as a single cell. Does this mean there is the same amount of energy in two batteries as what is found in a single cell? No, so we need a new unit: the Watt hour. That’s Volts times Amp hours, or more incorrectly, one joule per second hour.
Now it’s a question of the number of cells in a battery. What’s the terminology for the number of cells? S. If there are three cells in a battery, that battery has a 3S rating. You would think that C would be the best letter of the alphabet to use for this metric, but C is entirely different. Nothing here makes any sense at all.
What is C? That’s related to the number of amps a battery can discharge safely. If a 20C battery can discharge 2200mAh, it can deliver a maximum current of 44 A, with 20C times 2.2Ah being 44A.
So there you go. A complete description of something you can’t use logic and inference to reason through. Video below.
Continue reading “A Description of Maddening Battery Terminology”
We mentioned Pulse Width Modulation (PWM) when talking about [sprite_tm]’s marquee control. It’s a method of power control. While [sprite_tm] did it in software, [Afroman] sent along a very straight forward introduction to PWM using just a 555. Check out his video for coverage of this fundamental electrical design technique.